SUMMARYThe in vitro interaction of foot-and-mouth disease virus (FMDV) with an immune serum resulted in a fraction of virus which failed to be neutralized. This inability of antibody to neutralize the entire population of a virus preparation was studied with emphasis on the antigenic specificity of the antibody-virus reaction. Antibody to FMDV recognized multiple antigenic determinants. Immunoadsorbent fractionation of the serum into 12S subunit cross-reactive and 140S virion-specific antibody revealed that these multiple antigenic determinants are factors in determining the neutralizing ability of the antibody. Antibody specific to the infective 140S virion neutralized virus effectively, whereas antibody reactive with both the 140S virion and 12S non-infective component did so ineffectively. Neutralization was independent of viral aggregation, strain, or type heterogeneity, dissociation of the immune complex, heterogeneity of antibody class, or incubation time. The non-neutralized fraction of virus was not due to insufficient antibody in the system, was demonstrated to be complexed with antibody ('sensitized') and could be neutralized with anti-globulin serum. The findings demonstrate the heterogeneity of antibody specificity to FMDV in serum preparations and relate the importance of antibody specificity to the neutralization of virus in vitro.

SUMMARYThe reaction of foot-and-mouth disease virus (FMDV) with 12S subunit/140S virion cross-reactive (sensitizing) antibody was studied in order to elucidate the requirements for neutralization versus sensitization. The presence of sensitizing antibody in immune serum caused an atypical in vitro neutralization response curve and a non-neutralized fraction. Cell-associated (cytophilic) antibody was not present in the system. Dissociation of the immune complex was not a factor and sensitized virus adsorbed to host cells via the regutar virus receptor site(s). This finding led to the conclusion that sensitizing antibody is specific for non-critical sites. Dosing of the neutralization reaction mixtures with fractionated antibody of alternative antigenic specificities had an antagonistic effect on the neutralization response, suggesting steric hindrance. Cell receptor sites were a factor in sensitization since different host systems had different susceptibilities for sensitized antigen. The results suggest that in vitro neutralization of FMDV requires the attachment of multiple antibody molecules as proposed by the multi-hit theory of neutralization. The in vitro measurement of serum neutralizing activity as an indication of the in vivo immune response is discussed.

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