Host adaptation mechanisms of Influenza A viruses

Influenza A viruses are a member of Orthomyxoviridae possessing negative-sense, single-stranded, and 8 segmented RNA genomes. These 8 RNA segments encode individual specific viral proteins, and up until now a total of 11 proteins have been identified including alternate open reading frame in one genome. Because of this simplicity, utilization of many host systems is required during the viral infection cycle in a cell. Therefore adaptation to the host cell system is the most important issue for the influenza A virus infection and transmission. Although there are many factors which relate to the host adaptation in the viruses themselves, elucidation of mechanisms about changing viral receptor specificities is one of the most advanced research areas. Because the avian type viral receptor was found in the human lungs, the situation, such as co-existence of avian- and human-derived viral receptors in the human airway, may contribute to the human adaptation of avian derived influenza A viruses.