IL-12 signaling drives the differentiation and function of a T H 1-derived T FH1 -like cell population.

CD4 ⁺ T follicular helper (T _FH ) cells provide help to B cells and promote antibody-mediated immune responses. Increasing evidence supports the existence of T _FH populations that secrete cytokines typically associated with the effector functions of other CD4 ⁺ T cell subsets. These include T helper 1 (T _H 1)-biased T _FH (T _FH1 ) cells that have recognized roles in both immune responses to pathogens and also the pathogenesis of autoimmune disease. Given their apparent importance to human health, there is interest in understanding the mechanisms that regulate T _FH1 cell formation and function. However, their origin and the molecular requirements for their differentiation are unclear. Here, we describe a population of murine T _H 1-derived, T _FH1 -like cells that express the chemokine receptor Cxcr3 and produce both the T _H 1 cytokine interferon-γ and the T _FH -associated cytokine interleukin-21 (IL-21). Furthermore, these T _FH1 -like cells promote B cell activation and antibody production at levels indistinguishable from conventional IL-6-derived T _FH -like cells. Regarding their regulatory requirements, we find that IL-12 signaling is necessary for the differentiation and function of this T _FH1 -like cell population. Specifically, IL-12-dependent activation of STAT4, and unexpectedly STAT3, promotes increased expression of IL-21 and the T _FH lineage-defining transcription factor Bcl-6 in T _FH1 -like cells. Taken together, these findings provide insight into the potential origin and differentiation requirements of T _FH1 cells.