STAT5 inhibition of T_FH cell differentiation is mediated by Blimp-1. Prdm1^fl/fl SM cells were transduced with STAT5-CA RV and/or with a variant of Cre RV expressing the fluorescent protein mAmetrine or were not transduced but treated similarly (control). Data are a composite of three (A–C) or two (D) independent experiments, and n = 11–12/group (A–C) or 8/group (D). (A) Representative FACS plot gated on viable (7AAD^low) cells, with untransduced, singly transduced, and cotransduced cells boxed. (B–D) Transduced or control SM cells were adoptively transferred into C57BL/6J mice that were infected with LCMV 3–5 d later. Splenocytes were analyzed 8 d after infection. (B) Quantitation of SM cells as a percentage of total CD4⁺ T cells. (C) Representative FACS plots gated on SM cells, with T_FH cells (SLAM^low CXCR5^high) boxed. Quantitation of SM T_FH cells as a percentage of total SM cells. Data have been normalized so that the mean control SM T_FH percentage for each experiment is 100%. ***, P < 0.0001. (D) Representative FACS plots gated on SM cells, with GC T_FH cells (GL7^high CXCR5^high) boxed. Quantitation of SM GC T_FH cells as a percentage of total SM cells. Data have been normalized so that the mean control SM GC T_FH percentage for each experiment is 100%. *, P = 0.0330; ***, P < 0.0001. Error bars depict the standard error of the mean.