A fundamental function of CD4 + helper T (T H ) cells is the regulation of B cell-mediated humoral immunity. Development of T follicular helper (T FH ) cells that provide help to B cells is mediated by the cytokines interleukin-6 and interleukin-21 but is independent of T H 1, T H 2, and T H 17 effector cell lineages. Here, we characterize the function of Bcl6, a transcription factor selectively expressed in T FH cells. Bcl6 expression is regulated by interleukin-6 and interleukin-21. Bcl6 overexpression induced T FH -related gene expression and inhibited other T H lineage cell differentiation in a DNA binding-dependent manner. Moreover, Bcl6 deficiency in T cells resulted in impaired T FH cell development and germinal center reactions, and altered production of other effector T cell subsets. Our data thus illustrate that Bcl6 is required for programming of T FH cell generation.

After activation, CD4+ helper T (T(H)) cells differentiate into distinct effector subsets that are characterized by their unique cytokine expression and immunoregulatory function. During this differentiation, T(H)1 and T(H)2 cells produce interferon-gamma and interleukin (IL)-4, respectively, as autocrine factors necessary for selective lineage commitment. A distinct T(H) subset, termed T(HIL-17), T(H)17 or inflammatory T(H) (T(H)i), has been recently identified as a distinct T(H) lineage mediating tissue inflammation. T(H)17 differentiation is initiated by transforming growth factor-beta and IL-6 (refs 5-7) and reinforced by IL-23 (ref. 8), in which signal transduction and activators of transcription (STAT)3 and retinoic acid receptor-related orphan receptor (ROR)-gamma mediate the lineage specification. T(H)17 cells produce IL-17, IL-17F and IL-22, all of which regulate inflammatory responses by tissue cells but have no importance in T(H)17 differentiation. Here we show that IL-21 is another cytokine highly expressed by mouse T(H)17 cells. IL-21 is induced by IL-6 in activated T cells, a process that is dependent on STAT3 but not ROR-gamma. IL-21 potently induces T(H)17 differentiation and suppresses Foxp3 expression, which requires STAT3 and ROR-gamma, which is encoded by Rorc. IL-21 deficiency impairs the generation of T(H)17 cells and results in protection against experimental autoimmune encephalomyelitis. IL-21 is therefore an autocrine cytokine that is sufficient and necessary for T(H)17 differentiation, and serves as a target for treating inflammatory diseases.

Foxp3+ regulatory T (Treg) cells suppress different types of immune responses to help maintain homeostasis in the body. How Treg cells regulate humoral immunity, including germinal center reactions, is unclear. Here we identify a subset of Treg cells expressing CXCR5 and Bcl6, and localized in the germinal centers in mouse as well as human. The expression of CXCR5 on Treg cells depends on Bcl6. These CXCR5+Bcl6+ Treg cells are absent in thymus but can be de novo generated from CXCR5-Foxp3+ natural Treg precursors. Lack of CXCR5+ Treg cells leads to greater germinal center reactions. These results unveil a Bcl6-CXCR5 axis in Treg cells that undermines the development of follicular regulatory T (Tfr) cells that function to inhibit the germinal center reaction.

Regulatory T (Treg) and T helper 17 (Th17) cells were recently proposed to be reciprocally regulated during differentiation. To understand the underlying mechanisms, we utilized a Th17 reporter mouse with a red fluorescent protein (RFP) sequence inserted into the interleukin-17F (IL-17F) gene. Using IL-17F-RFP together with a Foxp3 reporter, we found that the development of Th17 and Foxp3(+) Treg cells was associated in immune responses. Although TGF-beta receptor I signaling was required for both Foxp3 and IL-17 induction, SMAD4 was only involved in Foxp3 upregulation. Foxp3 inhibited Th17 differentiation by antagonizing the function of the transcription factors RORgammat and ROR*. In contrast, IL-6 overcame this suppressive effect of Foxp3 and, together with IL-1, induced genetic reprogramming in Foxp3(+) Treg cells. STAT3 regulated Foxp3 downregulation, whereas STAT3, RORgamma, and ROR* were required for IL-17 expression in Treg cells. Our data demonstrate molecular antagonism and plasticity of Treg and Th17 cell programs.

SUMMARY Th17 cells have been recently discovered in both mouse and human. Although IL-1 has been shown to be important in human Th17 cell differentiation with little knowledge of the underlying mechanism, its function in mouse is less clear. Here we show that IL-1R1 expression in T cells, which was induced by IL-6, was necessary for Th17-mediated autoimmunity and for early Th17 differentiation in vivo. Moreover, IL-1 signaling in T cells was required in dendritic cell-mediated Th17 differentiation from naïve or regulatory precursors and IL-1 synergized with IL-6 and IL-23 to regulate Th17 differentiation and maintain cytokine expression in effector Th17 cells. Importantly, IL-1 regulated the expression of IRF4 and RORγt during Th17 differentiation; over-expression of these two factors resulted in IL-1-independent Th17 polarization. Our data thus indicate a critical role of IL-1 in Th17 differentiation and this pathway may serve as a novel target for Th17-mediated immunopathology.