Forkhead Transcription Factors Foxp1 and Foxp4 Regulate T Cell Development and Function

Transcription factors regulate T cell fates at every stage of development and differentiation. Members of the FoxP family of Forkhead transcription factors are essential for normal T lineage development; Foxp3 is required for regulatory T cell generation and function, and Foxp1 is necessary for the development of naïve T cells. FoxP family member Foxp4 is highly homologous to Foxp1 and has been shown to dimerize with other FoxP proteins. In this thesis, we report the first studies of Foxp4 in T lymphocytes. Using a CD4Cre-mediated conditional knockout approach we evaluated the roles for Foxp4 regulation in the T lineage. T cell development and homeostasis are normal in the absence of Foxp4. Despite effective control of infection with Toxoplasma gondii or acute Lymphocytic choriomeningitis virus in vivo, cytokine production during antigen-specific rechallenge is reduced in the absence of Foxp4. We conclude that Foxp4 is dispensable for T cell development, but necessary for normal memory T cell recall responses to antigen in acutely or chronically infected mice. Next we determined whether FoxP family members compensate for one another in Foxp1- or Foxp4-knockout models. We utilized a similar CD4Cre approach to delete both Foxp1 and Foxp4 in T cells. Foxp1/4-deficient T cells exhibit abnormal thymic development and T cell receptor signaling. Loss of Foxp1/4 results in significantly reduced T cell numbers, and altered T cell effector function, reminiscent of Foxp1-deficient T cells. Lastly, we examined the functions of Foxp1/4 in Foxp3+ regulatory T cells (Tregs). Tregs are critical for prevention of autoimmunity and controlling immune responses during infection. While conditional deletion of either Foxp1 or Foxp4 in T cells has little effect on Tregs, combined deletion results in abnormal Treg generation. Foxp1/4-deficient Tregs exhibited significant defects in both development and homeostasis. Under competitive conditions, double-deficient Tregs are at a significant developmental disadvantage relative to wild-type competitors. Furthermore, Foxp1/4-deficient Tregs exhibit impaired cytokine-induced STAT5 phosphorylation and reduced expression of Foxp3, suggesting Foxp1/4 is required for normal Treg generation. Together, these findings demonstrate that the FoxP family regulates multiple facets of T cell development and function, and actively contributes to the maintenance of immunological tolerance