Maintaining the balance. Novel molecular mechanisms regulating Foxp3 function

Abstract

A balanced immune response requires tight control of immune activation at various levels, which crucially involves the establishment of specific gene expression programs by key transcriptional regulators including the transcription factor Foxp3. As the driving factor of both development and functional maintenance of regulatory T (Treg) cells, Foxp3 is of critical importance for the generation of immunological tolerance. Foxp3 expression levels and transcriptional output are eminent determinants of tolerogenic mechanisms involving not only extrinsic Treg cell-mediated immune suppression, but also intrinsic modulation of conventional T (Tconv) cell activation, illustrating the importance of regulatory mechanisms controlling Foxp3 transcriptional activity. In the studies presented in this thesis, we sought to identify novel molecular mechanism regulating Foxp3 activity and functional output, and their potential to modulate immune responses through regulation of Treg cell suppressive function. The transcription factor TCF1, a known regulator of T cell development, is demonstrated to directly associate with Foxp3. In response to Wnt-mediated activation TCF1 negatively modulates Foxp3 activity and thereby Treg cell function. Wnt production was elevated in cells from the site of inflammation of patients with Juvenile Idiopathic Arthritis (JIA) and both in vitro and in vivo models to examine Treg cell suppressive function demonstrated that manipulation of the Wnt signaling cascade could successfully modulate Treg cell function. Further investigation of mechanisms involved in modulation of Treg cell function through post-translational modification of Foxp3 led to the identification of the deubiquitinase (DUB) USP7 as an important regulator of Foxp3 protein stability. Inhibition of Foxp3 deubiquitination, and in particular USP7 activity, promoted proteasomal degradation of Foxp3 protein, resulting in reduced Treg cell suppressive function. Finally, in the context of T cell acute lymphoblastic leukemia (T-ALL), Foxp3 can associated with oncogene LMO2. This competitive interaction negatively regulated complex formation of LMO2 with the oncogenic transcription factor TAL1, resulting in reduced TAL1 transcriptional activity, suggesting a role for Foxp3 in development and progression of T-ALL. The findings reported in this thesis provide novel insights in the molecular mechanisms that enable regulation of Foxp3 transcriptional activity in response to key regulatory signals present in the Treg cell microenvironment. The rapid, and moreover reversible, nature of the mechanisms described, could potentially allow for transient regulation of Foxp3 activity and consequently Treg cell suppressive function, to establish a balanced immune response and ultimately maintain immune homeostasis.