Homeostasis in the immune system is maintained by specialized regulatory CD4+ T cells (Treg) expressing transcription factor Foxp3. According to the current paradigm, high-affinity interactions between TCRs and class II MHC-peptide complexes in thymus “instruct” developing thymocytes to up-regulate Foxp3 and become Treg cells. However, the loss or down-regulation of Foxp3 does not disrupt the development of Treg cells but abrogates their suppressor function. In this study, we show that Foxp3-deficient Treg cells in scurfy mice harboring a null mutation of the Foxp3 gene retained cellular features of Treg cells including in vitro anergy, impaired production of inflammatory cytokines, and dependence on exogenous IL-2 for proliferation and homeostatic expansion. Foxp3-deficient Treg cells expressed a low level of activation markers, did not expand relative to other CD4+ T cells, and produced IL-4 and immunomodulatory cytokines IL-10 and TGF-β when stimulated. Global gene expression profiling revealed significant similarities between Treg cells expressing and lacking Foxp3. These results argue that Foxp3 deficiency alone does not convert Treg cells into conventional effector CD4+ T cells but rather these cells constitute a distinct cell subset with unique features.

Despite extensive research efforts to characterize peripheral regulatory T (Treg) cells expressing transcription factor Foxp3, their subset complexity, phenotypic characteristics, TCR repertoire and Ag specificities remain ambiguous. In this study, we identify and define two subsets of peripheral Treg cells differing in Foxp3 expression level and TCR repertoires. Treg cells expressing a high level of Foxp3 and TCRs not used by naive CD4+ T cells present a stable suppressor phenotype and dominate the peripheral Treg population in unmanipulated mice. The second Treg subset, expressing a lower level of Foxp3 and using TCRs shared with naive CD4+ T cells constitutes a small fraction of all Treg cells in unmanipulated mice and enriches Treg population with the same Ag specificities as expressed by activated/effector T cells. This Treg subset undergoes extensive expansion during response to Ag when it becomes a major population of Ag-specific Treg cells. Thus, Treg cells expressing TCRs shared with naive CD4+ T cells have a flexible phenotype and may down-regulate Foxp3 expression which may restore immune balance at the conclusion of immune response or convert these cells to effector T cells producing inflammatory cytokines.

In recent years the combined use of chemotherapy and immunotherapy, collectively termed chemoimmunotherapy, has emerged as a promising treatment option for patients with cancer. Antibiotics are commonly used to reduce infection-related complications in patients undergoing chemotherapy. Intriguingly, accumulating evidence has implicated gut microbiota as a critical determinant of host antitumor immune responses, raising the question as to whether the use of broad-spectrum antibiotics would invariably diminish tumor response to chemoimmunotherapies. We investigated the impact of antibiotics on the therapeutic outcomes of cyclophosphamide (CTX) chemotherapy and adoptive T-cell therapy (ACT) where CTX was used as the host-conditioning regimen in mice. We show that antibiotic prophylaxis dampened the endogenous T cell responses elicited by CTX, and reduced the efficacy of CTX against B-cell lymphoma. In the ACT setting, antibiotics administration impaired the therapeutic effects of adoptively transferred tumor-specific CD4+ T cells in mice with implanted colorectal tumors. In contrast, long-term antibiotic exposure did not affect the efficacy of ACT using CD19-targeting chimeric antigen receptor (CAR) T cells in mice with systemic B-cell lymphoma, although it correlated with prolonged CAR expression and sustained B-cell aplasia. Our study demonstrates that chemoimmunotherapies may have variable reliance on intestinal microbiota for T cell activation and function, and thus have different sensitivities to antibiotic prophylaxis. These findings may have implications for the judicial use of antibiotics in cancer patients receiving chemoimmunotherapies.

The gut microbiome is the largest source of intrinsic non–self-antigens that are continuously sensed by the immune system but typically do not elicit lymphocyte responses. CD4+ T cells are critical to sustain uninterrupted tolerance to microbial antigens and to prevent intestinal inflammation. However, clinical interventions targeting commensal bacteria–specific CD4+ T cells are rare, because only a very limited number of commensal-derived epitopes have been identified. Here, we used a new approach to study epitopes and identify T cell receptors expressed by CD4+Foxp3+ (Treg) cells specific for commensal-derived antigens. Using this approach, we found that antigens from Akkermansia muciniphila reprogram naïve CD4+ T cells to the Treg lineage, expand preexisting microbe specific Tregs, and limit wasting disease in the CD4+ T cell transfer model of colitis. These data suggest that the administration of specific commensal epitopes may help to widen the repertoire of specific Tregs that control intestinal inflammation.

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Immunotherapy is becoming an increasingly attractive therapeutic alternative for conventional cancer therapy. In recent years Foxp3+ regulatory T-cells (TR) were identified as the major obstacle to effective cancer immunotherapy. The abundance of these cells in peripheral blood is increased in patients with multiple types of cancer and their prevalence among tumor-infiltrating lymphocytes correlated with poor clinical prognosis. In contrast, removal or inactivation of TR cells led to enhanced antitumor immune response and better efficacy of cancer vaccines. We report that Bone Morphogenic Protein Receptor 1α (BMPR1α, Alk-3), is expressed by activated effector CD4+ and TR cells and modulates functions of both cell types. Bone Morphogenic Proteins (BMPs) belong to the transforming growth factor (TGF)-β family of cytokines that also include TGFβ and activins. BMPs play crucial roles in- embryonic development, tissue differentiation and homeostasis and development of cancer. It was demonstrated that BMPs and activins synergize with TGFβ to regulate thymic T-cell development, maintain TR cells and control peripheral tolerance. Inactivation of BMPR1α in T-cells results in impaired thymic and peripheral generation of TR cells. BMPR1α -deficient activated T-cells produced higher level of interferon (IFN)-γ than BMPR1α-sufficient T-cells. Moreover, transplanted B16 melanoma tumors grew smaller in mice lacking expression of BMPR1α in T-cells and tumors had few infiltrating TR cells and a higher proportion of CD8+ T-cells than wild-type mice.

Thymic central tolerance eliminates most immature T cells with autoreactive T cell receptors (TCR) that recognize self MHC/peptide complexes. Regardless, an unknown number of autoreactive CD4+Foxp3− T cells escape negative selection and in the periphery require continuous suppression by CD4+Foxp3+ regulatory cells (Tregs). Here, we compare immune repertoires of Treg-deficient and Treg-sufficient mice to find Tregs continuously constraining one-third of mature CD4+Foxp3− cells from converting to pathogenic effectors in healthy mice. These dormant pathogenic clones frequently express TCRs activatable by ubiquitous autoantigens presented by class II MHCs on conventional dendritic cells, including self-peptides that select them in the thymus. Our data thus suggest that identification of most potentially autoreactive CD4+ T cells in the peripheral repertoire is critical to harness or redirect these cells for therapeutic advantage.

Increased availability of homeostatic cytokines is considered a major mechanism by which lymphodepletion enhances the efficacy of adoptive T cell therapy (ACT). IL-7 is one such cytokine capable of augmenting the function of tumor-reactive CD8+ T cells. However, whether host-derived IL-7 plays a role in driving the proper function of CD4+ T cells in an ACT setting remains unclear. Here we report that lymphodepleting chemotherapy by cyclophosphamide (CTX) does not lead to increased availability of the endogenous IL-7 in mice. Despite of a paucity of IL-7 in the immune milieu, CTX preconditioning allowed adoptively transferred naïve tumor-specific CD4+ T cells to undergo effector differentiation and regain IL-7Rα expression, giving rise to IL-7-responsive polyfunctional CD4+ effector cells. Correspondingly, supplementation of exogenous recombinant IL-7 markedly amplified and sustained polyfunctional CD4+ effector cells, resulting in improved therapeutic outcome in a mouse lymphoma model. We further demonstrated that the immune-enhancing effects of IL-7 were also applicable to donor CD4+ T cells pre-activated under Th1 polarizing condition. These findings suggest caution in relying on the endogenous IL-7 to enhance donor T cell expansion and persistence after lymphodepleting chemotherapy, and highlight the usefulness of recombinant IL-7 as an adjuvant for adoptive immunotherapy.