The role of receptor binding specificity in pathogenicity of influenza virus

The receptor binding specificity of influenza viruses may be important for host restriction of human and avian viruses. The receptor binding protein, hemagglutinin (HA), from avian influenza viruses binds well to sialic acid attached to galactose via an alpha2,3 linkage (SAalpha2,3Gal), whereas human influenza virus HAs bind best to sialic acid attached to galactose via an alpha2,6 linkage (SAalpha2,6Gal). The potential of an avian influenza virus HA to acquire the ability to bind the human form of the receptor may be one requirement for an influenza virus of avian origin to propagate in the human population. This thesis shows that (1) single amino acid changes in the 1918 influenza virus hemagglutinin alters the receptor binding specificity from SAalpha2,6Gal to SAalpha2,3Gal; (2) This change in HA receptor binding specificity does not absolutely block infection in the ferret model of the human respiratory tract. Instead the SAalpha2,3Gal binding virus is attenuated in its ability to transmit between ferrets; and (3) The present work also demonstrates that sialyltransferase knock-out mice which lack a major SAalpha2,6Gal receptor for human influenza viruses, SAalpha2,6Gal-GlcNAc, can still be infected with SAalpha2,6Gal specific human influenza viruses. Overall human influenza viruses may not be absolutely restricted to using one type of SAalpha2,6, and that at high doses the virus can be more promiscuous in its use of sialic acid for entry.