Antigenic relationship of influenza-virus neuraminidases from Asian, Hong Kong, and Equi-2 strains

Preparation-conditioned changes of the antigenicity of influenza virus neuraminidases

The influenza virus strains A/Sing/1/57 (H2N2), A/Bel/42 (H0N1) and A/Bel/42 (HO)-A/Sing/1/57 (N2) were treated with bromelain under reducing conditions and with reducing agent alone, and the antigenicity of the neuraminidase (NA) of intact virus and of the split products was tested comparatively. It was found that the antigenicity of NA was influenced quantitatively and qualitatively by the preparation procedure. Antineuraminidase (AN) antibodies obtained after vaccination of guinea pigs with intact virus and with split products differed in their cross-reactivity with heterologous neuraminidases. In several cases, the quantity of AN antibody formation depended on the hemagglutinin (HA) dose present in the vaccines. The N2 NA on the recombinant virus was significantly more sensitive to treatment with reducing agent than was the N2 NA on the parent virus. AN antibodies directed against N2 NA on the recombinant differed qualitatively from that directed against N2 NA of parent virus. The results warrant the conclusion that the antigenicity of isolated NA or of NA on recombinant virus can differ from that of the NA on intact homologous virus and that such alterations could influence the determination of antigenic relationship between neuraminidases.

Kinetics of reactions of antihemagglutinin and antineuraminidase antibodies with H2N2 and H3N2 influenza virus strains and description of a modification of the photometric ACU method for titration of antineuraminidase antibodies

The isotherms describing the reactions of selected H2N2 and H3N2 virus strains with antihemagglutinin (AH) and antineuraminidase (AN) antibodies were established by use of a photometric hemagglutination inhibition test [antibody concentration unit (ACU) method]. It was found that the AN antibody isotherms had significantly higher values of the constant 1/N than did the AH antibody isotherms. This finding confirms for further virus strains the conclusion that the photometric ACU method can discriminate objectively between AN and AH antibodies. The results obtained when determining by use of the photometric ACU method the kinetics of reactions of AN antibodies oriented to A/Bel (HO)-A/Sing (N2) virus with the neuraminidases of H2N2 and H3N2 strains, and vice versa, indicated that the N2 neuraminidases of the test strains could be divided into the following groups: One group comprising the strains A/Sing/1/57 and A/AA/1/65, a second comprising the strains A/Hong Kong/1/68 and A/England/42/72 and a third represented by A/Port Chalmers/1/73. This finding indicated progressive antigenic variation of the neuraminidases of the strains tested. A modification of the photometric ACU method for the titration of AN antibodies oriented to N2 strains has been developed. The modified technique was found to be more sensitive and accurate than was AN antibody titration by means of enzyme inhibition and HI pattern test.