LEF-1 and TCF-1 orchestrate T follicular helper cell differentiation by regulating differentiation circuits upstream of Bcl6

T follicular helper (TFH) cells are specialized effector CD4+ T cells that help B cells develop germinal centers and memory. However, the transcription factors that regulate TFH differentiation remain incompletely understood. Here we report that selective loss of either Lef1 (LEF-1) or Tcf7 (TCF-1) resulted in TFH defects, while deletion of Lef1 and Tcf7 severely impaired TFH differentiation and germinal centers. Forced expression of LEF-1 enhanced TFH differentiation. LEF-1 and TCF-1 coordinated TFH differentiation by two general mechanisms. First, they established the responsiveness of naïve CD4+ T cells to TFH signals. Second, they promoted early TFH differentiation via the multipronged approach of sustaining expression of IL-6Rα and gp130, enhancing ICOS expression, and promoting expression of Bcl6.