Characterization of adaptation of an avian influenza a (H5N2) virus to a mammalian host

We have used the mouse model to monitor the acquisition of virulence of a n on-pathogenic influenza A virus upon adaptation to a new mammalian host. An avian strain, A/Mallard duck/Pennsylvania/10218/84 (H5N2) (Mld/PA/84) was adapted to mice by 23 serial lung-to-lung passages until a highly virulent mouse-adapted (MA) variant (Mld/PA/84-MA) emerged. This MA valiant was char acterized and compared to the parental strain as well as some of its interm ediate passage variants. MA variant caused bronchopneumonia in mice with a high mortality rate (the virulence of Mld/PA/84-MA measured as log (EID50/L D50) was 1.75), while the parental, avirulent strain Mld/PA/84 did not caus e illness and mortality in mice (log (EID50/LD50) was 7.25). Hemagglutinati on-inhibition (HAI) test with a set of hemagglutinin- (HA) specific monoclo nal antibodies (MAbs) revealed antigenic differences between the parental s train and MA variant. Mld/PA/84-MA reacted with HA-specific MAbs in higher titers than the parental strain. The HA genes of the parental strain Mld/PA /84, the 1(st), 3(rd), 8(th), and 15(th) intermediate passage variants: and Mld/PA/84-MA were sequenced. Three amino acid changes at positions 203, 27 3 and 320 were determined in the HA of MA variant. The first of them, Leu-- >Pro (320), appeared in the HA stem region at the 8(th) passage. Two other in the HA1 globular region (Ser-->Phe (203) and Glu-->Gly (273)) appeared a t the 15(th) passage. All of these substitutions were associated with the i ncrease of viral infectivity for mouse lungs and changes in the HA antigeni city. The potential role of these changes in HA with respect to the process of viral interspecies transmission and acquisition of virulence for new ho st is discussed.