CD4+ follicular helper T cells (TFH cells) are essential for germinal center (GC) responses and long-lived antibody responses. Here we report that naive CD4+ T cells deficient in the transcription factor Foxp1 ‘preferentially’ differentiated into TFH cells, which resulted in substantially enhanced GC and antibody responses. We found that Foxp1 used both constitutive Foxp1A and Foxp1D induced by stimulation of the T cell antigen receptor (TCR) to inhibit the generation of TFH cells. Mechanistically, Foxp1 directly and negatively regulated interleukin 21 (IL-21); Foxp1 also dampened expression of the costimulatory molecule ICOS and its downstream signaling at early stages of T cell activation, which rendered Foxp1-deficient CD4+ T cells partially resistant to blockade of the ICOS ligand (ICOSL) during TFH cell development. Our findings demonstrate that Foxp1 is a critical negative regulator of TFH cell differentiation.

Previously we have shown that transcription factor Foxp1 plays an essential role in maintaining naive T cell quiescence; in the absence of Foxp1, mature naive CD8+ T cells proliferate in direct response to homeostatic cytokine IL-7. Here we report that the deletion of Foxp1 in naive CD8+ T cells leads to enhanced activation of PI3K/Akt/mTOR signaling pathway and its downstream cell growth and metabolism targets in response to IL-7. We found that Foxp1 directly regulates Pik3ip1, a negative regulator of PI3K. In addition, we found that deletion of Foxp1 in naive CD8+ T cells results in increased expression levels of E2fs, the critical components for cell cycle progression and proliferation, in a manner that is not associated with increased phosphorylation of retinoblastoma protein (Rb). Taken together, our studies suggest that Foxp1 enforces naive CD8+ T cell quiescence by simultaneously repressing key pathways in both cellular metabolism and cell cycle progression.

While Th17 cells can protect against colonization by pathogenic organisms, they also have the potential to become pathogenic and promote autoimmune and inflammatory diseases. Mechanisms that control their pathogenic potential remain poorly understood. Here we show that Ndfip1, a co-activator of the E3 ubiquitin ligase Itch, restricts the frequency and pathogenicity of Th17 cells. Mice lacking Ndfip1 have increased numbers of Th17 cells, and this increase is cell intrinsic. We found that Ndfip1 restricts production of the proinflammatory cytokines in Th17 cells. Increased cytokine production correlated with reduced degradation and accumulation of RORγT. When transferred in vivo, Th17 cells lacking Ndfip1 were more likely to maintain their ability to make IL-17, were more potent proinflammatory cytokine producers, and were powerful inducers of colitis. Together our data support an essential role for Ndfip1 in degrading RORγT and suppressing Th17 lineage stability, proinflammatory cytokine production, and pathogenicity.

CD4+ T follicular helper (Tfh) cells are essential for germinal center (GC) and long-lived antibody responses. Studies have shown that the deletion of the inducible T cell co-stimulator (ICOS) results in severe defects of Tfh cell differentiation and germinal center responses. It has been shown that ICOS-ICOS ligand (ICOSL) interaction is not only critical for Bcl6 induction, but also essential for the follicular recruitment of activated CD4+ T cells by follicular bystander B cells that collectively form an ICOS-engaging field. Previously we have identified transcription factor Foxp1 as a critical negative regulator of Tfh cell differentiation. We now provide evidence showing that naïve CD4+ T cells deficient in both Foxp1 and ICOS still differentiate into Tfh cells at high frequencies with Bcl6 induced. Such results are confirmed in ICOSL-deficient recipient mice, in which Foxp1-deficient CD4+ T cell differentiate into Tfh cells. Taken together, our studies suggest that Foxp1 plays a profoundly dominant role in regulating Tfh cell differentiation.