Inflammation-Driven Reprogramming of CD4+Foxp3+ Regulatory T Cells into Pathogenic Th1/Th17 T Effectors Is Abrogated by mTOR Inhibition in vivo

Abstract

While natural CD4+Foxp3+ regulatory T (nTREG) cells have long been viewed as a stable and distinct lineage that is committed to suppressive functions in vivo, recent evidence supporting this notion remains highly controversial. We sought to determine whether Foxp3 expression and the nTREG cell phenotype are stable in vivo and modulated by the inflammatory microenvironment. Here, we show that Foxp3+ nTREG cells from thymic or peripheral lymphoid organs reveal extensive functional plasticity in vivo. We show that nTREG cells readily lose Foxp3 expression, destabilizing their phenotype, in turn, enabling them to reprogram into Th1 and Th17 effector cells. nTREG cell reprogramming is a characteristic of the entire Foxp3+ nTREG population and the stable Foxp3NEG TREG cell phenotype is associated with a methylated foxp3 promoter. The extent of nTREG cell reprogramming is modulated by the presence of effector T cell-mediated signals, and occurs independently of variation in IL-2 production in vivo. Moreover, the gut microenvironment or parasitic infection favours the reprogramming of Foxp3+ TREG cells into effector T cells and promotes host immunity. IL-17 is predominantly produced by reprogrammed Foxp3+ nTREG cells, and precedes Foxp3 down-regulation, a process accentuated in mesenteric sites. Lastly, mTOR inhibition with the immunosuppressive drug, rapamycin, stabilizes Foxp3 expression in TREG cells and strongly inhibits IL-17 but not RORγt expression in reprogrammed Foxp3− TREG cells. Overall, inflammatory signals modulate mTOR signalling and influence the stability of the Foxp3+ nTREG cell phenotype.

Inflammation and Lymphopenia Trigger Autoimmunity by Suppression of IL-2–Controlled Regulatory T Cell and Increase of IL-21–Mediated Effector T Cell Expansion

“…Factors, such as inflammatory cytokines or lymphopenia that support a stronger T eff expansion, increase the number of "pseudo-converted cells." Accordingly, other studies indicated that T reg instability was particularly observed in lymphopenic hosts (53) and within the gut microenvironment (Peyer's patches and mesenteric LN) (54,55). This is in accordance with our own observations where the greatest numbers of pseudo-converted cells were in CD28 2/2 recipients.…”

“…That Foxp3-expressing T cells represent a stable, terminally differentiated lineage has recently been questioned by a number of studies, where it was proposed that T regs , or a subpopulation of them, retain developmental plasticity (2,23,27,53,54). In an autoimmune setting, inflammatory signals or a lymphopenic environment could destabilize the T reg phenotype and allow for functional plasticity and reprogramming into Th1, Th17, or T FH effector cells (23,27,(53)(54)(55)(56)(57)(58)(59). T reg instability was, however, not supported by all studies (24) and remains controversial.…”

Akt-Dependent Enhanced Migratory Capacity of Th17 Cells from Children with Lupus Nephritis

“…These results are consistent with previous studies (36,37), additionally illustrating impaired nuclear translocation of RORg in the presence of rapamycin that alters Th17 differentiation (37).…”

Harnessing the plasticity of CD4+ T cells to treat immune-mediated disease

“…By contrast, mTOR deficiency favours the induction of FOXP3 expression in vitro, and mTORC2 seems to be largely dispensable for T Reg cell stability and function in vivo 84,85,88 . In addition, although a requirement for some mTORC1 activity in tT Reg cells has been demonstrated 88 , rapamycin treatment (which preferentially inhibits mTORC1) actually promotes T Reg cell stability 59,89 , and hyperactivation of mTORC1 in T Reg cells drives IL-17 production and loss of FOXP3 expression 90 . Furthermore, it seems that T Reg cell instability associated with increased mTOR activity is driven most prominently by activated mTORC2, which, by acting upstream of mTORC1, has the potential to activate parallel pathways to further destabilize T Reg cells 80,85 .…”