Measurements of the neuraminidase content of two recombinant influenza viruses [X-7 and X-7 (F1)] possessing A0 () haemagglutinin and A2 (/5) neuraminidase showed that particles of X-7 (F1) virus had more than twice as much A2 neuraminidase as those of X-7 virus.

Antibody to A0 haemagglutinin reacted to high titre in antihaemagglutinin, neutralization and complement-fixation tests with both viruses. Antibody to A2 neuraminidase (at the same concentration) did not inhibit haemagglutination by either virus and did not prevent X-7 virus from infecting cells. It did, however, reduce the size of X-7 virus plaques. Antineuraminidase neutralized, but only to a low titre, the infectivity of X-7 (F1) virus for cells of the chick allantonic membrane and completely inhibited plaque formation by this virus in tissue cultures. Antineuraminidase and antihaemagglutinin antibodies reacted with both viruses in complement-fixation tests to similar titres.

The biological ineffectiveness of antineuraminidase antibodies on the haemagglutinin activity or on the infectivity of these viruses was not due to lack of combination of antibody with the virus. The average avidity of the antineuraminidase antibody was high and not different from the antihaemagglutinin antibody (K≃2 × 10 c.g.s. units). X-7 virus has approximately one quarter and X-7 (F1) virus one half of the number of enzyme antigenic sites as haemagglutinin sites.


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