Antibody responses and protection against lethal MCMV infection by memory B cells adoptively transferred in RAG^−/− mice. (A) IgG anti-MCMV antibody responses in RAG^−/− mice adoptively transferred with 5 × 10⁶ naive B cells (○), 5 × 10⁶ B cells from MCMV-infected mice (•), or no cells (▴). Antibody titers (serum dilution, 1:100) at different time points after MCMV intraperitoneal infection with 10⁵ pfu MCMV-Smith strain are shown (mean ± SEM of 3 mice/group). (B) MCMV titers and (C) viral DNA copies in lung, spleen, and salivary gland 28 days after MCMV infection of RAG^−/− mice transferred with B cells from MCMV-infected mice (•), with naive B cells (○), and no cells (▴). Median values are shown. The dashed line in panel B indicates detection limit. (D) Survival curve of 5 RAG^−/− mice/group adoptively transferred with B cells from MCMV-infected mice (○), naive B cells (•), and no cells (- -▴- -). Mice that received memory B cells were protected against lethal MCMV infection for more than 100 days (P < .005).