Regulatory T cells (Tregs) play 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-tumor immunity and sterilizing immunity to chronic viral infections1. The transcription factor Foxp3 plays a major role in the development and programming of Treg cells2,3. The relative stability of Tregs at inflammatory disease sites has been highly contentious4-6. There is considerable interest in identifying pathways that control Treg stability as many immune-mediated diseases are characterized by either exacerbated or limited Treg function. Here we show that the immune cell-expressed ligand semaphorin-4a (Sema4a) and the Treg-expressed receptor neuropilin-1 (Nrp1) interact to potentiate Treg function and survival in vitro and in inflammatory sites in vivo. Nrp1 is dispensable for suppression of autoimmunity and maintenance of immune homeostasis, but is required by Tregs to limit anti-tumor immune responses and to cure established inflammatory colitis. Sema4a ligation of Nrp1 restrained Akt phosphorylation cellularly and at the immunologic synapse (IS) via phosphatase and tensin homolog (PTEN), which increased nuclear localization of the transcription factor Foxo3a. The Nrp1-induced transcriptome promoted Treg stability by enhancing quiescence/survival factors while inhibiting programs that promote differentiation. Importantly, this Nrp1-dependent molecular program is evident in intratumoral Tregs. Our data support a model in which Treg 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-mediated tumor-induced tolerance without inducing autoimmunity.

SummaryThe physiological basis and mechanistic requirement for the high immunoreceptor tyrosine activation motifs (ITAM) multiplicity of the T cell receptor (TCR)-CD3 complex remains obscure. Here we show that while low TCR-CD3 ITAM multiplicity is sufficient to engage canonical TCR-induced signaling events that lead to cytokine secretion, high TCR-CD3 ITAM multiplicity is required for TCR-driven proliferation. This is dependent on compact immunological synapse formation, interaction of the adaptor Vav1 with phosphorylated CD3 ITAMs to mediate Notch1 recruitment and activation and ultimately c-Myc-induced proliferation. Analogous mechanistic events are also required to drive proliferation in response to weak peptide agonists. Thus, the TCR-driven pathways that initiate cytokine secretion and proliferation are separable and co-ordinated by the multiplicity of phosphorylated TCR-CD3 ITAMs.

Regulatory T cell (Treg) stability has been primarily determined by the maintained expression of the transcription factor Forkhead box P3 (Foxp3). However, Tregs can exhibit instability while maintaining Foxp3 expression, requiring a re-examination of what defines Treg stability. Recent work suggests that the establishment and stability of Tregs is mediated by a number of mechanisms besides Foxp3 expression, such as epigenetic modifications, Foxo1/3a localization, expression of Eos and signaling via Neuropilin-1. Additional studies may help to define approaches that can undermine Treg stability in cancer or enhance Treg stability in transplantation, autoimmune or inflammatory diseases and therefore have substantial therapeutic utility. In this review, we will discuss how Treg stability is defined and the mechanisms utilized to maintain stability.

Brain-derived neurotrophic factor (BDNF) is implicated in the pathophysiology of major depression; mice lacking BDNF expression through promoter IV (BDNF-KIV) exhibit a depression-like phenotype. We tested our hypothesis that deficits caused by promoter IV deficiency (depression-like behavior, decreased levels of BDNF, and neurogenesis in the hippocampus) could be rescued by a 3-week treatment with different types of antidepressants: fluoxetine, phenelzine, duloxetine, or imipramine. Each antidepressant reduced immobility time in the tail suspension test without affecting locomotor activity in the open field test in both BDNF-KIV and control wild type mice, except that phenelzine increased locomotor activity in wild type mice and anxiety-like behavior in BDNF-KIV mice. The antidepressant treatments were insufficient to reverse decreased BDNF levels caused by promoter IV deficiency. No antidepressant treatment increased the hippocampal progenitors of either genotype, whereas phenelzine decreased the surviving progenitors in both genotypes. The antidepressant treatments differently affected the dendritic extension of hippocampal immature neurons: fluoxetine and imipramine increased extension in both genotypes, duloxetine increased it only in BDNF-KIV mice, and phenelzine decreased it only in wild type mice. Interestingly, a saline-only injection increased neurogenesis and dendrite extensions in both genotypes. Our results indicate that the behavioral effects in the tail suspension test by antidepressants do not require promoter IV-driven BDNF expression and occur without a detectable increase in hippocampal BDNF levels and neurogenesis but may involve increased dendritic reorganisation of immature neurons. In conclusion, the antidepressant treatment demonstrated limited efficacy; it partially reversed the defective phenotypes caused by promoter IV deficiency but not hippocampal BDNF levels.