Mature B cells are not necessary for cross-mucosal protection against C. trachomatis. (A) Groups of 7–8 C57BL/6 and µMT mice were intranasally immunized with C. trachomatis. Four weeks after immunization, these mice, naïve C57BL/6, and naïve µMT mice were challenged in the genital tract. Five days later, bacterial burden was determined by quantitative PCR. (B) Groups of 7–8 C57BL/6 mice were intranasally immunized. Four weeks later, serum was extracted from peripheral blood of these mice and naïve mice. Immune or naïve sera were injected i.v. into naïve mice. These mice were then infected in the genital tract and 5 days later bacterial burden was determined by quantitative PCR. These data are representative of 2 independent experiments. *: p ≤ 0.05.