Antigenic variants of influenza A virus (PR8 strain). IV. Serological characteristics of a second line of variants developed in mice given polyvalent vaccine

Antigenic variants of influenza A virus (PR8 strain) III. Serological relationships of a line of variants derived in sequence in mice given homologous vaccine

Seven variant strains of influenza A PR8-S virus, each derived from the previous one by serial passage in the lungs of mice immunized with the homologous agent have been produced. With the H.I. and neutralization procedures these variants showed a progressive serological deviation from the parent PR8-S virus. The seven variants provoked antibodies in varying titers to the preceding variants and the parent virus but not in relation to their position in the series. Thus, the seventh variant provoked significantly more antibody to the PR8-S virus than did the fifth variant. A possible explanation for this is presented. The first four variant viruses showed progressively less ability to react with antisera of the preceding variants and the PR8-S virus, and the three most recently derived variants showed essentially no ability to react with PR8-S and first variant antisera. The variant viruses remained antigenically stable through numerous lung passages in normal mice. Cross absorption tests revealed common antigenic components among the variant viruses and also individual characteristics which classify them as being different from one another. The implications of these findings in relation to studies by others have been discussed.

Antigenic variants of influenza A virus (PR8 strain). V. Virulence, antigenic potency, and cross-protection tests in mice of the original and second series

Two series of variants of influenza PR8-S virus have been described. While all retain the same degree of pathogenicity for mice and fertile eggs, there was a progressive loss in the ability of the variants to provoke antibody following vaccination or infection of mice and ferrets. The immunogenicity of the variants was, therefore, less than that of the original strain. Although little or no serological relationship could be demonstrated between some of the variants and the PR8-S virus a considerable degree of cross-immunity could be demonstrated in the cross-protection tests with these viruses if observations were based solely on death or survival of the mice. By employing the occurrence of lesions in the lung and the titer of virus in the lung 48 hours after challenge, the amount of cross-protection in mice could be related to the amount of serological cross-reaction. In general mice vaccinated with PR8-S virus were less resistant to infection with the variant viruses than mice vaccinated with variants and challenged with the PR8-S parent virus. The role of the immune environment of the host in the production of the variant influenza viruses with their serological differences, decreasing antigenicity, and persisting pathogenicity as well as the epidemiological implications of these findings with respect to epidemic influenza in man are discussed.

Antigenic drift in type A influenza virus: peptide mapping and antigenic analysis of A/PR/8/34 (HON1) variants selected with monoclonal antibodies

Variants of A/PR/8/34 (HON1) influenza virus, having hemagglutinin molecules with probably a single altered antigenic determinant, were isolated by growing the virus in the presence of the monoclonal hybridoma antibody PEG-1. The variants were analyzed by peptide mapping and characterized antigenically by using PEG-1 and four other monoclonal hybridoma antibodies to PR8 hemagglutinin. Peptide maps of the large hemagglutinin polypeptide, HA1, from 8 out of 10 variants showed a single changed peptide. This peptide from two of the variants was analyzed, and in each case a serine residue in the wild-type hemagglutinin was replaced by leucine in the variant. Although these eight variants showed identical peptide maps, one could be discriminated antigenically from the others with one of the hybridomas. (The peptide maps represented about one-third of the HA1 molecule.) Of the other two variants, one gave the same HA1 map as the wild type, but could be distinguished antigenically from wild-type virus by two of the hybridomas. The other was unique, and could be distinguished, both antigenically and by peptide mapping, from the other variants. Since a large number of the variants selected with PEG-1 showed the same peptide change, it is likely that this alteration in amino acid sequence (serine to leucine) was responsible for the inability of the variants to bind PEG-1 monoclonal antibody. We do not know, however, whether the changed amino acids were located within the antigenic sites or whether the change occurred somewhere else in the hemagglutinin molecule and altered the determinants through conformational changes.