B cells and CD4⁺ T cells, but not CD8⁺ T cells and IFN-γ, are essential for MNV-3 protective immunity.

<p><b>A</b>) Groups of mice of the indicated knockout strains (n = 7 mice per condition tested in at least two independent experiments) were inoculated with mock inoculum (black bars) or 10⁴ TCID₅₀ units of MNV-3 (grey bars). Six weeks later, all mice were challenged with 10⁷ TCID₅₀ units of MNV-3. One day later, animals were perfused, the indicated organs harvested, and viral burden determined by plaque assay. The data for all mice in each group were averaged. Limits of detection are indicated by dashed lines. The mock→MNV-3 infection group was compared to the MNV-3→ infection group for each mouse strain for statistical analysis. <b>B</b>) The mock→MNV-3 (1°) infection viral titers were divided by the MNV-3→ (2°) infection titers for the mice presented in panel A to determine the fold-reduction in titers as a quantitative measure of protective immunity. <b>C</b>) The same experiment was carried out for MNV-1; shown only are the fold-reductions in viral titers.