FIG. 4.

Serum virus-neutralizing antibody titers in CD4⁺ T-cell-deficient C57BL/6 mice i.m. immunized with formalin-inactivated PR8 virus. CD4⁺ T-cell-deficient mice were immunized i.m. with formalin-inactivated PR8 virus (10 μg/mouse) at day 0 and boosted at day 15 with the same dose. Different dilutions of serum samples from immunized mice were mixed with approximately 100 PFU of PR8 virus and incubated for 1 h at room temperature. The mixtures were then used to infect a monolayer of MDCK cells, and a standard plaque reduction assay was performed. The neutralizing-antibody titer of the serum is considered the highest dilution that was found to reduce the number of plaques by 50% or more. ⧫, control, serum from unimmunized CD4⁺ T-cell-deficient mice; ■, serum from CD4⁺ T-cell-deficient C57BL/6 mice 15 days after priming; ▴, serum from CD4⁺ T-cell-deficient C57BL/6 mice 10 days after boosting.