Antibody titers and class-switch recombination. (A) M17 is up-regulated by IL-4. Isolated splenocytes were MACS depleted of CD43⁺ cells and subsequently activated with the indicated stimuli. Following the isolation of total RNA 48 hours later, RT-PCR was performed using intron-spanning primers for the M17 and β-actin genes. Naive B indicates naive B cells in vitro; GC, GC B cells; and H₂0, water control. A representative experiment is shown. (B) Antibody titers in the serum of unimmunized wild-type and M17^–/– mice were determined in an ELISA assay. Each circle represents one mouse. Black bars indicate the geometric means. Closed circles indicate wild-type mice; open circles, M17-deficient mice. (C) In vitro stimulation of isolated B cells of M17^–/– mice and wild-type controls. B cells were induced to undergo CSR with the indicated stimuli. The percentage of class-switched cells was determined 4 days later by flow cytometry. Numbers in the graphs represent the percentages of switched cells. A representative experiment is shown. (D) Percentage of IgA⁺ GC B cells in the Peyer patches of M17^–/– mice and wild-type controls. The differences in percentages of IgA⁺ B cells were not statistically significant (P = .15).