The role of NF-κB transcription factors in the development and function of regulatory T cells

Foxp3 expressing CD4⁺ regulatory T cells (Tregs) are critical for the maintenance of immune homeostasis. Despite an increasing understanding of Treg biology, much remains to be learnt about the transcriptional programs that control the development and function of these cells. Recent work has established an important role for the NF-κB pathway, in particular c-Rel, in the development of thymic Tregs (tTregs). However, the greater loss of Treg numbers with deletion of factors upstream of cRel in the NF-κB pathway suggests that there are additional roles for other NF-κB transcription factors in Treg development. To investigate this possibility studies were undertaken using a number of loss-of-function approaches. The development of Tregs was assessed in the absence of RelA using Rela-/- HSC chimeras, which revealed fewer Tregs in the thymi of HSC chimeras when RelA was missing. This reduction in Treg numbers does not appear to be intrinsic to Rela-/- Tregs, and does not result from TNF-dependent killing of Rela-/- Tregs. Whilst the mechanism has yet to be elucidated, the impact that the loss of RelA has on haematopoietic antigen presenting cells in the thymus could account for this defect in Rela-/- Treg development. Surprisingly, when the nuclear activity of canonical NF-κB proteins was examined in resting Tregs, it was found the Tregs constitutively had high levels of NF-κB/RelA activity, similar to stimulated conventional CD4⁺ T cells. As this high level of continual RelA activity was present in nucleus of splenic as well as thymic Tregs, it was hypothesized that RelA also plays a role in the function and/or homeostasis of Tregs. To assess this hypothesis, a second murine model that inactivated RelA specifically in Foxp3⁺ Tregs (Foxp3creRelafl/fl) was employed. Treg specific deletion of RelA resulted in the development of disseminated autoimmune pathology early in the lifespan of Foxp3creRelafl/fl mice. Whilst Foxp3creRelafl/fl Tregs were unable to prevent the onset of autoimmune disease in these mice, no reduction in Treg numbers was discovered, nor was any defect in the in vitro suppressive ability of Foxp3creRelafl/fl Tregs identified. Investigation of RelA-deficient Tregs did however reveal a decrease in a subpopulation of Tregs with an effector phenotype. These effector Tregs are thought to be particularly important for maintaining immune homeostasis in peripheral tissues, the loss of which could explain the pathology in Foxp3creRelafl/fl mice. In agreement with this notion, micorarray analysis of Tregs from Rela+/+ and Rela-/- chimeras revealed a subset of 10 genes differentially expressed in Rela-/- cells that are associated with an ‘effector gene signature’ in Tregs. Finally, mice with a genetic inactivation of c-Rel have reduced numbers of tTregs yet similar numbers of peripherally induced Tregs (pTregs). Coupled with the finding that the expression of GITR and CD5 is altered on Rel-/-thymic Treg precursors and Tregs, this confirmed that c-Rel is critical for the development of thymic Treg. Collectively, the data presented in this thesis indicated that the canonical NF-κB transcription factors c-Rel and RelA have distinct roles in the thymic development and function of Tregs.