The dynamics of CD4(+) effector T cells (Teff cells) and CD4(+)Foxp3(+) regulatory T cells (Treg cells) during diabetes progression in nonobese diabetic mice was investigated to determine whether an imbalance of Treg cells and Teff cells contributes to the development of type 1 diabetes. Our results demonstrated a progressive decrease in the Treg cell:Teff cell ratio in inflamed islets but not in pancreatic lymph nodes. Intra-islet Treg cells expressed reduced amounts of CD25 and Bcl-2, suggesting that their decline was due to increased apoptosis. Additionally, administration of low-dose interleukin-2 (IL-2) promoted Treg cell survival and protected mice from developing diabetes. Together, these results suggest intra-islet Treg cell dysfunction secondary to defective IL-2 production is a root cause of the progressive breakdown of self-tolerance and the development of diabetes in nonobese diabetic mice.

After being activated by antigen, helper T lymphocytes switch from a resting state to clonal expansion. This switch requires inactivation of the transcription factor Foxo1, a suppressor of proliferation expressed in resting helper T lymphocytes. In the early antigen-dependent phase of expansion, Foxo1 is inactivated by antigen receptor-mediated post-translational modifications. Here we show that in the late phase of expansion, Foxo1 was no longer post-translationally regulated but was inhibited post-transcriptionally by the interleukin 2 (IL-2)-induced microRNA miR-182. Specific inhibition of miR-182 in helper T lymphocytes limited their population expansion in vitro and in vivo. Our results demonstrate a central role for miR-182 in the physiological regulation of IL-2-driven helper T cell-mediated immune responses and open new therapeutic possibilities.

It is believed that memory CD8+ T cells are maintained in secondary lymphoid tissues, peripheral tissues, and BM by homeostatic proliferation. Their survival has been shown to be dependent on IL-7, but it is unclear where they acquire it. Here we show that in murine BM, memory CD8+ T cells individually colocalize with IL-7+ reticular stromal cells. The T cells are resting in terms of global transcription and do not express markers of activation, for example, 4-1BB (CD137), IL-2, or IFN-γ, despite the expression of CD69 on about 30% of the cells. Ninety-five percent of the memory CD8+ T cells in BM are in G0 phase of cell cycle and do not express Ki-67. Less than 1% is in S/M/G2 of cell cycle, according to propidium iodide staining. While previous publications have estimated the extent of proliferation of CD8+ memory T cells on the basis of BrdU incorporation, we show here that BrdU itself induces proliferation of CD8+ memory T cells. Taken together, the present results suggest that CD8+ memory T cells are maintained as resting cells in the BM in dedicated niches with their survival conditional on IL-7 receptor signaling.

A progressive waning in Foxp3+ regulatory T cell (Treg) functions is thought to provoke autoimmunity in the NOD model of type 1 diabetes (T1D). A deficiency in IL-2 is one of the main triggers for the defective function of Tregs in islets. Notably, abrogation of the ICOS pathway in NOD neonates or BDC2.5-NOD (BDC2.5) mice exacerbates T1D, suggesting an important role for this costimulatory pathway in tolerance to islet Ags. Thus, we hypothesize that ICOS selectively promotes Foxp3+ Treg functions in BDC2.5 mice. We show that ICOS expression discriminates effector Foxp3− T cells from Foxp3+ Tregs and specifically designates a dominant subset of intra-islet Tregs, endowed with an increased potential to expand, secrete IL-10, and mediate suppressive activity in vitro and in vivo. Consistently, Ab-mediated blockade or genetic deficiency of ICOS selectively abrogates Treg-mediated functions and T1D protection and exacerbates disease in BDC2.5 mice. Moreover, T1D progression in BDC2.5 mice is associated with a decline in ICOS expression in and expansion and suppression by intra-islet Foxp3+ Tregs. We further show that the ICOS+ Tregs, in contrast to their ICOS− counterparts, are more sensitive to IL-2, a critical signal for their survival and functional stability. Lastly, the temporal loss in ICOS+ Tregs is readily corrected by IL-2 therapy or protective Il2 gene variation. Overall, ICOS is critical for the homeostasis and functional stability of Foxp3+ Tregs in prediabetic islets and maintenance of T1D protection.

OBJECTIVE-In this study, we asked whether a possible quantitative or qualitative deficiency in naturally occurring Foxp3 ϩ CD4 ϩ regulatory T-cells (nT reg ), which display potent inhibitory effects on T-cell functions in vitro and in vivo, may predispose to the development of type 1 diabetes. RESEARCH DESIGN AND METHODS-We assessed the frequency and function of Foxp3 ϩ nT reg cells in primary and secondary lymphoid tissues in the NOD animal model of type 1 diabetes. RESULTS-We show that the cellular frequency of Foxp3ϩ nT reg cells in primary and secondary lymphoid tissues is stable and does not decline relative to type 1 diabetes-resistant mice. We show that thymic and peripheral CD4 ϩ CD25 ϩ T-cells are fully functional in vivo. We also examined the functional impact of CD4 ϩ Foxp3 ϩ nT reg cells on the development of autoimmune diabetes, and we demonstrate that nT reg cells do not affect the initial priming or expansion of antigen-specific diabetogenic T-cells but impact their differentiation in pancreatic lymph nodes. Moreover, CD4ϩ Foxp3 ϩ nT reg cells also regulate later events of diabetogenesis by preferentially localizing in the pancreatic environment where they suppress the accumulation and function of effector T-cells. Finally, we show that the nT reg cell functional potency and intra-pancreatic proliferative potential declines with age, in turn augmenting diabetogenic responses and disease susceptibility.CONCLUSIONS-This study demonstrates that Foxp3-expressing nT reg cells in NOD mice regulate diabetogenesis, but temporal alterations in nT reg cell function promote immune dysregulation and the onset of spontaneous autoimmunity.