ABSTRACT
CD4(+) follicular helper T cells (T(FH) 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 T(FH) 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 T(FH) 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 T(FH) cell development. Our findings demonstrate that Foxp1 is a critical negative regulator of T(FH) cell differentiation.
Subject(s)
Cell Differentiation , Forkhead Transcription Factors/physiology , Repressor Proteins/physiology , T-Lymphocytes, Helper-Inducer/cytology , Animals , CD4-Positive T-Lymphocytes/immunology , Inducible T-Cell Co-Stimulator Protein/genetics , Interleukins/genetics , Mice , Mice, Inbred C57BL , Receptors, Antigen, T-Cell/physiologyABSTRACT
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. In this study, we report that the deletion of Foxp1 in naive CD8(+) T cells leads to enhanced activation of the PI3K/Akt/mammalian target of rapamycin signaling pathway and its downstream cell growth and metabolism targets in response to IL-7. We found that Foxp1 directly regulates PI3K interacting protein 1, a negative regulator of PI3K. Additionally, 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. 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.