p53−/−, aid−/−, and wild-type littermate mice were inoculated i.p. with 2×106 pfu of polyoma virus strain A2. Splenic germinal center B cells were analyzed for surface isotype expression by flow cytometry 12–20 days after inoculation. (A) FACS plots show surface Ig expression on B220+ GL7+ splenic B cells from an experiment in which cells were harvested 14 days after infection. Percentages of switched cells are depicted next to the gates. (B) Data points and bars represent individual mice and means (n=7 for wild-type, n=5 for p53−/−) of the percentages of isotype switched cells within the B220+ GL7+ splenic germinal center B-cell subset. Three independent experiments were performed using: 3, 2, and 2 wild-type mice, 1, 2, and 2 p53−/− mice, and in the second experiment (shown) 2 aid−/− mice were included. Significance was determined by the Mann-Whitney U test.