Pathogen persistence after clinical cure is a hallmark of many chronic infections. Previously, we showed that naturally occurring CD4+CD25+ regulatory T (nTreg) cells rapidly accumulate within chronic dermal sites of Leishmania major infection where they suppress anti-pathogen CD4+ T cell responses, favor parasite persistence and dermal pathology, and consequently control concomitant immunity. Here, we postulated that chemokines might direct nTreg cell homing in sites of infection and show that CD4+CD25+ nTreg cells, compared with normal CD4+ T cells, preferentially express the CCR5 chemokine receptor, which enables them to migrate in response to CCR5 ligands in vitro. We show that in contrast to their wild-type (WT) counterparts, CCR5−/− CD4+CD25+ nTreg cells resulted in an increased magnitude of parasite-specific, interferon γ–producing CD4+ T cells within infection sites, dramatically reduced parasite numbers, and potent resistance to infection, a finding consistent with the clinical outcome of infected CCR5−/− mice. Interestingly, this resistance was related to an inefficient migration of CCR5−/− nTreg cells to infected dermal sites compared with WT nTreg cells. Thus, this study shows that CCR5 directs the homing of CD4+CD25+ nTreg cells to L. major–infected dermal sites where they promote the establishment of infection and long-term survival of the parasite in the immune host.

Naturally occurring CD4 + CD25 + regulatory T cells (Treg) are potent suppressors of CD4 + and CD8 + T cell responses in vitro and inhibit several organ-specific autoimmune diseases. While most in vitro studies suggest that CD4 + CD25 + Treg cells adopt a cytokine-independent but cell contact-dependent mode of T cell regulation, their precise mechanism of suppression in vivo remains largely unknown. Here we examine the functional contribution of Treg cell-derived TGF-b1 and effector T cell responsiveness to TGF-b in CD4 + CD25 + T cell-mediated suppression of inflammatory bowel disease (IBD). We show that CD4 + CD25 + Treg cells from either TGF-b1 +/+ or neonatal TGF-b1 -/-mice can suppress the incidence and severity of IBD as well as colonic IFN-c mRNA expression induced by WT CD4 + CD25 -effector T cells. Furthermore, TGF-b-resistant Smad3 -/-CD4 + CD25 + Treg cells are equivalent to WT Treg cells in their capacity to suppress disease induced by either WT or Smad3 -/-CD4 + CD25 -effector T cells. Finally, anti-TGF-b treatment exacerbates the colitogenic potential of CD4 + CD25 -effector T cells in the absence of CD4 + CD25 + Treg cells.Together, these data demonstrate that in certain situations CD4 + CD25 + T cells are able to suppress intestinal inflammation by a mechanism not requiring Treg cell-derived TGF-b1 or effector T cell/Treg cell responsiveness to TGF-b via Smad3. IntroductionCD25 (IL-2Ra chain)-expressing CD4 + T cells, which constitute 5-10% of normal CD4 + T cells, play a critical role in the induction and maintenance of peripheral tolerance [1,2]. These naturally occurring CD4 + CD25 + regulatory T (Treg) cells consist of an anergic lymphocyte population with potent immunosuppressive functions in vivo, as the depletion of CD25-expressing CD4 + T cells correlates with increased immunity to tumors, grafts and intracellular pathogens and provokes the induction of multiple inflammatory diseases including autoimmmune gastritis (AIG) and inflammatory bowel disease (IBD) [3,4]. Currently, the mechanism(s) by which CD4 + CD25 + Treg cells mediate suppression in vitro and in vivo is unclear. Following TCR activation, CD4 + CD25 + Treg cells suppress the in vitro proliferation and cytokine production of CD4 + and CD8 + T cells in an antigen nonspecific, but contact-dependent manner that does not appear to involve modulation of APC function [5][6][7]. The role of regulatory cytokines in CD4 + CD25 + Treg cell control of inflammation in vivo appears to be tissue and/ or context-dependent. While CD4 + CD25 + Treg cells from IL-4 -/-and IL-10 -/-mice are as effective as WT CD4 + CD25 + T cells in mediating suppression of AIG, CD4 + CD25 + Treg cells from IL-10 -/-mice cannot control IBD induced by antigen-experienced effector T cells [8,9]. Similarly, IL-10 -/-CD4 + CD25 + Treg cells, in contrast to WT cells, fail to attenuate effector T cell responses to Leishmania major in resistant C57BL/6 mice [10].TGF-b1 is a potently suppressive cytokine that plays a critical role in the regulation of immune function, a...

Natural FOXP3+CD4+CD25High regulatory T cells are critical in immunological self-tolerance. Their characterization in humans is hindered by the failure to discriminate these cells from activated effector T cells in inflammation. To explore the relationship between FOXP3 expression and regulatory function at the clonal level, we used a single-cell cloning strategy of CD25-expressing CD4+ T cell subsets from healthy human donors. Our approach unveils a functional heterogeneity nested within CD4+CD25HighFOXP3+ T cells, and typically not revealed by conventional bulk assays. Whereas most cells display the canonical regulatory T (Treg) cell characteristics, a significant proportion of FOXP3+ T cells is compromised in its suppressive function, despite the maintenance of other phenotypic and functional regulatory T hallmark features. In addition, these nonsuppressive FOXP3+ T cells preferentially emerge from the CD45RO+ memory pool, and arise as a consequence of a rapid downregulation of FOXP3 expression upon T cell reactivation. Surprisingly, these dysfunctional Treg cells with unstable FOXP3 expression do not manifest overt plasticity in terms of inflammatory cytokine secretion. These results open a path to an extensive study of the functional heterogeneity of CD4+CD25HighFOXP3+ Treg cells and warrant caution in the sole use of FOXP3 as a clinical marker for monitoring of immune regulation in humans.

Nonobese diabetic (NOD) mice serve as a model of spontaneous type 1 diabetes (T1D), a T cell-mediated autoimmune disease leading to the destruction of pancreatic insulin-producing beta islet cells. A possible deficiency in regulatory T (T(reg)) cell development or function may promote the activation, expansion, and recruitment of autoreactive T cells and the onset of T1D. Naturally occurring CD4(+)CD25(+) T(reg) (nT(reg)) cells, which typically display potent inhibitory effects on T cell functions in vitro and in vivo, may be defective at controlling autoimmunity in T1D. We have examined the relative contribution of CD4(+)CD25(+) nT(reg) cells in the immune regulation of T1D in the NOD mouse model. CD4(+)CD25(+) T cells represent 5-10% of CD4(+) thymocytes or peripheral T cells from prediabetic neonatal NOD mice, are anergic to TCR signals, and potently suppress activated T cells in a contact-dependent and cytokine-independent fashion in vitro. Unlike total CD4(+) T cells, prediabetic CD25(+)-depleted CD4(+) T cells are potently diabetogenic when transferred in immunodeficient NOD mice. Co-transfer of CD4(+)CD25(+) T cells from thymocytes or peripheral lymphoid tissues of neonatal NOD mice dramatically halts disease development and beta-islet cell lymphocytic infiltration, even when T1D is induced by CD4(+) T cells from BDC2.5 transgenic or diabetic NOD mice. Finally, we show that CD4(+)CD25(+) T(reg) preferentially accumulate in inflamed pancreatic environments, where they potently inhibit the antigen-specific expansion and cytokine effector functions of diabetogenic T cells. Thus, CD4(+)CD25(+) T cell-mediated regulation is operative in the prediabetic neonatal T cell repertoire and can suppress the diabetogenic process and control the onset of T1D.