Influenza virus drift. Antigenic structure. Typical Ags of influenza A viruses are hemagglutinin and neuraminidase; The classification of influenza viruses is based on the combination of these proteins. Molecular genetics of influenza virus
Typical Ags of influenza viruses type A - ; The classification of influenza viruses is based on the combination of these proteins. In particular, 13 Ags are isolated from the influenza A virus various types hemagglutinin and 10 types of neuraminidases. Antigenic differences among viruses of types A, B and C determine differences in the structures of NP and M proteins. All strains of type A viruses have group (S-) Ag, detected in RTGA. Type-specific Ag - hemagglutinin and neuraminidase; variation in their structure leads to the emergence of new serological variants, often in the dynamics of one epidemic outbreak (Fig. 26 –2 ). Changes in antigenic structure can occur in two ways:
Y Layout, Figure 26-02.
Rice. 26 –2 . Mechanism diagram, causing antigenic shift and antigenic drift of influenza viruses. Explanations in the text.
Antigenic drift. Causes minor changes in the structure of Ag caused by point mutations. To a greater extent, the structure of hemagglutinin changes. Drift develops in the dynamics of the epidemic process and reduces the specificity of immune reactions that have developed in the population as a result of previous circulation of the pathogen.
Antigenic shift. Causes the emergence of a new antigenic variant of the virus, unrelated or distantly antigenically related to previously circulating variants. Presumably, antigenic shift occurs as a result of genetic recombination between human and animal virus strains or latent circulation in a virus population that has exhausted its epidemic capabilities. Every 10–20 years, the human population is renewed, but the immune “layer” disappears, which leads to the formation of pandemics.
R. G. WEBSTER and W. G. LEIVER i(R. G. WEBSTER and W. G. LAYER)
I. INTRODUCTION
The influenza virus type A1 is unique among the causative agents of human infectious diseases due to its ability to change its own antigenic structure so strongly that the specific immunity acquired in response to infection with one strain provides very little or no protection against the next emerging virus. Due to this Due to the variability of the virus, influenza continues to be one of the main epidemic human diseases.
Two types of antigenic variation have been found in influenza viruses: antigenic drift (Burnet, 1955) and significant antigenic shift. Antigenic drift is characterized by relatively small changes that occur mainly within a certain family of strains, each of which can be easily correlated with all other strains of this family in relation to both internal and surface antigens. Among the influenza A virus strains that infect humans, each subsequent variant replaces the previous one. This is possibly due to the selective advantage that new antigenic variants have in overcoming host immunological barriers. Antigenic drift is characteristic of influenza viruses not only A, IAO and B.
The second type of antigenic variation, which has only been described for virus A, involves more unexpected and dramatic changes. These are called significant antigenic shifts2. These shifts occur at intervals of 10-15 years (see Chapter 15) and are marked by the emergence of antigenically “new” viruses to which the population has no immunity, and these are precisely the “viruses” that cause significant influenza pandemics.
These “new” viruses have HA1 and NA subunits that are completely different from those circulating among humans before the emergence of the new virus. A significant shift may occur in one or both surface antigens; Two influenza pandemics have been described, caused by viruses belonging to each of these two categories (see Chapter 15).
Influenza is also a natural infection of some animals and birds. Viruses, exclusively type A, have been isolated from pigs, horses, and a variety of birds, including chickens, ducks, turkeys, quail, pheasants, and terns (McQueen et al., 1968; Pereira, 1969; World Health Organization, 1972). Previously, it was believed that the surface of an influenza virus particle consists of a mosaic of antigens that are part of all strains of this type, and that antigenic variation results from the movement of these antigens from prominent to recessed positions and vice versa. Later, another mechanism of antigenic drift was proposed. It is currently believed that changes are constantly occurring in the amino acids that make up the antigenic determinants of the HA and NA subunits. They are the result of the selection of mutants that exhibit changes in the amino acid sequence of polyeltide subunits, caused in turn by mutation of the viral RNA. Significant antigenic shifts, as a result of which “new” viruses arise, are probably due to a different mechanism. The hemagglutinating and neuraminidase subunits of these “new” viruses are antigenically completely different from the subunits of viruses circulating among humans before the emergence of new strains. We believe that the “new” virus is not the result of a mutation of a previous human influenza virus, but arises from genetic recombination between a human virus and one of the many strains of influenza A virus whose natural hosts are animals or birds. Having emerged, the “new” virus replaces the “old” one, which completely disappears from the human population.
Significant antigenic shifts in influenza B viruses have not yet been identified. Pereira (1969) suggested that the lack of significant antigenic changes in influenza B viruses may be a consequence of the absence of such influenza viruses among lower animals and birds.
Antigenic variation involves only the HA and NA subunits; The internal proteins of the virus (nucleoirotein antigen and matrix or membrane M protein) are largely constant. Of the two surface antigens, HA is the more important because antibodies to this antigen neutralize the infectivity of the virus.
II. FLU IN HISTORICAL ASPECT (see also Chapter 15)
A. EVIDENCE OF ANTIGENIC CHANGES
Influenza-like illness has been frequently reported in past centuries (Hirsch, 1883); the disease occurred either in the form of pandemics, affecting a very large part of the population and spreading almost throughout the world, or as local outbreaks. Until 1933, when the influenza virus was first isolated (from humans. Ed.)1, it was impossible to say with certainty whether a given pandemic was actually caused by an influenza virus. However, the characteristics of the epidemics described in historical documents indicate that these epidemics could well be caused by influenza viruses. Although other infectious diseases may have symptoms characteristic of influenza, only influenza causes sudden epidemics that last several weeks and disappear just as suddenly (Burnet, White, 1972). Serological studies conducted on elderly people also indicate about previous influenza epidemics that occurred in not so distant times (Mulder, Mazurel, 1958).
The earliest known influenza epidemic was recorded in Germany in 1170 (Hirsch, 1883), and from other historical sources it is possible to compile a fairly complete list of epidemics in Europe since 1500. Only the most severe epidemics will be mentioned here. More details can be found in Hirsch (1883), Creighton (1891, 1894), Burnet and Clarke (1942), and Burnet and White (1972).
Epidemic of 1781-1782 began in Asia in 1781, and then by early 1782 spread through Russia to Europe. This epidemic caused relatively few deaths, but its peculiarity was that the disease more often affected middle-aged people than children and the elderly. Quite severe epidemics also occurred in 1803, 1833, 1837 and 1847. Epidemic of 1847-1848 began in Eastern Russia in March 1-847 and reached Europe and England in the winter of 1847-1848. This epidemic has caused many deaths, especially among the elderly.
The 1889 pandemic also came to Europe from Russia, reaching England and America in early 1890. The disease spread at the speed of travellers. After the virus emerged in 1889, there were four further waves of infection in each of the following years. The second and third outbreaks caused many deaths, especially among children and the elderly. Serological (Mulder and Mazurel, 1958) and other studies (Pereira, 1969) suggest that viruses related to influenza viruses of Asia, Hong Kong and equine serotype 2 were present at that time.
The most severe influenza pandemic occurred in 1918-1919. The exact location of this pandemic is not known, but Burnet and Clarke (1942) believe that the virus could have developed independently in Asia and Europe or could have been introduced to Europe (by Chinese workers. The pandemic occurred in waves and killed an average of 20 to 50 million people. "human lives, mainly young people. The 1918-1919 pandemic was probably caused by a strain of influenza A virus related to the swine influenza virus. This was first suggested by Laidlaw (1935) and Shope (1936), but it is possible that this virus was transferred from humans to pigs, and not in the opposite direction.Intensive studies of the decline with age of antibodies to the swine influenza virus in human sera, carried out by Davenport et al. (1953-1964), Hennessy et al. (1965), give reason to believe that the virus that caused epidemic of 1918-1919 1pg., serologically related to the swine influenza virus.
The large number of deaths led Burnet and Clarke (1942) to suggest that the virus may have had unusual virulence. According to other researchers (Zhdanov et al., 1958; Kilbourne, 1960), the reasons for the high mortality from secondary bacterial infections could be war conditions and the lack of antibiotics. It seems likely, however, that some virus mutants were highly virulent, because the pandemic virus of 1781 “g., which also affected young people, did not cause such a high mortality rate.
B. ANTIGENIC CHANGES IN THE VIRUS IN THE PERIOD AFTER 1933
After the identification of the first influenza virus, which was designated H0N1 (World Health Organization, 1971), antigenic shifts occurred in 1947, when the H1N1 virus appeared (for example, A/FM/1/47), in 1957, when the H2N2 viruses (for example, A/Singapore/1/57), and in 1968, when the Hong Kong virus (A/Hong Kong/1/68) appeared. The antigenic shift in 1947 consisted of a change in the hemagglutinating antigen (from H0N1 to H1N1); in 1957, both HA and NA were antigenically completely different from the antigens of viruses of previous years (from H1N1 to H2N2), and in 1968 a variant Hong Kong showed significant antigenic difference in HA (from H2N2 to H3N2).
The Asian strain of influenza virus (H2N2), which first appeared in a Chinese province in 1957, contained HA and NA subunits that were completely antigenically different from the H0N1 and H1N1 subunits of influenza viruses previously circulating in humans. This strain of influenza virus caused a pandemic unprecedented in history (Burnet and White, 1972), but the number of deaths was small. The next and so far the last influenza pandemic was caused by the A/Hong Kong/68 virus, in which the NA subunits were similar to those of the “old” Asian A2 virus, and the HA subunits were completely antigenically different from those of the “old” Asian strain (Coleman et al., 1968; Schulman, Kilbourne, 1969; Webster, Laver, 1972).
B. GENERAL PROPERTIES OF PREVIOUS PANDEMICS
The pandemic nature of influenza in humans indicates that, at irregular intervals, humanity is affected by viruses that have new antigenic determinants. The above information indicates that these pandemics often originate in Southeast Asia and spread at the speed of travellers. Most pandemics caused increased mortality among children and the elderly, but at least two pandemics (1781 and 1918) caused increased mortality among young people.
III. PROPERTIES OF THE FLU VIRUS GENOME
The influenza virus has a fragmented genome, consisting of at least seven single-stranded RNA fragments. This fragmentation allows the genome to rearrange (“recombine”) during mixed infections with different strains (see Chapter 7) and may be fundamental to antigenic variation influenza virus. After mixed infection of cells with two different influenza A viruses, viral recombinants are formed with a high frequency. The high frequency of recombinations between influenza A viruses was first demonstrated by Burnet (Burnet, Lind, 1949, 1951) and repeatedly confirmed by other researchers working in this area (Hirst , Gotlieb, 1953, 1955; Simpson, Hirst, 1961; Simpson, 1964; Sugiura, Kilbourne, 1966) It was found that the recombination frequency can reach up to 97%.
The high frequency of recombination between influenza viruses allows the formation of antigen-hybrid viruses during a mixed infection in experiments both in vitro and in vivo. For the first time, biochemical confirmation of this was given by Laver and Kilbourne (1966), who discovered that the genetically stable recombinant X7 virus, isolated from cells mixed with influenza virus strains NW-S (H0N1) and RI/5+ (H2N2), possesses HA subunits of the virus H0N1, and the NA subunits of the H2N2 virus. Many other such recombinant influenza A viruses have subsequently been isolated and, in fact, they can be created “in the right order” (Webster, 1970) (see also 39). The formation of new influenza strains by recombination between animal (or avian) and human viruses is discussed in section VII. Evidence has been obtained that the strains of viruses that cause influenza pandemics can arise in nature in this way. Recombinations between influenza B viruses are also possible (Perry, Burner, 1953; Perry et al., 1954; Ledinko, 1955; Tobita, Kilbourne, 1974), but recombination between influenza viruses of types A and B has never been discovered.
IV. HEMAGGLUTININ SUBUNITS
AND NEURAMINIDASES AS HIGHLY VARIABLE
ANTIGENS
The hemagglutinating and neuraminidase activities of the influenza virus are associated with various subunits (Laver and Valentine, 1969; Laver, 1973), which form a layer of “spikes” on the surface of viral particles (32).
Hemagglutiin is the main surface antigen. It is responsible for the interaction of the virus with the cell surface and for the induction of neutralizing antibodies. Variability of the hemagglutinating antigen contributes to the emergence of new influenza viruses.
The NA enzyme is the second virus-specific surface antigen of the influenza virus particle. Antigenically, NA is completely different from NA (Seto, Rott, 1966; Webster, Laver, 1967). NA antibodies do not neutralize the infectivity of the virus (except at very high concentrations). but they greatly slow down the release of the virus from infected cells (Seto and Rott, 1966; Webster and Laver, 1967; Kilbourne et al., 1968; Becht et al., 1971; Dowdle et al., 1974), and these antibodies may play an important role role in reducing viral replication in vivo and preventing spread
infections (Schulman et al., 1968). While normal variability is also inherent in NA, variations in this antigen are perhaps less significant for the epidemiology of influenza.
Hemagglutinating subunits are glycoprotein rod-shaped structures, triangular in cross-section, with a relative molecular weight of approximately 215,000 (33). They are "monovalent" and (interact with
cell receptors at only one end (Laver and Valentine, 1969). Isolated subunits are highly immunogenic when administered to animals in the presence of an adjuvant. Each virus particle contains approximately 400 HA subunits (Tiffany Blough, 1970; Schulze, 1973; Layer, 1973).
HA subunits consist of two polyleptides with relative molecular weights of about 25,000 and 55,000 (Cottpans et al., 1970; Schulze, 1970; Laver, 1971; Skehel and Schild, 1971; Stanley and Haslam, 1971; Skehel, 1971, 1972; Klenk et al., 1972). They are designated as heavy and light cholypeptides HA1 and HA2. Oi6e, these chains are synthesized as a single tulle-peltide precursor with a molecular weight of about 80,000, which in some cells is cleaved into light and heavy polypeptides (Lazarowitz et al., 1971, 1973; Skehel, 1972; Klenk et al. ., 1972). In intact subunits, the heavy and light chains are connected by disulfide bonds, forming a dimer, and each HA subunit consists of two or three such dimers (Laver, 1971).
The HA subunit has hydrophobic and hydrophilic ends (34). The hydrophilic end is responsible for the biological activity of the subunit, while the hydrophobic end communicates with the lipids of the viral envelope. The hydrophobic properties of the subunit are apparently associated with the C-terminus by molding polypeptide chain (HA2) (Skehel, Waterfield, 1975) (OM. Chapter 3).
The neuraminidase subunit is a sglycoprotein structure with a relative molecular weight of about 240,000. It consists of square, box-shaped heads measuring 8-8-4 wells, to the center of which is attached a thread with a diffuse tail or with a small head at the end (, 35) (Laver and Valentine, 1969; Wrigley et al., 1973). The isolated subunits have full enzymatic activity and are highly immunogenic when administered to animals with an adjuvant. Each virus particle contains approximately 80 NA subunits (Schulze, 1973; Laver, 1973). However, the number of NA subunits in a viral particle can vary depending on the strain (Webster et al., 1968; Webster and Laver, 1972; Palese and Schulman, 1974), as well as on the type of host cell on which the virus was grown
NA subunits consist of four glycosylated lolipeptides with a relative molecular weight of about 60,000, linked to each other by disulfide bonds located in the filament or in its tail (see also Chapter 4). In most strains, these 4 polypeptides appear to be identical. However, in some strains, NA may consist of two types of polypeptides slightly different in size (Webster, 1970a; Skehel, Schield, 1971; Bucher, Kilbourne, 1972; Laver, Baker, 1972; Lazdins et al., 1972; Downie, Laver, 1973; Wrigley et al., 1973).
The active site of the enzyme and antigenic determinants are localized in various regions of the head of the NA subunit (Ada et al., 1963; Fazekas de St. Groth, 1963), and these heads have hydrophilic properties. The “tail” of NA is hydrophobic and serves to attach the subunit to the lipid shell of the virus (Laver, Valentine, 1969) (see "29).
A. ISOLATION AND SEPARATION OF SUBUNITS ON AND NA FROM EACH OTHER
For some influenza virus strains, pure, intact HA and NA subunits can be obtained by electrophoresis on cellulose acetate strips after destruction of viral particles with SDS (Laver, 1964, 1971; Laver and Valentine, 1969; Downie, 1973). The success of isolating any of these subunits using this technique depends on their resistance to denaturation by SDS at room temperature. According to this criterion, influenza viruses can be divided into four groups.
1. Viruses with HA subunits resistant to denatured alcohol
tions SDS. When this type of virus is destroyed by SDS and elec
trophoresis on cellulose acetate strips. all viral proteins,
“besides HA subunits, they migrate as anions. Hemaggluti-
nin migrating as a cation can be isolated in pure
form with complete restoration of biological activity
under conditions that do not destroy covalent bonds [for example
measures: A/Bel/42 (H0N1)].
2. Viruses with NA subunits that are resistant to denature
tions SDS. Pure, active NA subunits can be you
separated from these viruses by the method described above (eg
measures: B/LEE/40).
3. Viruses in which neither HA nor NA are resistant to dena
turation SDS. In this case, all viral proteins migrate
as anions and none of the surface subunits can
can be isolated using the described methods [for example:
A/NWS/33 (H0N1)].
4. Viruses that have both HA and NA subunits
resistant to SDS denaturation. For these viruses, both sub
units during electrophoresis - migrate as cations
and cannot be divided in this way [for example:
A/Singapore? 1/57 (H2N2)].
The HA and NA subunits of the latter group of viruses can be isolated, as shown in 36. An avian influenza virus (A/petrel/Australia/1/72(Hay6Mau5)) was isolated, which was stable to SDSHAHNA (Downie and Laver, 1973). In progress cellulose acetate electrophoresis, they moved together as cations (see 31, top) and could not be separated in this way. In this regard, the two types of these subunits were separated genetically using recombination (Webster, 1970b). To obtain recombinants, parental viruses with HA or NA subunits sensitive to SDS denaturation. SDS-stable avian virus HA and NA subunits were then isolated from SDS-degraded recombinant virus particles by electrophoresis on cellulose acetate strips (em. 31, IB middle and bottom). pure subunits needed for chemical analysis and preparation of “monospecific” antisera.
HA and NA subunits can also be isolated from certain strains of influenza virus by treating viral particles with schroteolytic enzymes (Noll et al., 1962; Seto et al., 1966; Compans et al., 1970; Brand and Skehel, 1972; Wrigley et al. al., 1973). With this method, the separation of surface subunits from viral particles occurs, apparently, as a result of digestion of the hydrophobic (ends of the polypentide chain, which are responsible for attaching the subunits to the lipid layer of the viral envelope. However, partial digestion should also occur other regions of the HA subunit, as a result of which hemagglutinating activity is disrupted and some antigenic determinants are lost.
B. SEPARATION OF HEMAGLUTININ POLYPEPTIDES (HA1 AND HA2)
The light and heavy chains of hemagglutinating subunits can be separated by SDS-polyacrylamide body electrophoresis. However, for preparative purposes, the best separation is achieved by density gradient centrifugation of guanidine hydrochloride-dithiothriethol (Laver, 1971), carried out under conditions in which disulfide bonds are broken, or by tel filtration in a solution of guanidine hydrochloride-dithiothriethol (Webster, 1970a). This separation is apparently based on the significant hydrophobicity of the light polypeptide chain. During centrifugation in a concentrated solution of guanidine hydrochloride - dithiothriethol, this light polypeptide digests it faster than the heavy chain, and during gel filtration the light chain comes out first, apparently due to the fact that even in such a strongly dissociating environment the light chain does not exists iB as a monomer.
These remarks apply only to “HA subunits obtained from a virus grown on cells in which complete proteolytic cleavage of the precursor occurs.”
of the HA polypeptide into NAL and HA2. Moreover, the heavy and light polypeptides (HA1 and HA2) of the HA subunits produced by proteolytic digestion cannot be separated in this manner, possibly because digestion destroys the hydrophobic regions of the light chain (Skehel, Laver, unpublished data ).
B. PROPERTIES OF NA1 AND NA2
The light and heavy polypeptide chains of influenza A virus strain BEL (H0N1) had a similar polypeptide composition, except that the heavy polypeptide contained significantly more proline than the light chain (Laver and Raker, 1972). However, the peptide maps of the tryptic cleavage products of these two chains were completely different, indicating different amino acid sequences in these chains (Laver, 1971). Both polypeptide chains contain carbohydrates, but analysis of glucosamine suggests that the heavy polypeptide contains many more carbohydrates than the light chain. The heavy chain was found to contain 9.4% N-acetylglucosamine, as well as neutral sugars; so it probably contains about 20% carbohydrates.
D. NUMBER OF DIFFERENT SPECIFIC VIRUSES
ANTIGENIC DETERMINANTS ON THE SURFACE
SUBUNITS PER
Number of different virus-specific antigens
determinants on hemalglutinating subunits of the virus
influenza unknown (on the surface of hemagglutinating
subunits there are also determinants specific
to the host cell). Recent experiments have shown
however, that the hemagglutinating subunits of the Gon strain
Kong (H3N2) human influenza virus have at least
at least two, and possibly more, different virus-specific
ical antigenic determinants (Laver et al., 1974).
This has been demonstrated as follows: hemagglu
tin subunits were derived from influenza virus
Hong Kong (A/Hong Kong/68, H3N2) and its antigenic variant
A/Memphis/102/72, which arose as a result of antigenic
drift. Immunodiffusion tests showed that the subunits
Hong Kong/68 virus variants have at least two
various types antigenic determinants, while va
riant 1972 carries, apparently, at least three times
personal determinants (37).
The hemagglutinating subunits of viruses A/Hong Kong/68 and A/Memphis/102/72 had one common determinant. Antibodies to this determinant cross-reacted with both viruses in immunodiffusion, heme agglutination inhibition, and neutralization tests. Antibodies to other determinants did not show any significant serological cross-reactions between the Hong Kong/68 and Memphis/72 viruses. Thus, it is obvious that in the process of anti-
genetic drift, the Hong Kong influenza virus has undergone significant changes in one of its “specific” determinants. Data from Laver et al. (1974) (suggest that different antigenic determinants are localized on the same HA subunit and that viral particles do not possess a mixture of antigenically distinct subunits.
D. LOCALIZATION OF HOST CELL ANTIGEN
Although the first descriptions of a host cell antigen in an influenza virus (Knight, 1944, 1946) were met with some skepticism, their existence is now firmly established. The presence of such antigens was detected by a number of serological methods, including precipitation reactions (Knight, 1944), immunodiffusion reactions (Howe et al., 1967), complement fixation (Smith et al., 1955), hematglutination inhibition (Knight, 1944; Harboe et al. , 1961; Harboe, 1963a) and the method of blocking the inhibition of hemagglutination (Harboe, 1963b; Laver, Webster, 1966). The host cell antigen consists mainly of carbohydrates and is bound to the polypeptides of the HA and NA subunits. No connections between the host antigen (and carbohydrates) and the internal proteins of the viral particle were detected.
One of the mysterious features of the host antigen of influenza viruses is that it is detected in viruses grown in the allantois cavity of chicken or turkey embryos (Harboe, 1963a), but not in viruses grown, for example, in the allantois cavity of duck embryos or in the lungs of mice or in various cell cultures. Viruses grown on these cells were not at all inhibited in the themagglutination inhibition reaction by antisera obtained against extracts from uninfected host cells. This is probably due to the fact that the virus grown in these cells "Contains carbohydrates" of the host cell, but for some reason they either do not have antigenic properties or antibodies directed against them do not inhibit heme agglutination.
E. ROLE OF THE HOST CELL ANTIGEN
The carbohydrate component may play a very important role in the assembly of the viral envelope. Isolated NA and HA subunits aggregate in the absence of SDS. This gives reason to believe that these subunits have both hydrophobic and hydrophilic ends (Laver and Valentine, 1969) and, perhaps, the carbohydrate component of the host cell determines the hydrophobicity of one end of the HA and NA subunits.
G. ANTIGENIC VARIABILITY OF SUBUNITS
HEMAGGLUTININS AND NEURAMINIDASES DETECTED
MONOSPECIFIC ANTISERUMS
Until recently, it was believed that the V-antigen, or the envelope of the influenza virus particle, was something indivisible, but this is not so. It is now known that the V antigen consists of HA, NA and the host cell viral antigen. In none of the previously published works on the antigenic relationships between influenza viruses is this<не принималось во внимание, <в результате чего уровни реакций перекреста ■между данными вирусами зависели от используемых тестов. Так, широко используемая штаммоспецифическая реакция связывания комплемента выявляла перекрестные реакции окзк между нейраминидазными, так и между гемагглютипи-рующими антигенами, :в то время как реакция перекреста между нейраминидазным"и антигенами может выявляться также и в РТГА. Это происходит потому, что в интактном вирусе может возникать «стерическая нейтрализация» нейр-аминидазной активности антителами к гемагглютинину и наоборот (Laver, Kilbourne, 1966; Schulman, Kilbourne, 1969; Easterday et al., 1969; Webster, Darlington, 1969).
Antigenic drift of individual influenza virus antigens can be studied after separating these antigens from the viral particle (Webster and Darlington, 1969) or by “genetically separating these antigens (Kilbourne et al., 1967). Thus, now with the use of monospecific antisera “to these two antigens, “it is possible to conduct detailed serological studies of the antigenic drift of individual influenza virus antigens.
V. MECHANISM OF ANTIGENIC DRIFT
(MINOR ANTIGENIC
CHANGE)
A. INTRODUCTION
The two distinct manifestations of antigenic variation observed among influenza A viruses, namely the sudden emergence of new antigenic subtypes and gradual drift within one subtype, are probably unrelated to each other.
It is generally accepted that drift—the sequential replacement of influenza A viruses by antigenically new strains—is the result
tat interaction of mutational variability of the virus and immunological selection
The importance of this selection mechanism is confirmed by the experimental production of antigenic variants by propagation of influenza viruses in the presence of small amounts of antiseizure (Burnet, Lind, 1949; Archetti, Horsfall, 1950; Isaacs, Edney, 1950; Edney, 1957; Laver, Webster, 1968) or in partially immune animals (Gerber et al.
1955, 1956; Magill, 1955; Hamre et al., 1958). Epidemiological
The observations are also consistent with such a mechanism, which
which offers a reasonable explanation for the disappearance of the mouth
emerging strains from the human population.
Several hypotheses have been put forward to explain the mechanism of antigenic drift. One of them (Francis, 1952, 1955, 1960; Jensen et al., 1956; Jensen, 1957) suggests that the surface of the influenza virus consists of a mosaic of antigens belonging to all strains of a given type, but present in individual antigenic strains in different proportions or in different places. Antigenic variability should be a consequence of the displacement of these antigens on the viral envelope from the protruding to the “Hidden position.” According to another hypothesis (Hilleman, 1952; Magil, Jotz, 1952; Andrewes,
1956, 1957; Takatsy, Furesz, 1957), antigens gradually
are located in the course of variability. Both of these hypotheses require
the existence of a relatively large number of antigens
but different protein molecules on the surface of the vi
Jensen et al. (1956) found that in each of the many strains in the vast collection of influenza A viruses available for research in 1953, the number of antigens present in different quantities and/or locations reached up to 18. Extension of these data to many new variants discovered since then would seem to lead to "the assumption of an even greater number of antigens in each virus, especially if accepted, and apparently
Well, it makes sense that the strains isolated from humans, pigs, horses and birds are part of the same complex.
The existence of such a large number of individual protein molecules in influenza viruses cannot be linked to the coding capacity of viral RNA (Laver, 1964). In addition, electron microscopic (Lafferty, Oertelis, 1963), immunochemical (Fazekas de St. Groth, 1961, 1962; Fazekas de St. Groth, Webster, 1963, 1964) and “biochemical (Laver, 1964) data are more consistent with the presence of a very limited number of antigenically distinguishable protein molecules on the viral envelope.
Based on recent experiments, it is assumed that antigenic drift is the result of selection of an immune population of mutant viral particles with “altered antigenic determinants, and therefore with advantages in growth in the presence of antibodies (Table 26). Moreover, it was “turned out that There are changes in the sequence of amino acids in polypeptides of hemagglutinating units of antigenic mutants isolated by selection by antibodies in an in vitro system (Laver, Webster, 1968) (Fig. 38).
Peptide maps have revealed that during natural antigenic drift there are also changes in the amino acid sequence of both the light and heavy polypeptide chains (39).
These results suggest that antigenic variation among influenza viruses is associated with changes in the amino acid sequence of their antigenic proteins. Although some of the changes in the sequence may be random, having little or no effect on antigenic determinants, it is likely that some of these changes affect antigenic determinants
HA subunits, making them less able to strictly “fit” the corresponding antibody molecules. The experiment, however, does not show whether these changes exist specifically in the antigenic determinants of viral proteins or in some other regions of the molecule.
Influenza viruses exhibit asymmetric crossover reactions in the RTGA. Fazekas de St.-Groth (1970) named viruses
which behave in a similar way, “older” and “younger” strains. Moreover, he “suggested (Fazekas de St. Groth, 1970) that in the process of natural antigenic drift, “older” influenza viruses replace “younger” strains. The last assumption “is confirmed only by very” sparse data.
B. IS IT POSSIBLE TO FORESEE THE DIRECTION OF DRIFT"
The ability of the influenza virus to undergo antigenic changes remains a major concern. Each new variant must be isolated and identified before vaccine production can begin, so each new variant has the potential to infect large numbers of people before it can be controlled with vaccines.
In this regard, attempts have been made to predict antigenic drift in the laboratory, but not entirely successfully. Hannoun and Fazekas de St. Groth at the Pasteur Institute in Paris, strain A/Hong Kong/68 (H3N2) was passaged in the presence of small concentrations of antiserum. After several such growth cycles, a variant was obtained that was no longer subject to antigenic mutations under these experimental conditions. This variant , the authors suggested, represented the end point of evolution within the NZ serotype, and was thus a virus of emergence (which might have been expected after 1970). This assumption was supported by the discovery that the London variant of influenza virus, isolated for the first time , in 1972 (A/England/42/72), was antigenically very similar to the first mutant that Hannoun and Fazekas de St. Groth obtained in their laboratory a year earlier (Fazekas de St. Groth, Hannoun, 1973) .
It was hoped that vaccines derived from the final "older" variant would provide protection against all NZ variants that might arise in humans. However, influenza A viruses subsequently isolated in 1973 and 1974 (e.g. A/Port Chalmers/1/73), which were antigenically different from the A/England/42/72 strain, were also significantly different from the artificially produced variant, suggesting that under natural conditions the drift did not go in the predicted direction.
In any case, the variant obtained in the laboratory by passage in the presence of antiserum experienced drift only in NA, whereas natural variants exhibit drift in both NA and NA. Thus, this attempt to prepare the “future” vaccine, lotidimoma, was unsuccessful.
B. POSSIBILITY OF SIGNIFICANT CHANGES IN CERTAIN ANTIGENIC DETERMINANTS DURING ANTIGENIC DRIFT
In section IV, it was shown that the HA subunits of the Hong Kong influenza virus possess at least two types of antigenic determinants and that in the process of evolution, through the antigenic drift of the Hong Kong influenza virus, a virus was formed (A/Memphis/102/72), in which one of these antigenic determinants
the termiyaant experienced a significant antigenic change (comparable in magnitude to the antigenic shift), while the other “drifted” (om. 37). We called the first of these determinants “specific” and the second “common” for these two viruses<(Laver et al., 1974).
Antibodies to the “specific” determinant do not detect any cross-reactions between the two viruses in immunodiffusion, HRT or neutralization of infectivity tests. Another determinant(s) was common to both viruses (although some antigenic drift occurred in this determinant), and cross-reactions were found between Hong Kong/68 and Memphis/72 viruses due to the same antibodies to this “common” determinant(s).
Different IB animals react to different determinants to varying degrees when immunized with the same preparation of isolated HA subunits. These variations in the immunological response may explain the variability in crossover reactions sometimes observed between two viruses when tested with different sera.
Despite the significant antigenic change in IB ONE
from determinants, peptide maps of heavy and light poly
peptides (HA1 and HA2) of HA subunits of Hong Kong/68 viruses
■and Memphis/72 were largely similar (see
39), on the basis of which it is assumed that in the process
evolution of the Hong Kong virus and education. Meme variant
fis/72 in the amino acid sequence of these polypeptides
only relatively small changes occur. Izme
differences occur in the peptide maps as heavy (HA1),
and light (HA2) polypeptide chains; some of them
may be random changes, others are selected
under the pressure of antibodies.
D. ANTIGENIC CHANGES IN NEURAMINIDASE
Antigenic drift observed in neuraminidase antigen
not influenza viruses of both type A and type B (Paniker, 1968;
Schulman, Kilbourne, 1969; Schild et al., 1973; Curry et al.
1974). It probably occurs through selection (under pressure
antibodies) mutants that have an altered sequence
amino acid content in NA subunit polypeptides
(Kendal, Kiley, 1973). So far it has not been possible to achieve anti
genetic drift in the laboratory. Antibodies to NA are not neutral
the infectivity of the virus is known; therefore it is likely that
variability of this antigen is less important for survival
virus than the variability of HA (Seto, Rott, 1966; Dowdle et al.,
E. ANTIGENIC VARIABILITY OF INFLUENZA VIRUSES TYPE B
Antigenic drift occurs among influenza B viruses to approximately the same extent as among influenza A viruses, but the significant antigenic shifts seen in them were not found among influenza B strains. Antigenic drift (includes changes in both antigens - NA nd NA (Chakraverty, 1972a, b; Curry et al., 1974).The mechanism of antigenic variation of B strains is probably similar to that inherent in influenza A viruses, but “no biochemical studies have been carried out.
E. ANTIGENIC CHANGES IN BIRD AND ANIMALS INFLUENZA VIRUSES
Antigenic changes among influenza viruses infecting lower mammals and birds have not been well studied and little information is available about them. Based on some results, however, it can be assumed that antigenic drift also occurs in strains (mammalian and avian influenza), but to a lesser extent than in influenza viruses that infect humans.
Antigenic drift has been observed in swine and equine influenza viruses (erotype 2) (Meier-Ewert et al., 1970; Pereira et al., 1972), but there is no data on antigenic drift in avian influenza viruses. Perhaps the reason for this is that birds, especially domestic birds, live shorter lives than humans or horses. In humans, each subsequent variant of the influenza A virus quickly completely replaces the previous one, but viruses that differ from each other often circulate simultaneously among animals and birds.
VI. MECHANISM OF ANTIGENIC SHIFT (SIGNIFICANT ANTIGENIC CHANGES)
During antigenic changes of another kind, the surface subunits of the virus experience significant antigenic shifts. With these major shifts, there is a sudden and complete change in one or both surface antigens, so that “new” viruses arise to which there is no immunity in the population. These are the very viruses that cause influenza pandemics.
Human H2N2 influenza viruses provide a natural system for studying the molecular aspects of significant antigenic shifts. The viruses that emerged in humans in 1957 had HA and NA subunits that were completely antigenically different from those of the H1N1 strains. H2N2 viruses
experienced antigenic drift until 1968, when a “new” pandemic strain emerged; Hong Kong. .A2 viruses (H2N2) and the Hong Kong strain (H3N2) originated in China. The Hong Kong virus had the same NA as the previous A2 viruses, but an antigenically different NA (Coleman et al., 1968; Schulman and Kilbourne, 1969). This was clearly demonstrated using specific antisera to isolated HA subunits of representatives of type A2 influenza viruses (grown in chicken embryos. These monospecific sera were used in RTGA with viruses grown in duck embryos (Webster, Laver, 1972), which eliminated the problems of steric suppression of hemagglutinating antibodies to NA and host cell antigen, which can occur when using sera to whole viruses.
The results of these tests (Table 27) showed that the serological correspondence between the hemagglutinin antigens of the “old” A2/Asia strains isolated between 1957 and
1968, and there was no Hong Kong virus (1968). Among the three Hong Kong strains isolated during the first 3 years of the influenza pandemic, there was little or no variation (Webster and Laver, 1972). Where then did the “new” HA subunits of the Hong Kong influenza virus come from? There appear to be two possible reasons for the formation of “new” hemagglutinating subunits: either they arose as a result of mutation from an existing human influenza virus or came from some other source, such as animal or avian influenza viruses.
A single mutation of the “old” influenza A2/Asia virus could cause the polypeptide chains of the HA subunits to fold so that completely new ones are formed
antigenic determinants. If the HA subunits of the Hong Kong influenza virus were obtained by such a mutation from earlier A2 type viruses, then the sequence of amino acids in the polypeptides of the “old” and “new” subunits should be close. A complete shift in one of the antigenic determinants of the HA subunits, which occurred during the process of antigenic drift, was previously described, and this “shift” in one of the determinants is not accompanied, apparently, by any significant general changes in the follower of “H” Osti amino acids in HA polypeptides. However, if the “new” subunits do not arise through mutation and selection, but come from the animal influenza virus, then their polypeptide chains may differ significantly in amino acid sequence from the lolipaptide chains of the “old” A2/Asia viruses.
HA subunits were isolated from three strains of influenza A2/Asia obtained in 1968 before the onset of the Hong Kong influenza pandemic, and from three strains of Hong Kong influenza virus isolated in different parts of the world in 1968, 1970 and 1971. Due to antigenic drift, the three viruses isolated at the end of the A2/Asia period exhibit significant antigenic differences. On the other hand, the three Hong Kong strains that were isolated during the first 3 years of the new pandemic show almost no antigenic variation.
HA subunits isolated from each of these six viral strains were dissociated by treatment with guanidine hydrochloride and dithiothreitol and their light and heavy targets were separated by centrifugation (Laver, 1971). Each of the isolated polypeptide targets was trypsinized and triltic peptides were mapped. The maps showed that the polypeptide chains from the hemagglutinating subunits of the “old” A2 viruses, isolated in 1968, differed significantly in amino acid composition from the lolileptid chains of the “new” Hong Kong strains! (40 and 41). At the same time, it was assumed that the “new” polypeltides were not obtained by mutation from the “old” ones (Laver, Webster, 1972).
One explanation for this result is that a frameshift mutation results in polypeptides with completely different amino acid sequences. However, it seems unlikely that such a mutation, if it occurs, would result in polypeltides capable of forming a functional hemagglutinating unit. Second, mutations may occur affecting mainly the basic amino acids, so that the maps of tristic peptides could differ significantly without any significant change in the overall amino acid sequence of the lolyletides.
Data have now been obtained indicating that some animal influenza viruses are possible precursors of the Hong Kong strain of human influenza virus. Two strains of influenza virus, A/horse/Miami/1/63 (Heq2Neq2) ■and A/duck/Ukraine /1/63 (Hav7Neq2), isolated from horses and ducks in 1963, i.e., 5 years before the emergence of Hong Kong influenza in humans, was shown to be antigenically similar to the Hong Kong strain (Coleman et al., 1968; Masurel, 1968; Kaplan, 1969; Zakstelskaja et al., 1969; Tumova, Easterday, 1969; Kasel et al., 1969).
The HA subunits of horse and duck viruses gave cross-reactions in the RTGA and in the immunodiffusion test with the subunits of the Hong Kong strain of human influenza virus A/Hong Kong/1/68 (H3N2). Moreover, the peptide maps of the light chains of the equine, duck, and human viruses were almost identical, leading to the assumption that the light chains from these three strains have almost identical amino acid sequences (Laver and Webster, 1973). This is clearly visible from 42, where the peptide maps of lolipeptide light chains from the HA subunits of the Hong Kong influenza virus and from the duck//Ukraine and horse/Miami strains (2nd serotype) are almost identical and significantly different from the map of lolipeptide light chains from the “old” virus Asia/68.
These results suggest that equine and avian viruses and the human Hong Kong strain virus may have arisen by genetic recombination from a common progenitor, and suggest an alternative mechanism to mutation to explain the origin of Hong Kong influenza virus.
Recent studies have shown that wild bird sera contain antibodies directed against antigens present in influenza viruses that infect humans (World Health Organization, 1972). In addition, influenza viruses have recently been isolated from wild birds distant from human populations, suggesting influenza has been a natural infection of birds for many thousands of years (Downie and Laver, 1973).
Rasmussen (1964) was the first to suggest that pandemic influenza viruses arise from such animal viruses as a result of the process of recombination. Subsequently, Tumova and Pereira (1965), Kilbourne (1968) and Easterday et al. (1969) obtained antigen-hybrid viruses by genetic recombination in vitro between human influenza viruses and strains of animal and avian influenza viruses.Recently, Webster et al (1971, 1973) simulated the emergence of a new pandemic strain of influenza virus in in vivo experiments (these will be described below).
VII. ADDITIONAL EVIDENCE,
CONFIRMING THE ROLE OF THE PROCESS
RECOMBINATIONS IN THE ORIGIN OF NEW
PANDEMIC FLU VIRUSES
The biochemical data presented do not support the theory that the HA antigen of the Hong Kong virus was due to a single mutation from previous Asian strains. Therefore, one may ask whether there is any evidence obtained from in vitro or in vivo laboratory studies or especially from observational
in natural conditions, which would support a theory suggesting that new viruses arise through recombination.
A. DATA OBTAINED FROM IN VITRO STUDIES
Antigenic hybrids (recombinants) of many influenza A viruses of mammals and birds were isolated after mixed infection of chicken embryos or cell cultures with various influenza A viruses (Tumova, Pereira, 1965; Kilbourne, Schulman, 1965; Kilbourne et al., 1967; Kilbourne, 1968; Easterday et al., 1969). These studies are summarized in reviews by Kilbourne et al. (1967), and Webster and La-ver (1971). It is now obvious that recombinant influenza A viruses with mixed surface antigens (Webster, 1970b) or growth potential (Kilbourne, Murphy, 1960; Kilbourne et al., 1971) or other biological characteristics (McCahon, Schild, 1971) can be made to order.
Thus, “new” influenza viruses can be created in the laboratory, but only recently has evidence been obtained that recombination and selection of “new” viruses can also occur in vivo under natural conditions (Webster et al., 1971).
B. DATA OBTAINED FROM STUDY IN THE IN VIVO SYSTEM
1. Demonstration of recombination in the in system
Kilbourne (1970) noted that recombination between two different strains of influenza A viruses has not yet been demonstrated in intact animals, even under experimental conditions. In order to find out whether recombination can occur in vivo, two systems were used. In the first, only one of the parental viruses multiplied in the host animal, and in the second, both parental viruses multiplied. The animals were injected with large doses of the parental viruses and on the 3rd day When at least one of the viruses was multiplied, the animals were killed. Lung suspensions were examined directly in the allantois membranes for the presence of recombinant (antigen-hybrid) viruses; parental viruses were suppressed with specific antisera (Webster, 1970b).
In the first system, pigs were injected with a mixture of swine influenza virus - HH"C (A/pig/Wisconsin/1/67) and fowl fever virus type A - HPV (Denmark/27) (43). The latter does not release infectious virus after administration to pigs . Lung suspensions collected through
In the second system, where both viruses replicated, turkeys were infected with VChV and the turkey influenza virus - VGI (A/I "ndyuk/Massachusetts/3740/65). As (it was indicated, in the allantoion membrane system antigenic hybrids with VGI (G) were isolated -VChP (N) (Hav6Neql) and VChP (N)-VGI(1Ch) (Havl-N2).
There are two possible objections to the idea that the described recombination occurs in vivo. First, recombination may occur in the cell culture system used for virus selection; second, it is unknown whether these antigenic hybrids were genetically stable and were not simply phenotypically mixed particles.
The first objection can be ignored, since the selection of antigenically hybrid viruses was carried out directly at very high concentrations of antibodies, which should neutralize the parent viruses. To obtain more rigorous evidence that anti-(HHH) hybrid viruses do not arise by isolation from outside an infected host, it was necessary to obtain mixed harvest virus plaques from a plaster suspension, to isolate individual plaques, and to characterize virus samples obtained from individual plaques. 25% plaques isolated from a suspension of lungs of turkeys mixed with HPV + HIV were recombinant viruses. Hybrid viruses were not isolated from control cultures infected with an artificial mixture of both parental viruses.
The genetic stability of recombinant viruses was established by “introducing cloned antigen hybrid viruses into animal hosts (Webster et al., 1971). For example, chickens infected with an antigen-hybrid virus carrying HPV(H)-CVI(N), (HavliN2), died from a transient infection, and the virus, again isolated from the lungs of these birds after 3 days, was a pure culture of the virus, possessing B4n(H)-(Havl-N2). Other antigen-tibrid viruses were also newly isolated from animals and turned out to be genetically stable.
2. Natural transmission of the virus and selection
The studies described have shown that two different strains of influenza A virus can recombine in vivo if they are simultaneously injected into the same animal.
The simultaneous administration of large doses of two different influenza A viruses to animals is, however, an artificial system that probably does not exist in nature. To investigate whether recombination could occur under more natural conditions, two different influenza A viruses were allowed to spread simultaneously in a flock of susceptible birds as follows: two turkeys infected with HIV (A/i-ndkj/Vieconsin/66 (Hav6N2]) , were placed in a flock of 30 sensitive protected turkeys. 2 days later, two more turkeys infected with HPV were introduced into the same flock. Two turkeys from the flock were slaughtered daily and lung samples were examined for the presence of each of the parental and antigen-hybrid viruses in the allantois membranes, and by plaque isolation and virus identification (Webster et al., 1971).IPV spread rapidly among protected birds and was detected 3 days after introduction; AIV was not detected until 9 days after introduction into a flock of infected birds (Webster et al. . Experiments of this kind were carried out three times, and in each experiment, antigenic hybrids were isolated on the 9-10th day; these hybrids possessed VChP (N)-VGI (N), but no reverse -hybrids were isolated. The isolated recombinant virus probably had a growth advantage over the parent viruses; in each experiment, this virus was isolated as dominant from one or more birds. In order for a “new” strain of influenza virus to appear in nature through this kind of recombination and become an epidemic strain, the “new” virus must have some selective advantage. This selective advantage may be the possession of antigens to which the population is generally not immune, but the virus must also have the ability to transfer to susceptible hosts. Both possibilities were studied in the experiments presented. For example, by the time the recombinant virus was already present, normal birds were introduced into the flock, but the recombinants failed to become the dominant strain, and all normal contacting birds died from infection caused by the parental HPV.
3. Selection and transmission of a “new” influenza virus in an in vivo system
If we hypothesize that new strains of influenza A viruses may arise naturally through recombination, it is important to show how these viruses can be selected to become dominant or new epidemic strains. A possible mechanism of selection may be that recombination and selection take place<в иммунных животных. Опыты Webster и Campbell (1974) показали, что рекомбинация и селекция «нового» штамма -вируса гриппа может происходить у индеек с низкими уровнями антител к НА одного родительского вируса и к NA другого родительского вируса (45).
Turkeys with low levels of antibodies to NA CIV (A/indkj/Wisconsin/bb) and to NA CIV were subjected to mixed infection with CIV and CIV. 1-2 days after mixed infection, both parental viruses and a recombinant influenza virus carrying HPV (H)-HIV (N) were present in the tracheas of turkeys. On day 6 after mixed infection, only the recombinant B4n(H)iBrH(N) virus was present. On the 7th day “after a mixed infection, the turkeys died, and only recombinant influenza viruses with HPV (H)-HIV (N) were isolated. All viruses were isolated at extreme dilutions from allantois membranes or from embryos, and no antibodies were used for selection of recombinant viruses. All non-immune birds introduced into the flock on the 5th day died from a transient infection and from “they were isolated only (recombinant influenza viruses.
After mixed infection of nonimmune or hyperimmune turkeys, there was no sequestration of the recombinant influenza virus. Thus, a mixed infection of birds that have low levels of antibodies to the NA of one virus and to the NA of another provides ideal conditions for the selection of recombinants. Following infection, both parent viruses replicate to a limited extent, thereby stimulating the immune system itself, which eliminates the parent viruses. In this way, recombinants can be selected and, provided that they have the necessary virulence properties and the ability to be transmitted to other individuals, these recombinants can cause an epidemic disease.
These experiments show that, under relatively natural conditions, recombination occurs between different influenza A viruses and that new viruses may have a selective advantage over both parental strains. These experiments do not prove that all new influenza viruses of lower mammals, birds and humans arise by this mechanism, but they establish that this mechanism is one of the ways “through which new” viruses appear.
B. DATA ON THE RECOMBINATION OF INFLUENZA VIRUSES IN NATURE
The above experiments leave no doubt that new strains of influenza virus can be “obtained in vitro and in vivo, and suggest that similar processes may also occur in nature. Is there, however, any evidence that recombination in occurs in nature? This evidence is indirect and includes: 1) antigenic correspondences between influenza viruses isolated from humans and from lower mammals and birds; 2) the absence of a strict host range for influenza viruses.
1. Antigenic relationships between influenza viruses of humans, lower mammals and birds
Evidence suggesting that recombination between human and animal influenza viruses is possible in nature comes from the finding that some influenza viruses from humans, lower mammals, and birds have similar, if not identical, surface antigens.
a) Antigenic relationships due to NA. The NA of some avian influenza viruses is antigenically very similar to the NA of early human influenza viruses. For example, the duck virus (A/uzha/Germany/1868/68) has an NA similar to the NA of the human viruses HOS and H1N1 (Schild and Newman, 1969). Influenza viruses isolated from pigs also carry an NA antigen, which is related to the NA antigen of human viruses
H0N1 (Meier-Ewert et al., 1970). Similarly, HIV (A/indkj/MA/65) has an NA similar, if not identical, to that of human influenza viruses H2N2 (Pereira et al., 1967; Webster and Pereira, 1968; Schild and Newman, 1969). Other avian influenza viruses have NA antigens, ■ closely related to the NA of equine influenza viruses types 1 and 2 (Webster and Pereira, 1968; World Health Organization, 1971). Thus, the NA of VChP (A/ VChP/ Holland/27) is similar to the NA of equine influenza virus type 1 (A/ losha, d/ Prague/1/57). These interspecies relationships are used in the revised nomenclature of influenza viruses (World Health Organization, 1971). There are eight different subtypes of avian influenza viruses and four of them have NA antigens related to the NA antigens of human and equine influenza viruses.
b) Antigenic matches caused by the HA antigen. Fewer similar examples were found with influenza viruses isolated from lower mammals and birds, which would have HA antigens related to the HA antigens of human viruses. The correspondence between the HAs of the Hong Kong, duck/Ukraine/63 and horse/type 2 viruses was discussed above. Recently, it was found that a virus isolated from ducks in Germany (A/ut-ka/Germany/1225/74) has an HA similar to the HA viruses influenza family Asia. Thus, as more viruses are isolated, the number of detected matches increases.
2. Circle of hosts
Influenza A viruses are not always strictly defined
high specificity to the host (see Easterday, Tumova, 1971;
Webster, 1972). For example, the Hong Kong influenza virus was
isolated from pigs, dogs, cats, baboons and gibbons. Viru
Influenza A/Hong Kong (H3N2) viruses have also recently been isolated
from chickens and calves (Zhezmer, 1973). These viruses are experimental
but were transferred to calves and chickens; in all cases
the virus replicated in the host from which it was isolated
linen. Thus, the calf influenza virus caused a respiratory infection
tion in calves, and the chicken influenza virus replicated, but not
showed signs of disease in chickens (Schild, Campbell, Web
russ of Hong Kong influenza could not replicate in chickens.
In the case of the Hong Kong influenza virus, it is clear that this virus
has adapted to cause natural infection
tion from other owners, and thus conditions were created
when double infection and genetic
interaction
D. SUMMARY OF DATA SUPPORTING A POINT
VIEWS ABOUT THE EMERGENCE OF NEW STRAINS
FLU VIRUS BY RECOMBINATION
1. Influenza pandemics in humans are caused only by viruses
mi influenza type A, and only influenza viruses of this type were
isolated from lower mammals and birds. Influenza viruses
type B constantly recombine in vitro, but in nature they can
Such a combination of genetic information may not occur
mation [which would allow "the emergence of a pandemic
strain of influenza virus type B. Recombinations between viruses
Influenza types A and B were not shown.
2. Biochemical data presented earlier, as follows:
indicate the unlikely possibility of occurrence
“new” pandemic strains of influenza virus by
means mutation from previous influenza viruses
person.
3. New influenza viruses that can cause a pandemic
may arise through recombination and selection under conditions
in vivo experiment.
4. Based on antigenic and biochemical correspondences
vii between hemalglutinating and neuraminidase an
tigens of human influenza viruses, lower mammals
and birds suggest that genetic exchanges exist
and in nature.
The evidence presented is circumstantial; More direct evidence may be obtained if future pandemic strains are found to have antigens identical to those already isolated from domestic or wild animal influenza viruses (see also Chapter 15).
VIII. FUTURE ANTIGENE CHANGES
FLU VIRUSES AND CAPABILITIES
VARIABILITY PREDICTIONS
AND DISEASE CONTROL
A. POSSIBLE EXPLANATIONS FOR THE CYCLICAL NATURE OF THE PANDEMIC
Based on the study of antibodies in the sera of elderly people, it can be assumed that an influenza virus similar to the Hong Kong thrip virus existed among people in earlier times and may have been the cause of the influenza pandemic of the late 19th century (see section II). elderly people - antibodies to NA of equine influenza viruses type 2 and Asia were also detected in low titers. Antibodies to NA of influenza viruses. Hong Kong or Asia were not detected in the same ayatis-vortok, while antibodies to NA of equine influenza virus
type 2 have been identified. This suggests that viruses with similar HA subunits but different NA subunits are responsible for the previous and current epidemics. Epidemiological data have led to the belief that pandemic human influenza viruses appear cyclically. The lack of data on NA homology makes it unlikely that the same Hong Kong influenza virus exists at the end of the 19th century and again in 1968. It seems more likely that the influenza virus that existed at the end of the 19th century had an HA subunit that showed some antigenic similarity to the Hong Kong influenza virus, but carried a completely different NA antigen. Based on serological data, this NA is antigenically related to equine influenza NA type 2. A new cycle of influenza viruses may occur as a result of the emergence of viruses from some animal reservoir, with or without the participation of recombination, when herd immunity “no longer protects the human population from it.
Another phenomenon associated with the emergence of new influenza strains is the apparent disappearance of previous strains. It could simply be due to a lack of interest in collecting samples of influenza viruses that are no longer dangerous to the majority of society (Fenner, 1968), but this explanation is unlikely, since experience has shown that human influenza viruses do not coexist in nature for any long period of time. time. The disappearance of strains that appeared as a result of antigenic drift can be explained by self-eradication; serologically new virus increases the level of older antibodies, thereby preventing the spread of the old virus. The disappearance of older strains (Fazekas de St. Groth, 1970) of each subtype after a significant antigenic shift is less clear and does not yet have a satisfactory explanation.
B. POSSIBILITIES FOR CONTROL OF ANTIGENIC CHANGES IN THE INFLUENZA VIRUS IN THE FUTURE
The biological, biochemical and immunological data presented above provide only indirect evidence that significant antigenic shifts in human influenza viruses occur through recombination. More definitive data will be obtained if reassortment between different influenza viruses can be detected in nature to produce a new pandemic strain. The rarity of such an event effectively rules out this possibility. An alternative approach to this problem is to isolate influenza viruses from animal populations before the next one emerges. pandemic strain for humans, i.e.
creating a “bank” of influenza viruses. After the emergence of the next strain that causes a pandemic among people, this virus can be compared with the viruses in the “bike”, and it will be possible to obtain data about its occurrence. Wildlife populations as sources of new influenza viruses have been largely ignored. Bird populations around the world live in high-density colonies for longer periods than mammals or humans. Interestingly, eight different subtypes of avian influenza viruses have already been identified, six of them - from domestic birds. It is therefore logical to begin the search for influenza viruses in nature in large bird colonies, especially at the end of the nesting season. Such ecological studies will help establish the number of different subtypes of influenza virus that exist in nature and may eventually reveal how new strains are emerging.If there are only a limited number of influenza A viruses, then in the future it will be possible to think about controlling these viruses, which represent a huge disaster for humans.
LITERATURE
Ada G. L., Lind P. E., Laver W. G. J. gen. Microbiol., 1963, v. 32, p. 225.
Andrewes S. N. Calif. Med., 1956, v. 84, p. 375.
Andrewes S H. N. engl. J. Med., 1957, y. 242, p. 197.
Andrewes S. N . In: Perspectives in Virology (M. Pollard, ed.); New York,
Wiley, 1959, p. 184-196.
Archetti I. , Horsfall F. L. J. exp. Med., 1950, v. 92, p. 441. Becht H., Hammerling U., Rott R. Virology, 1971, v. 46, p. 337. Brand S M., Skehel J. J. Nature (London ). New Biol., 1972, v. 238, p. 145. Bucher D. J., Kilbourne E. D. J. Virol., 1972, v. 10, p. 60. Burnet F. M. “Principles of Animal Virology”, 1st ed. New York , 1955, p. 380. Burnet F. M., Clarke E. Influenza, Melbourne , Walter and Eliza Hall Inst., 1942.
Burnet F. M., Lind P. E. Aust. J. Sci., 1949, v. 22, p. 109.
Burnet F. M., Lind P. E. J. gen. Microbiol., 1951, v. 5, p. 67.
Burnet F. M., White D. O. Natural History of Infectious Disease, 4th ed. London - New York, Cambridge Univ. Press, 1972, p. 202-212.
Chakraverty P. Bull. Wld Hlth Org., 1972a, v. 45, p. 755.
Chakraverty P. Bull. Wld Hlth Org., 1972b, v. 46, p. 473.
Chu C.-M. J. Hyg., Epidemiol., Microbiol., Immunol., 1958, v. 2, p. 1.
Coleman M. T ., Dowdle W. R., Pereira H. G., Schitd G. C, Chang W. K- Lancet, 1968, v. 2, p. 1384.
Compans R. W., Klenk H. D., Caliguiri L. A., Choppin P. W. Virology, 1970
Influenza A/H1N1 as a typical emerging infection: General characteristics of influenza viruses, variability, emergence of new pandemic strains
Influenza viruses - RNA viruses - belong to the family. Orthomyxoviridae and are divided into viruses A, B and C (Table 1).
Table 1.
Comparative characteristics of influenza viruses
| Criteria | Type A | Type B | Type C |
| Severity of the disease | ++++ | ++ | + |
| Natural reservoir | Eat | No | No |
| Human pandemics | Calls | Doesn't call | Doesn't call |
| Human epidemics | Calls | Calls | Does not cause (only sporadic diseases) |
| Antigenic changes | Shift, drift | Drifting | Drifting |
| Segmented genome | Yes | Yes | Yes |
| Sensitivity to rimantadine | Sensitive | Not sensitive | Not sensitive |
| Sensitivity to zanamivir | Sensitive | Sensitive | - |
| Surface glycoproteins | 2 (HA, NA) | 2 (HA, NA) | 1(HA) |
The influenza virus has a spherical shape and size of 80-120 nm. The core is a single-stranded negative strand of RNA, consisting of 8 fragments that encode 11 viral proteins.
Influenza A viruses are widespread in nature and infect both humans and a wide range of mammals and birds. Influenza viruses types B and C have been isolated only from humans.
Epidemially significant are 2 subtypes of influenza A virus - H3N2 and H1N1 and influenza virus type B (A.A. Sominova et al., 1997; O.M. Litvinova et al., 2001). The result of such co-circulation was the development of influenza epidemics of various etiologies in different countries during the same epidemic season. The heterogeneity of the population of epidemic viruses also increases due to the divergent nature of the variability of influenza viruses, which leads to the simultaneous circulation of viruses belonging to different evolutionary branches (O.M. Litvinova et al., 2001). Under these conditions, prerequisites are created for the simultaneous infection of humans by various pathogens, which leads to the formation of mixed populations and reassortment both between viruses of co-circulating subtypes and among strains within the same subtype (O.I. Kiselev et al., 2000).
The classification of influenza virus types is based on antigenic differences between two surface glycoproteins - hemagglutinin (HA) and neuraminidase (NA). According to this classification, influenza viruses are divided into 3 types - influenza viruses type A, type B and type C. There are 16 HA subtypes and 9 NA subtypes.
Rice. 1. Classification of influenza A viruses and types of animals and birds - intermediate and final hosts in the chain of transmission of infection to humans.
Subtype 16 (H16) of hemagglutinin was recently discovered
Note: ∗ NA 7 and NA 7-NA8 were also detected in horses
In Fig. 1 shows the subtypes of influenza A viruses and their intermediate hosts and natural reservoirs (migratory birds). The main hosts of influenza A viruses include those species that are associated with influenza.
In the human population, only three subtypes of influenza A viruses have been identified so far: HA1, HA2 and HA3. Moreover, viruses contain only two types of neuraminidase - NA1 and NA2 (Fig. 1). Their stable circulation has been proven over the past century, starting with the 1918 pandemic (R.G. Webster et al., 1978; K.G. Nicholson et al., 2003).
Influenza A viruses (to a lesser extent B) have the ability to change the structure of HA and NA. The influenza A virus is characterized by two types of variability:
- point mutations in the viral genome with a corresponding change in HA and NA (antigenic drift);
- complete replacement of one or both surface glycoproteins (NA and NA) of the virus through reassortment/recombination (antigenic shift), as a result of which a fundamentally new variant of the virus appears that can cause influenza pandemics.
For influenza B virus, antigenic variability is limited only by drift, because it apparently has no natural reservoir among birds and animals. The influenza C virus is characterized by greater stability of the antigenic structure and only local outbreaks and sporadic cases of the disease are associated with it.
Of some interest emergence of new strains of influenza virus in the human population and associated pandemics (Fig. 2). In Fig. Figure 2 presents the main antigenic shifts associated with pan-epidmias of the twentieth century caused by influenza A viruses:
- in 1918, the pandemic was caused by the H1N1 virus;
- in 1957 - H2N2 strain A/Singapore/1/57;
- in 1968 - H3N2 strain A/Hong Kong/1/68;
- in 1977 - H1N1 strain A/USSR/1/77 (many scientists did not consider this as a pandemic, but with the appearance of this strain, a situation arose with the simultaneous co-circulation of 2 strains of influenza A virus - H3N2 and H1N1).
In 1986, in China, the A/Taiwan/1/86 virus caused a widespread epidemic of influenza A/H1N1, which lasted until 1989. Drift variants of this virus survived until 1995, causing local outbreaks and sporadic cases of the disease. According to the results of molecular biological studies, multiple mutations arose in the genome of the A/H1N1 virus during these years. In 1996, two antigenic variants of the A/H1N1 influenza virus appeared: A/Bern and A/Beijing, their feature was not only antigenic, but also geographic disunity. Thus, in Russia, the influenza A/Bern virus took an active part in the influenza epidemic of 1997-98. During the same season, circulation of strains of the A/Beijing virus was registered in the east of the country. Subsequently, in 2000-2001. influenza A/H1N1 virus became the causative agent of the influenza epidemic in Russia. Modern influenza A/H1N1 viruses have low immunogenic activity; fresh isolated virus isolates interact only with the erythrocytes of mammals (human group 0 and guinea pigs).
Rice. 2. The emergence of new strains of influenza virus in the human population and associated pandemics
Influenza A viruses have undergone significant genetic changes over the past century, resulting in global pandemics with high mortality rates in humans. The largest influenza pandemic (H1N1) was in 1918-1919. ("Spaniard"). The virus, which appeared in 1918, has undergone a pronounced drift; its initial (Hsw1N1) and final (H1N1) variants are considered shift. The virus caused a devastating epidemic that claimed 20 million lives (half of the dead were young people aged 20 to 50 years (M.T. Osterholm, 2005).
Research by J.K. Tanbenberger et al., (2005) showed that the virus that caused the 1918 pandemic was not a reassortant between the avian influenza virus and the human influenza virus - all 8 genes of the H1N1 virus were more similar to variants of the avian virus than to the human one (Fig. .3). Therefore, according to R.B. Belshe (2005) avian influenza virus must infect (bypassing the intermediate host) humans, transmitted from person to person.
It is important to note that avian influenza viruses “participate” in the emergence of new “human” influenza viruses, which are characterized by high pathogenicity and the ability to cause pandemics (E.G. Deeva, 2008). These viruses (H1N1, H2N2 and H3N2) had a different set of internal genes, the origin of which indicates their phylogenetic relationship with avian and swine viruses.
What are the mechanisms of origin of pandemic strains and what biological characteristics are necessary for the emergence of a highly pathogenic virus with pandemic potential?
Influenza A viruses are characterized by a high frequency of occurrence of reassortants as a result of mixed infection, which is due to the segmentation of the viral genome. The predominance of a reassortant of a certain gene composition is considered the result of selection, in which from an extensive set of different reassortants the one that is most adapted to reproduction under given conditions is selected (N.L. Varich et al., 2009). Strain-specific properties of genomic segments can have a strong influence on the gene composition of reassortants under non-selective conditions. In other words, a distinctive feature of influenza viruses is that frequent and unpredictable mutations occur in eight of the gene segments, especially the HA gene. Reassortment plays an important role in the emergence of new viral variants, particularly in the origin of pandemic strains. And sometimes the possibility of a virus with higher virulence emerging during a pandemic cannot be ruled out.
Modern research has shown that the gene structure of the new A/H1N1 virus is complex and, as we noted in the introduction, its composition includes the genes of swine flu that affects pigs in North America; genes for swine flu, which affects pigs in Europe and Asia; avian influenza genes; human influenza genes. Essentially, the genes for the new virus come from four different sources. A micrograph of the influenza A/H1N1 virus is shown in Fig. 4.
Rice. 4. Microphotograph of influenza A/H1N1 virus
WHO published “Guidelines for Influenza Laboratories” and presented new data on the viral gene sequence and their length of the reassortant new influenza A/H1N1 virus (isolate A/California/04/2009): HA, NA, M, PB1, PB2, RA, NP, NS. These data indicate the formation of a new pandemic variant of the virus, creating universal vulnerability to infection due to the lack of immunity. It is becoming clear that pandemic variants of the influenza virus arise through at least two mechanisms:
- reassortment between animal/avian and human influenza viruses;
- direct adaptation of the animal/avian virus to humans.
To understand the origin of pandemic influenza viruses, it is important to study the properties of the natural reservoir of infection and the evolutionary paths of this family of viruses when changing hosts. It is already well known and can be argued that waterfowl are a natural reservoir of influenza A viruses (adapted to these intermediate hosts for many centuries), as evidenced by the carriage of all 16 HA subtypes of this virus. Through bird feces, which can survive in water for more than 400 days (Bird flu..., 2005), viruses can be transmitted to other animal species when drinking water from a reservoir. (K. G. Nicholson et al., 2003). This is confirmed by phylogenetic analysis of nucleic acid sequences of different subtypes of influenza A viruses from different hosts and from different geographical regions.
Analysis of nucleoprotein gene sequences showed that avian influenza viruses evolved with the emergence of 5 specific host lineages: viruses of wild and domestic horses, gulls, pigs and humans. Moreover (!) the human and swine influenza viruses form a so-called sister group, which indicates their close relationship and, naturally, a common origin. The predecessor of the human influenza viruses and the classic swine virus appear to have been entirely of avian origin. In the countries of Central Asia, for known reasons, pork is not popular, and these animals are practically absent from livestock farming. This leads to the fact that (unlike China, for example), this region does not have the main intermediate host in the domestic animal population - pigs, therefore the probability of the “emergence” of pandemic viruses in the Central Asian region is lower than in China, which practically follows from data on the analysis of their origin (Avian influenza, 2005). A permanent source of genes for pandemic influenza viruses exists (in a phenotypically unchanged state) in the natural reservoir of viruses of waterfowl and migratory birds (R.G. Welster, 1998). It should be borne in mind that the predecessors of the viruses that caused the Spanish flu pandemic (1918), as well as the viruses that were the source of the genome of the Asia/57 and Hong Kong/68 pandemic strains, still circulate among the wild bird population with minor mutational changes (Influenza birds..., 2005).
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The first mention of influenza was noted many centuries ago - back in 412 BC
AD a description of the influenza-like illness was made by Hippocrates. Also
influenza-like outbreaks were noted in 1173. First documented
a flu pandemic that has killed many
lives, happened in 1580.
In 1889-1891, a moderate pandemic occurred, caused by a virus of the H3N2 type.
The infamous "Spanish Flu" caused by the H1N1 virus occurred in 1918-1920.
This is the worst known pandemic
Taking more than 20 million lives. From "Spanish flu"
20-40% of the world's population was seriously affected. Death was extremely
fast. A person could still be absolutely healthy in the morning, but by noon he would fall ill and
died by nightfall. Those who did not die in the first days often died from complications,
caused by influenza, such as pneumonia. An unusual feature of the "Spanish flu" was
that it often affected young people (usually influenza primarily
children and the elderly suffer).
The causative agent of the disease, the influenza virus, was discovered by Richard Shope in 1931.
The influenza A virus was first identified by English virologists Smith,
Andrews and Laidlaw (National Institute for Medical Research, London) in 1933
year. Three years later, Francis isolated the influenza B virus.
In 1940, an important discovery was made - the influenza virus could be
cultured on chicken embryos. Thanks to this, new
opportunities for studying the influenza virus.
Influenza C virus was first isolated by Taylor in 1947.
There was a pandemic in 1957-1958
Which was called the "Asian flu", caused by the H2N2 virus. Pandemic
began in February 1957 in the Far East and quickly
spread throughout the world. In the US alone, people died during this pandemic.
more than 70,000 people.
In 1968-1969, a moderately severe "Hong Kong flu" occurred, caused by
H3N2 virus. The pandemic began in Hong Kong in early 1968. Most often
The virus affected older people over 65 years of age. Total number
The death toll from this pandemic was 33,800.
A relatively mild pandemic occurred in 1977-1978
Called the "Russian" flu. Influenza virus (H1N1) that caused this pandemic
already caused an epidemic in the 50s.
Therefore, those born after 1950 were the first to suffer.
Influenza pathogens belong to the orthomyxovirus family, which includes 3 genera of viruses influenza: A, B, C. Influenza viruses contain RNA, an outer shell in which 2 antigens are located - hemagglutinin and neuraminidase, which can change their properties, especially in type A virus. Changes in hemagglutinin and neuraminidase cause the emergence of new subtypes of the virus that usually cause more severe and widespread diseases.
According to the International Nomenclature, the designation of virus strains includes the following information: genus, place of isolation, isolate number, year of isolation, type of hemagglutinin (H) and neuraminidase (N). For example, A/Singapore/l/57/H2N2 denotes a genus A virus isolated in 1957 in Singapore, which has the H2N2 antigen variant.
Influenza pandemics are associated with type A viruses. Influenza B viruses do not cause pandemics, but local “waves” of increased incidence may affect one or more countries. Influenza C viruses cause sporadic cases of illness. Influenza viruses are resistant to low temperatures and freezing, but quickly die when heated.
Orthomyxoviruses - influenza viruses A, B, C
Structural features.
Orthomyxoviruses are enveloped (supercapsid, “dressed”) viruses, the average size of virions is from 80 to 120 nm. Virions are spherical in shape. The genome is represented by single-stranded segmented (fragmented) negative RNA. The virion has a supercapsid containing two glycoproteins protruding above the membrane in the form of protrusions (spikes) - hemagglutinin (HA) and neuraminidase (NA). Influenza A viruses have 17 antigenically different types of hemagglutinin and 10 types of neuraminidases.
Classification of influenza viruses is based on the differences between nucleoprotein antigens (division into viruses A, B and C) and surface proteins HA and NA. The nucleoprotein (also called S-antigen) is constant in its structure and determines the type of virus (A, B or C). Surface antigens (hemagglutinin and neuraminidase - V-antigens), on the contrary, are variable and determine different strains of the same type of virus. Changes in hemagglutinin and neuraminidase cause the emergence of new subtypes of the virus, which usually cause more severe and more widespread diseases
Main functions of hemagglutinin:
Recognizes the cellular receptor - mucopeptide;
Responsible for the penetration of the virion into the cell, ensuring the fusion of the membranes of the virion and the cell; (Hemagglutinin provides the ability of the virus to attach to the cell.)
Its antigens have the greatest protective properties. Changes in antigenic properties (antigenic drift and shift) contribute to the development of epidemics caused by new Ag variants of the virus (against which herd immunity has not been sufficiently developed).
Neuraminidase responds for the dissemination of virions, together with hemagglutinin determines the epidemic properties of the virus.
Neuraminidase is responsible, firstly, for the ability of a viral particle to penetrate the host cell, and, secondly, for the ability of viral particles to exit the cell after reproduction.
The nucleocapsid consists of 8 vRNA segments and capsid proteins that form a helical strand.
Life cycle of the virus.
Replication of orthomyxoviruses is primarily realized in the cytoplasm of the infected cell; viral RNA synthesis occurs in the nucleus. In the nucleus, three types of virus-specific RNA are synthesized on vRNA: positive template mRNAs (a template for the synthesis of viral proteins), full-length complementary cRNA (a template for the synthesis of new negative virion RNAs) and negative virion vRNAs (the genome for newly synthesized virions).
Viral proteins are synthesized on polyribosomes. Next, viral proteins in the nucleus bind to vRNA, forming a nucleocapsid. The final stage of morphogenesis is controlled by the M protein. The nucleocapsid, passing through the cell membrane, is first covered with M protein, then with the cellular lipid layer and supercapsid glycoproteins HA and NA. The reproduction cycle lasts 6-8 hours and ends with the budding of newly synthesized virions.
Antigenic variability.
(Antigenic variability of influenza viruses. The variability of the influenza virus is well known. This variability of antigenic and biological properties is a fundamental feature of influenza viruses types A and B. Changes occur in the surface antigens of the virus - hemagglutinin and neuraminidase. Most likely this is an evolutionary mechanism of virus adaptation to ensure survival. New virus strains, unlike their predecessors, are not bound by specific antibodies that accumulate in the population. There are two mechanisms of antigenic variability: relatively small changes (antigenic drift) and strong changes (antigenic shift).
The modern division of orthomyxoviruses into genera (or types A, B and C) is associated with the antigenic properties of the main nucleocapsid proteins (nucleocapsid protein - phosphoprotein NP) and the viral envelope matrix (M protein). In addition to differences in NP and M proteins, orthomyxoviruses are distinguished by the highest antigenic variability due to the variability of the surface proteins HA and NA. There are two main types of changes - antigenic drift and antigenic shift.
Antigenic drift is caused by point mutations that change the structure of these proteins. The main regulator of the epidemic process during influenza is population (collective) immunity. As a result of its formation, strains with altered antigenic structure (primarily hemagglutinin) are selected, against which antibodies are less effective. Antigenic drift maintains the continuity of the epidemic process.
(Antigenic drift - occurs between pandemics in all types of viruses (A, B and C). These are minor changes in the structure of surface antigens (hemagglutinin and neuraminidase) caused by point mutations in the genes that encode them. Typically, such changes occur every year. As a result, epidemics occur, since protection from previous contacts with the virus remains, although it is insufficient.)
However, another form of antigenic variability has been discovered in influenza A viruses - antigenic shift(shift) associated with a change from one type of hemagglutinin (or neuraminidase) to another, i.e. on the emergence of a new antigenic variant of the virus. This is rarely observed and is associated with the development of pandemics. Over the entire known history of influenza, only a few antigenic phenotypes have been identified that cause influenza epidemics in humans: HoN1, H1N1, H2N2, H3N2, i.e. only three types of hemagglutinin (HA1-3) and two neuraminidase (NA 1 and 2). Influenza viruses type B and C cause disease only in humans, influenza A viruses cause disease in humans, mammals and birds. The most variable influenza A viruses have the greatest epidemic role. Influenza C viruses lack neuraminidase; these viruses usually cause a milder clinical picture.
There is an opinion that antigenic shift is the result of genetic exchange (recombination) between human and animal influenza viruses. It has not yet been definitively established where, during the inter-epidemic period - outside the human population (in birds or mammals) or in the human population (due to long-term persistence, local circulation) viruses that have temporarily exhausted their epidemic capabilities are preserved.
Birds are considered the primary and main hosts of influenza A viruses, in which, unlike humans, viruses with all 17 types of HA and 10 types of NA are common. Wild ducks are the natural hosts of influenza A viruses, in which the pathogen is located in the gastrointestinal tract and does not cause noticeable damage to the hosts. Viruses exhibit their pathogenic properties when they move to other birds and mammals. Among mammals, the greatest importance is attached to pigs, which are considered an intermediate host and are compared to a “mixing vessel”.
(Modern human influenza viruses are weakly transmitted to animals. All influenza A pandemics since 1930 began in China, the main gateway of spread is Siberia (mass migrations of birds).
Н1N1- 1930 Identified in humans, pigs, whales (1972), domestic and wild birds. The famous “Spanish flu” pandemic is associated with it. This type has become widespread again since 1977.
H2N2 has been detected since 1957. in humans and birds. Epidemics associated with these viruses came periodically. Now both types are identified in parallel.
H3N2 was identified in 1963. (Hong Kong).
Virus A/Singapore/1/57 (H2N2) has three genes from Eurasian avian influenza viruses, virus A/Hong Kong/1/68 (H3N2) contains 6 genes from the “Singapore” virus and two from birds. These data confirm that humanity receives new epidemic types of influenza A viruses from birds, the primary host. The immediate forecast is the possibility of the emergence of new epidemic variants of the influenza A virus that have hemagglutinin HA5 or 7 (replacement of one or two amino acids in their structure is sufficient).)
The family of orthomyxoviruses (Greek orthos - correct, tukha - mucus) includes influenza viruses types A, B, C, which, like paramyxoviruses, have an affinity for mucin. Influenza A viruses infect humans and some species of animals (horses, pigs, etc.) and birds. Influenza viruses types B and C are pathogenic only for humans. The first human influenza virus was isolated from humans in 1933 by W. Smith, C. Andrews and P. Ladow (WS strain) by infecting white ferrets. Later, this virus was classified as type A. In 1940, T. Francis and T. Megill discovered the influenza virus type B, and in 1949, R. Taylor discovered the influenza virus type C. When classifying influenza viruses, there have always been certain difficulties associated with with their antigenic variability. Influenza viruses are divided into three types A, B and C. Type A includes several subtypes that differ from each other in their antigens - hemagglutinin and neuraminidase. According to the WHO classification (1980), human and animal influenza viruses type A are divided into 13 antigenic subtypes based on hemagglutinin (H1-H13) and 10 based on neuraminidase (N1-N10). Of these, human influenza viruses type A include three hemagglutinins (HI, H2 and NZ) and two neuraminidases (N1 and N2). For the type A virus, the subtype of hemagglutinin and neuraminidase is indicated in parentheses. For example, influenza A virus: Khabarovsk/90/77 (H1N1).
