Summary of recent advancesFoxp3 + regulatory T cells (T regs ) contribute significantly to the maintenance of peripheral tolerance, but they ultimately fail in autoimmune diseases. The events that lead to T reg failure in controlling autoreactive effector T cells (T effs ) during autoimmunity are not completely understood. In this review, we discuss possible mechanisms for this subversion as they relate to type 1 diabetes (T1D) and multiple sclerosis (MS). Recent studies emphasize (i) the role of inflammatory cytokines, such as IL-6, in inhibiting or subverting T reg function, (ii) the issue of T reg plasticity, (iii) the possible resistance of autoimmune T cells to T reg -mediated control, and (iv) T reg -associated inhibitory cytokines TGFβ, IL-10 and IL-35 in facilitating T reg suppressive activity and promoting T reg generation. These recent advances place a large emphasis on the local tissue specific inflammatory environment as it relates to T reg function and disease development.

SUMMARY Type I diabetes is a T cell-mediated autoimmune disease, characterized by lymphocytic infiltration of the pancreatic islets. It is currently thought that islet antigen-specificity is not a requirement for islet entry and that diabetogenic T cells can recruit a heterogeneous bystander T cell population. We tested this assumption directly by generating TCR retrogenic mice expressing two different T cell populations. By combining diabetogenic and non-diabetogenic and/or non-autoantigen specific T cells, we demonstrate that bystander T cells cannot accumulate in the pancreatic islets. Autoantigen specific T cells which accumulate in islets, but do not cause diabetes, were also unaffected by the presence of diabetogenic T cells. Additionally, 67% of TCRs cloned from NOD islet-infiltrating CD4+ T cells were able to mediate cell-autonomous islet infiltration and/or diabetes when expressed in retrogenic mice. Therefore islet entry/accumulation appears to be a cell-autonomous and tightly-regulated event and is governed by islet antigen specificity.

SUMMARY Regulatory T (Treg) cells, driven by the Foxp3 transcription factor, are responsible for limiting autoimmunity and chronic inflammation. We showed that a well-characterized Foxp3gfp reporter mouse, which expresses an N-terminal GFP-Foxp3 fusion protein, is a hypomorph that causes profoundly accelerated autoimmune diabetes on a NOD background. Although natural Treg cell development and in vitro function are not markedly altered in Foxp3gfp NOD and C57BL/6 mice, Treg cell function in inflammatory environments was perturbed and TGFβ-induced Treg cell development was reduced. Foxp3gfp was unable to interact with the histone acetyltransferase Tip60, the histone deacetylase HDAC7, and the Ikaros family zinc finger 4, Eos, which led to reduced Foxp3 acetylation and enhanced K48-linked polyubiquitylation. Collectively this results in an altered transcriptional landscape and reduced Foxp3-mediated gene repression, notably at the hallmark IL-2 promoter. Loss of controlled Foxp3-driven epigenetic modification leads to Treg cell insufficiency that enables autoimmunity in susceptible environments.

Decoding the molecular composition of individual Ngn3 + endocrine progenitors (EPs) during pancreatic morphogenesis could provide insight into the mechanisms regulating hormonal cell fate. Here, we identify population markers and extensive cellular diversity including four EP subtypes reflecting EP maturation using high-resolution single-cell RNA-sequencing of the e14.5 and e16.5 mouse pancreas. While e14.5 and e16.5 EPs are constantly born and share select genes, these EPs are overall transcriptionally distinct concomitant with changes in the underlying epithelium. As a consequence, e16.5 EPs are not the same as e14.5 EPs: e16.5 EPs have a higher propensity to form beta cells. Analysis of e14.5 and e16.5 EP chromatin states reveals temporal shifts, with enrichment of beta cell motifs in accessible regions at later stages. Finally, we provide transcriptional maps outlining the route progenitors take as they make cell fate decisions, which can be applied to advance the in vitro generation of beta cells.

LAG-3 (CD223) is a CD4 homolog that is required for maximal regulatory T cell function and for the control of CD4+ and CD8+ T cell homeostasis. Lag3−/− NOD mice developed substantially accelerated diabetes with 100% incidence. Adoptive transfer experiments revealed that LAG-3 was primarily responsible for limiting the pathogenic potential of CD4+ T cells, and to a lesser extent CD8+ T cells. Lag3−/− mice exhibited accelerated, invasive insulitis, corresponding to increased CD4+ and CD8+ T cell islet infiltration and intra-islet proliferation. The frequencies of islet antigen reactive chromogranin A-specific CD4+ T cells and IGRP-specific CD8+ T cells were significantly increased in the islets of Lag3−/− mice, suggesting an early expansion of pathogenic clones which is normally restrained by LAG-3. We conclude that LAG-3 is necessary for regulating CD4+ and CD8+ T cell function during autoimmune diabetes, and thus may contribute to limiting autoimmunity in disease-prone environments.

Interleukin (IL)-35 is a newly identified inhibitory cytokine used by T regulatory cells to control T cell–driven immune responses. However, the therapeutic potential of native, biologically active IL-35 has not been fully examined. Expression of the heterodimeric IL-35 cytokine was targeted to β-cells via the rat insulin promoter (RIP) II. Autoimmune diabetes, insulitis, and the infiltrating cellular populations were analyzed. Ectopic expression of IL-35 by pancreatic β-cells led to substantial, long-term protection against autoimmune diabetes, despite limited intraislet IL-35 secretion. Nonobese diabetic RIP-IL35 transgenic mice exhibited decreased islet infiltration with substantial reductions in the number of CD4+ and CD8+ T cells, and frequency of glucose-6-phosphatase catalytic subunit–related protein-specific CD8+ T cells. Although there were limited alterations in cytokine expression, the reduced T-cell numbers observed coincided with diminished T-cell proliferation and G1 arrest, hallmarks of IL-35 biological activity. These data present a proof of principle that IL-35 could be used as a potent inhibitor of autoimmune diabetes and implicate its potential therapeutic utility in the treatment of type 1 diabetes.

Autoreactive T cells infiltrating the target organ can possess a broad TCR affinity range. However, the extent to which such biophysical parameters contribute to T cell pathogenic potential remain unclear. In this study, we selected eight InsB9-23-specific TCRs cloned from CD4+ islet-infiltrating T cells that possessed a relatively broad range of TCR affinity to generate non-obese diabetic (NOD) TCR retrogenic mice. These TCRs exhibited a range of 2D affinities (~10−4 - 10−3 μm4) that correlated with functional readouts and responsiveness to activation in vivo. Surprisingly, both higher and lower affinity TCRs could mediate potent insulitis and autoimmune diabetes suggesting that TCR affinity does not exclusively dictate or correlate with diabetogenic potential. Both central and peripheral tolerance mechanisms selectively impinge on the diabetogenic potential of high affinity TCRs, mitigating their pathogenicity. Thus, TCR affinity and multiple tolerance mechanism converge to shape and broaden the diabetogenic T cell repertoire, potentially complicating efforts to induce broad, long-term tolerance.

Inhibitory receptors are pivotal in controlling T cell homeostasis because of their intrinsic regulation of conventional effector T (Tconv) cell proliferation, viability and function. However, the role of Inhibitory receptors on regulatory T (Treg) cells remains obscure, as they could be required for suppressive activity and/or limit Treg cell function. We evaluated the role of Lymphocyte Activation Gene-3 (LAG3, CD223) on Treg cells by generating mice in which LAG3 is absent on the cell surface of Treg cells in a murine model of Type 1 Diabetes. Surprisingly, mice that lacked LAG3 expression on Treg cells exhibited reduced autoimmune diabetes, consistent with enhanced Treg cell proliferation and function. Whereas the transcriptional landscape of peripheral wild-type (WT) and Lag3-deficient Treg cells was largely comparable, substantial differences between intra-islet Treg cells were evident and involved a subset of genes and pathways that promote Treg cell maintenance and function. Consistent with these observations, Lag3-deficient Treg cells out-competed WT Treg cells in the islets but not in the periphery in co-transfer experiments due to enhanced IL2-Stat5 signaling and increased Eos expression. Our study suggests that LAG3 intrinsically limits Treg cell proliferation and function at inflammatory sites, promotes autoimmunity in a chronic autoimmune-prone environment and may contribute to Treg cell insufficiency in autoimmune disease.