Development of a localized hemolysis-in-gel assay for Vi antigen: characterization of the Vi-specific PFC response of nude and normal mice.

An assay to detect specific plaque-forming cells (PFC) to Vi antigen (Vi) was developed and the optimal conditions for sensitization of sheep erythrocytes (SE) and plaque development were determined. Using PFC and passive hemagglutination (PHA) assays, Vi-specific immune responses of athymic (nude) and normal mice were characterized. Vi was found to elicit only IgM PFC. No discernable secondary response was detected following a second injection of antigen. Nude and normal mice responded in a quantitatively similar manner to all doses of Vi tested and responded similarly on varying days following immunization. Also, both nude and normal mice produced the greatest number of Vi-specific PFC 4 days following immunization with an optimally immunogenic dose of Vi (1.0 microng/mouse). These results indicate that functional thymus-derived cells are not necessary to elicit an immune response against Vi antigen.