Antigen-specific B-cell help from CXCR5^-/- T helper cells is impaired in vivo. (A) Serum TNP-specific IgG1 was determined by ELISA from sublethally irradiated mice that received TNP-activated B cells isolated from third donors alone or together with either naive or activated CXCR5^-/-DO11.10 or CXCR5^+/+DO11.10 cells. Data represents mean + SD derived from 3 mice of each group. Similar results were obtained in 2 to 3 additional experiments; ^***P < .001. (B) TNP-specific IgG1 in supernatants of in vitro cultures of in vivo activated B cells in the absence or presence of activated CXCR5^-/-DO11.10 or CXCR5^+/+DO11.10 cells. Supernatants were collected after 10 days and pooled from 6 wells. Similar data were obtained in 2 additional experiments. For further details see “Materials and methods” and “Results.”