Virus de la grippe et barrière d'espèce

Wild aquatic birds represent the main reservoir and genetic diversity of influenza viruses that allows the barrier species crossing, with virus-host interactions and host-host interactions. There is a specificity of receptor molecules that governs virus entry into cells: hemagglutinin molecules of avian influenza viruses preferentially bind to one form of sialic acid molecule in the host cell membrane [sialic acid (SA)-a-2,3-Gal-terminated saccharides] and the hemagglutinins on human influenza viruses prefer another (SA-a-2,6-Gal-terminated saccharides). The genetic diversity of flu strains is the consequence of reassortants and mutations, due to the segmented RNA genome and RNA polymerase infidelity. As an illustration, the A(H1N1)2009 pandemic is a triple-reassortant between one swine strain, 2 avian strains and 1 human strain. Swines, that possess avian and human strain receptors on their tracheal cells, have been considered as an intermediate host for the adaptation of avian influenza viruses to humans or as mixing vessels for the generation of genetically reassortant viruses. Evolution patterns among swine viruses include evolution of host adaptation, antigenic drift and genetic reassortment as the species barrier crossing needs adaptation to the new host. In that way, the flu virus has developped strategies to improve fitness and pathogenicity with mutation selection particularly on the polymerase gene (PB1, PB2, PA and NP) in respect to fonctionnal contraints. Some mutations are signature of species such as at residue 627 on PB2: in the avian virus, this residue is usually glutamic acid, whereas in mammalian influenza virus it is lysine, suggesting that this residue might be important in determining species range. Adaptative mutations found in ribonucleoprotéins offer optimisation of interaction between the virus and microcellular environment that could influence each step of the replicative cycle. In that way, the threat of a new flu pandemic will persist for a long time.