FVII inhibitor-producing mice have increased GC formation and increased GC TFH cells in response to F8 immunization. FVIII^null mice were given 5 weekly IV FVIII injections. Five days after the last immunization, inhibitor titers were measured by Bethesda assay. Representative animals with inhibitors and no inhibitors were euthanized for splenocyte analysis. Splenocytes were stained for CD3, CD4, CD19, CD44, CD62L, B220, Foxp3, CXCR5, PD-1, Fas, GL-7, ICOS, and CD40L and analyzed by flow cytometry. (A) Percentage of B220⁺ B cells bearing a GC phenotype (Fas⁺ GL-7⁺) in saline-injected control mice, non–inhibitor-producing mice and inhibitor-producing mice. (B) Percentage and total number of activated TFH cells expressing GL-7 in in spleen. (C) Representative histograms and quantification (n = 6 per group) of expression of ICOS and CD40L on naive, CXCR5⁻PD-1⁻ effector, GL-7⁻ TFH, and GL-7⁺ TFH cells. *P < .05. MFI, mean fluorescence intensity.