Rechallenge with FVIII antigen induces recall responses of TFH cells. Eight to 10 weeks after the last round of IV FVIII immunization, pairs of mice bearing similar inhibitor titers were rechallenged with 50 U/kg rhF8 or saline. Five days after boost, the mice were euthanized. Splenocytes were stained for CD3, CD4, CD19, CD44, CD62L, Foxp3, CXCR5, and PD-1 and analyzed by flow cytometry. (A) FVIII inhibitor titers. (B) Percentage of CXCR5⁺PD-1⁺ TFH cells among effector CD4 helper cells. (C) Percentage of activated TFH cells among total CD4 T cells. (D) Total number of activated TFH cells per spleen was compared between saline-treated and rhF8-boosted animals. Paired mice of similar inhibitor titers are represented by the same symbol. *P < .05; **P < .01.