Influenza viruses belong to Orthomyxoviridae family and are negative-sense single-stranded RNA viruses causing acute respiratory disease in a multitude of hosts all over the world. Influenza viruses were recognized as early as the 16th century and the first pandemic officially documented was in 1580 [1].
Influenza viruses evolved to form mainly four types: alphainfluenza virus (influenza A), betainfluenza (influenza B), gammainfluenza (influenza C), and deltainfluenza (influenza D) which again diverged to subtypes and lineages, affecting multiple mammalian species worldwide, including humans.
Influenza viruses undergo antigenic drift—acquiring frequent mutations in HA and NA, which enables the virions to evade the pre-existing immunity to cause seasonal epidemics/epizootics, and antigenic shift—undergoing gene reassortments causing pandemics.
The most important IAV human pandemics: 1918 Spanish flu (H1N1), 1957–1958 Asian flu (H2N2), 1968 Hong Kong flu (H3N2), and 2009 swine-origin H1N1 emerged during the last century [1].
Structurally, IAV and IBV genomes have eight RNA segments, whereas ICV and IDV have only seven segments. IAV has hemagglutinin (HA), neuraminidase (NA), matrix proteins (M1, M2), and NP (ribonucleoprotein) as structural proteins; 3 subunits of the RNA polymerase complex, polymerase basic protein 1 (PB1), polymerase basic protein 2 (PB2), and polymerase acidic protein (PA); and 3 nonstructural proteins, NS1, NS2/NEP (nuclear export protein), and PB1-F2.
Studies have shown that NS2 and M1 protein form complexes that can be detected in purified virions and cell lysates of virus-infected cells [2,3]. Hence, NS2 and (probably) NS1 of IAV are not considered as non-structural proteins, as these proteins can be detected in virions [4].
IBV possesses six structural proteins, HA, NA, NB, M2, M1, NP and NS2; 3 subunits of RNA polymerase complex, PA, PB1, and PB2; and nonstructural protein NS1 [5].
ICV and IDV have 4 structural proteins, M2, M1, NP, and the hemagglutinin–esterase fusion (HEF) protein that replaces the HA and NA of IAV or IBV; 3 subunits of RNA polymerase complex, P3, PB1, and PB2; and 2 nonstructural proteins, NS1 and NS2.
IAV has several subtypes based on the HA and NA proteins. Currently, there are 18 HA and 11 NA subtypes, of which H1 to H16 and N1 to N9 have been isolated from birds; the subtypes H17, H18, N10, and N11 have been identified in bats [6,7]. Out of these, only three HA (H1, H2, H3) and two NA (N1, N2) subtypes have been associated with human epidemics and are capable of sustained transmission [8]. KTS. lähdeartikkeli:
2019 Jun 17;11(6):561. doi: 10.3390/v11060561
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