Dario A. A. Vignali

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 remains unclear. In this study, we selected eight InsB_9–23-specific TCRs cloned from CD4⁺ islet-infiltrating T cells that possessed a relatively broad range of TCR affinity to generate NOD TCR retrogenic mice. These TCRs exhibited a range of two-dimensional 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 mechanisms converge to shape and broaden the diabetogenic T cell repertoire, potentially complicating efforts to induce broad, long-term tolerance.

Summary: Regulatory T cells (Tregs) prevail as a specialized cell lineage that has a central role in the dominant control of immunological tolerance and maintenance of immune homeostasis. Thymus‐derived Tregs (tTregs) and their peripherally induced counterparts (pTregs) are imprinted with unique Forkhead box protein 3 (Foxp3)‐dependent and independent transcriptional and epigenetic characteristics that bestows on them the ability to suppress disparate immunological and non‐immunological challenges. Thus, unidirectional commitment and the predominant stability of this regulatory lineage is essential for their unwavering and robust suppressor function and has clinical implications for the use of Tregs as cellular therapy for various immune pathologies. However, recent studies have revealed considerable heterogeneity or plasticity in the Treg lineage, acquisition of alternative effector or hybrid fates, and promotion rather than suppression of inflammation in extreme contexts. In addition, the absolute stability of Tregs under all circumstances has been questioned. Since these observations challenge the safety and efficacy of human Treg therapy, the issue of Treg stability versus plasticity continues to be enthusiastically debated. In this review, we assess our current understanding of the defining features of Foxp3⁺ Tregs, the intrinsic and extrinsic cues that guide development and commitment to the Treg lineage, and the phenotypic and functional heterogeneity that shapes the plasticity and stability of this critical regulatory population in inflammatory contexts.

Foxp3⁺ regulatory T cells (T_regs) have a well-characterized role in limiting autoimmunity and dampening deleterious immune responses. However, a potential consequence of the immunosuppressive function of T_regs can be the limitation of protective immunity to infectious pathogens. Parasitic infections are a potent stimulus for the generation of T_reg responses, which may be beneficial to both the parasite and the host by promoting persistence of infection and limiting immune-mediated pathology, respectively. In this study, we explore the functional role of T_regs post–low-dose infection with the intestinal helminth parasite Trichuris muris, which yields a chronic infection because of inefficient induction of Th2 responses. Early T_reg depletion postinfection resulted in expedited worm clearance, and was associated with reduced Th1-mediated inflammation of the intestinal environment. Interestingly, this protective immunity was lost, and worm burden enhanced if T_regs were depleted later once the infection was established. Early and late T_reg depletion resulted in enhanced Th2 and reduced Th1 cytokine and humoral responses. Blockade of the Th2 cytokine IL-4 resulted in a moderate increase in Th1 but had no effect on worm burden. Our findings suggest that T_regs preferentially limit Th2 cell expansion, which can impact infections where clear immune polarity has not been established. Thus, the impact of T_reg depletion is context and time dependent, and can be beneficial to the host in situations where Th1 responses should be limited in favor of Th2 responses.

Interleukin-35 is a potent suppressive cytokine of the IL-12 family. Although other members of the IL-12 family are produced mainly by antigen-presenting cells (APCs), IL-35 is produced by regulatory T (Treg) cells and suppresses cell proliferation. It has been shown to play an important role in many disease models and has been recently shown to have additional functions aside from inhibition of proliferation, including inducing its own expression in non-Treg cells. In this chapter, we discuss the history and current status of IL-35 biology, as well as suggest where the field might move in the future.

Neuropilin-1 (Nrp1) on regulatory T (Treg) cells is shown to interact with semaphorin-4a (Sema4a) to promote a program of Treg-cell stability and survival, in part through PTEN-mediated modulation of Akt signalling; Nrp1-deficient Treg cells can maintain immune homeostasis but fail to suppress in inflammatory sites, such as tumours, providing an attractive immunotherapeutic target for the treatment of cancers. Regulatory T cells (Treg) constitute a barrier to effective anti-tumour immunity. Their depletion can induce reduction and clearance of many tumours, but as the cells perform an important balancing role in the immune system, depletion also results in unchecked autoimmunity and death. This paper describes an interaction between semaphorin-4a — an activator for T-cell-mediated immunity — and the neuropilin receptor Nrp1 on Treg cells that is required for Treg cells to limit anti-tumour immune responses and to cure established inflammatory colitis, but is dispensable for suppression of autoimmunity and maintenance of immune homeostasis. It remains to be determined whether it is feasible to limit tumour growth by targeting Treg cells without unleashing autoimmunity. The two biological activities may be inseparable, but this work points to ways in which this important system can be further characterized. Regulatory T cells (Treg cells) have a crucial role in the immune system by preventing autoimmunity, limiting immunopathology, and maintaining immune homeostasis1. However, they also represent a major barrier to effective anti-tumour immunity and sterilizing immunity to chronic viral infections1. The transcription factor Foxp3 has a major role in the development and programming of Treg cells2,3. The relative stability of Treg cells at inflammatory disease sites has been a highly contentious subject4,5,6. There is considerable interest in identifying pathways that control the stability of Treg cells as many immune-mediated diseases are characterized by either exacerbated or limited Treg-cell function. Here we show that the immune-cell-expressed ligand semaphorin-4a (Sema4a) and the Treg-cell-expressed receptor neuropilin-1 (Nrp1) interact both in vitro, to potentiate Treg-cell function and survival, and in vivo, at inflammatory sites. Using mice with a Treg-cell-restricted deletion of Nrp1, we show that Nrp1 is dispensable for suppression of autoimmunity and maintenance of immune homeostasis, but is required by Treg cells to limit anti-tumour immune responses and to cure established inflammatory colitis. Sema4a ligation of Nrp1 restrained Akt phosphorylation cellularly and at the immunologic synapse by phosphatase and tensin homologue (PTEN), which increased nuclear localization of the transcription factor Foxo3a. The Nrp1-induced transcriptome promoted Treg-cell stability by enhancing quiescence and survival factors while inhibiting programs that promote differentiation. Importantly, this Nrp1-dependent molecular program is evident in intra-tumoral Treg cells. Our data support a model in which Treg-cell stability can be subverted in certain inflammatory sites, but is maintained by a Sema4a–Nrp1 axis, highlighting this pathway as a potential therapeutic target that could limit Treg-cell-mediated tumour-induced tolerance without inducing autoimmunity.

The physiological basis and mechanistic requirements for a large number of functional immunoreceptor tyrosine-based activation motifs (ITAMs; high ITAM multiplicity) in the complex of the T cell antigen receptor (TCR) and the invariant signaling protein CD3 remain obscure. Here we found that whereas a low multiplicity of TCR-CD3 ITAMs was sufficient to engage canonical TCR-induced signaling events that led to cytokine secretion, a high multiplicity of TCR-CD3 ITAMs was required for TCR-driven proliferation. This was dependent on the formation of compact immunological synapses, interaction of the adaptor Vav1 with phosphorylated CD3 ITAMs to mediate the recruitment and activation of the oncogenic transcription factor Notch1 and, ultimately, proliferation induced by the cell-cycle regulator c-Myc. Analogous mechanistic events were also needed to drive proliferation in response to weak peptide agonists. Thus, the TCR-driven pathways that initiate cytokine secretion and proliferation are separable and are coordinated by the multiplicity of phosphorylated ITAMs in TCR-CD3.

Cytokine signals are essential for generating a robust and specialized immune response. These signals are typically transmitted via canonical STAT homodimers. However, the number of STAT molecules utilized by cytokine signaling cascades within immune cells are limited, and so the mechanism used to deliver complex signals remains elusive. Heterodimerization of STAT proteins is one potential mechanism for signals to be modified downstream of the receptor and may play an important role in dictating the targets of specific cytokine signaling. In this review, we discuss our current understanding of the prevalence of STAT heterodimers, how they are formed and what their physiologic role may be in vivo.