Neutralization of Foot-and-Mouth Disease Virus. II. Further Parameters Related to the Sensitization of the 140S Virion by Antibody

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Abstract
The 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 vs. sensitization. The presence of sensitizing antibody in [guinea pig] 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 [fetal porcine kidney MVPK-1] cells via the regular virus receptor site(s). Evidently, 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 immune response is discussed.