The influenza virus-infected host cell, a target for the immune response.

The exposure of some viral antigens at the surface of infected host cells was studied as an essential stimulus for the immune response during influenza virus infections. Only antibodies directed against the HA1 of the haemagglutinin were bound to the cell surface, anti-HA2 antibodies did not gain access to the membrane. Attempts to purify the cell-associated haemagglutinin (formerly called "viromicrosomes" by, R. Rott) indicated the formation of a special form of truncated haemagglutinin. This antigen and the nucleoprotein was purified from infected chorioallantoic membranes by immunoaffinity chromatography. NP-specific epitopes could not be defined on the native NP molecule which seems to be in solution in a discoordinate and partially polymeric array. Three non-overlapping epitopes were assigned to proteolytic peptides of the NP. Purified NP could induce cross-reactive cytotoxic T-cells in mice, but these animals were not protected against a challenge infection. CTL induced by exogenous stimulation or intracellular synthesis of the NP were restricted by MHC class I in both cases. NP could be incorporated into ISCOM after amphiphatic modification of the molecule. NP-ISCOM conferred some protection to experimental mice, but did not induce CTL demonstrable in vitro.