Changes in the antigenic specificity of influenza hemagglutinin in the course of adaptation to mice.

Influenza virus strains A/USSR/90/77 (H1N1) and A/USSR/92/77 (H1N1) have been adapted to mice by successive intranasal passages. In the course of adaptation a loss (for A/USSR/90/77) or a reduction (for A/USSR/92/77) of the reactivity toward two monoclonal antibodies in a panel of seven monoclones has been registered. The changes emerged after two or three passages, long before the acquisition of the lethal effect for mice. Possible significance of the phenomenon for the antigenic variation of influenza A virus is discussed.