Background/Purpose: T follicular helper (Tfh) cells are a new subset of T cells that regulate B cell function and exert an important role in the pathogenesis of autoimmune diseases. However, their relationship with other helper lineages and the mechanisms that direct their specification are incompletely understood. As STAT family transcription factors play pivotal roles in specifying T cell lineages, we investigated the roles of STAT proteins in Tfh cell differentiation.

Methods: Naive CD4+ T cells isolated from wild type and STAT deficient mice were activated and cultured with various cytokines. Cytokine production and expression of cell surface molecules and transcription factors were assessed by flow cytometry and qPCR. Genome-wide targets of STATs and epigenetic modifications in Tfh-like cells were evaluated by chromatin immunoprecipitation sequencing (ChIP-seq) and DNaseI hypersensitivity sequencing (DNase-seq). Expression levels of mRNA were determined by microarray analysis.

Results: IL-12 acting STAT4 directly induced IL-21- and IFN-γ-producing cells, which share features of both Tfh and Th1 cells. Although IL-4 acting STAT6 had no effect for induction of IL-21 and IFN-g, IL-6 acting STAT3 generated cells that express IL-21 and not IFN-g. STAT4-indued IL-21 in turn acted through STAT3 in a positive feedback loop to maximize IL-21 production. Using ChIP-seq and DNase-seq, we found that both STAT3 and STAT4 directly bound to multiple genes involved in Tfh cell development including Il21 and Bcl6, regulating gene expression and epigenetic modifications. Thus, STAT3 and STAT4 redundantly serve to induce IL-21, Bcl6 and other Tfh cell molecules. However, STAT4 also induced the transcription factor T-bet that repressed Bcl6, thereby attenuating the Tfh-like phenotype. IFN-γ-dependent activation of STAT1 induced T-bet, which produced a biphasic effect: early on it promotes, but later it inhibits, Tfh-like phenotype. Finally, the Bcl6 locus remained accessible in fully polarized effector T cells, suggesting that although Bcl6 expression can vary with different states of differentiation; this locus remains poised for transcription even in the absence of active transcription.

Conclusion: These results highlight the importance of STAT-mediated gene regulation, which underlies plasticity of Tfh cells. Like Th17 cells, Tfh cells are a fluid subset and their differentiation represents a dynamic balance of signals mediated by STATs. Thus, modulation of STAT-mediated gene regulation in Tfh cells should offer opportunities for the treatment of autoimmune diseases.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/dynamic-regulation-of-t-follicular-helper-cell-differentiation-through-stat-signaling/