c-REL and IκBNS Govern Common and Independent Steps of Regulatory T Cell Development from Novel CD122-Expressing Pre-Precursors

Dissecting the metabolic signaling pathways by which microbial molecules drive the differentiation of regulatory B cells

The host-microbiome axis has been implicated in promoting anti-inflammatory immune responses. Yet, the underlying molecular mechanisms of commensal-mediated IL-10 production by regulatory B cells (Bregs) are not fully elucidated. Here, we demonstrate that bacterial CpG motifs trigger the signaling downstream of TLR9 promoting IκBNS-mediated expression of Blimp-1, a transcription regulator of IL-10. Surprisingly, this effect was counteracted by the NF-κB transcription factor c-Rel. A functional screen for intestinal bacterial species identified the commensal Clostridium sporogenes, secreting high amounts of short-chain fatty acids (SCFAs) and branched-chain fatty acids (BCFAs), as an amplifier of IL-10 production by promoting sustained mTOR signaling in B cells. Consequently, enhanced Breg functionality was achieved by combining CpG with the SCFA butyrate or the BCFA isovalerate thereby synergizing TLR- and mTOR-mediated pathways. Collectively, Bregs required two bacterial signals (butyrate and CpG) to elicit their full suppressive capacity and ameliorate T cell-mediated intestinal inflammation. Our study has dissected the molecular pathways induced by bacterial factors, which might contribute not only to better understanding of host-microbiome interactions, but also to exploration of new strategies for improvement of anti-inflammatory cellular therapy.

Many Faces of Regulatory T Cells: Heterogeneity or Plasticity?

Regulatory T cells (Tregs) are essential for maintaining the immune balance in normal and pathological conditions. In autoimmune diseases and transplantation, they restrain the loss of self-tolerance and promote engraftment, whereas in cancer, an increase in Treg numbers is mostly associated with tumor growth and poor prognosis. Numerous markers and their combinations have been used to identify Treg subsets, demonstrating the phenotypic diversity of Tregs. The complexity of Treg identification can be hampered by the unstable expression of some markers, the decrease in the expression of a specific marker over time or the emergence of a new marker. It remains unclear whether such phenotypic shifts are due to new conditions or whether the observed changes are due to initially different populations. In the first case, cellular plasticity is observed, whereas in the second, cellular heterogeneity is observed. The difference between these terms in relation to Tregs is rather blurred. Considering the promising perspectives of Tregs in regenerative cell-based therapy, the existing confusing data on Treg phenotypes require further investigation and analysis. In our review, we introduce criteria that allow us to distinguish between the heterogeneity and plasticity of Tregs normally and pathologically, taking a closer look at their diversity and drawing the line between two terms.

Regulatory T Cells for Control of Autoimmunity

Regulatory T (Treg) cells, which specifically express the master transcription factor FoxP3, are indispensable for the maintenance of immunological self-tolerance and homeostasis. Their functional or numerical anomalies can be causative of autoimmune and other inflammatory diseases. Recent advances in the research of the cellular and molecular basis of how Treg cells develop, exert suppression, and maintain their function have enabled devising various ways for controlling physiological and pathological immune responses by targeting Treg cells. It is now envisaged that Treg cells as a "living drug" are able to achieve antigen-specific immune suppression of various immune responses and reestablish immunological self-tolerance in the clinic.

NF-κB in control of regulatory T cell development, identity, and function

Regulatory T cells (Treg cells) act as a major rheostat regulating the strength of immune responses, enabling tolerance of harmless foreign antigens, and preventing the development of pathogenic immune responses in various disease settings such as cancer and autoimmunity. Treg cells are present in all lymphoid and non-lymphoid tissues, and the latter often fulfill important tasks required for the physiology of their host organ. The activation of NF-κB transcription factors is a central pathway for the reprogramming of gene expression in response to inflammatory but also homeostatic cues. Genetic mouse models have revealed essential functions for NF-κB transcription factors in modulating Treg development and function, with some of these mechanistic insights confirmed by recent studies analyzing Treg cells from patients harboring point mutations in the genes encoding NF-κB proteins. Molecular insights into the NF-κB pathway in Treg cells hold substantial promise for novel therapeutic strategies to manipulate dysfunctional or inadequate cell numbers of immunosuppressive Treg cells in autoimmunity or cancer. Here, we provide an overview of the manifold roles that NF-κB factors exert in Treg cells.

c-Rel employs multiple mechanisms to promote the thymic development and peripheral function of regulatory T cells in mice

The NF-κB transcription factor c-Rel is a critical regulator of Treg ontogeny, controlling multiple points of the stepwise developmental pathway. Here, we found that the thymic Treg defect in c-Rel-deficient (cRel^-/- ) mice is quantitative, not qualitative, based on analyses of TCR repertoire and TCR signaling strength. However, these parameters are altered in the thymic Treg-precursor population, which is also markedly diminished in cRel^-/- mice. Moreover, c-Rel governs the transcriptional programme of both thymic and peripheral Tregs, controlling a core of genes involved with immune signaling, and separately in the periphery, cell cycle progression. Last, the immune suppressive function of peripheral cRel^-/- tTregs is diminished in a lymphopenic model of T cell proliferation and is associated with decreased stability of Foxp3 expression. Collectively, we show that c-Rel is a transcriptional regulator that controls multiple aspects of Treg development, differentiation, and function via distinct mechanisms.

A distal Foxp3 enhancer enables interleukin-2 dependent thymic Treg cell lineage commitment for robust immune tolerance

Activation of the STAT5 transcription factor downstream of the Interleukin-2 receptor (IL-2R) induces expression of Foxp3, a critical step in the differentiation of regulatory T (Treg) cells. Due to the pleiotropic effects of IL-2R signaling, it is unclear how STAT5 acts directly on the Foxp3 locus to promote its expression. Here, we report that IL-2 - STAT5 signaling converged on an enhancer (CNS0) during Foxp3 induction. CNS0 facilitated the IL-2 dependent CD25+Foxp3- precursor to Treg cell transition in the thymus. Its deficiency resulted in impaired Treg cell generation in neonates, which was partially mitigated with age. While the thymic Treg cell paucity caused by CNS0 deficiency did not result in autoimmunity on its own, it exacerbated autoimmune manifestations caused by disruption of the Aire gene. Thus, CNS0 enhancer activity ensures robust Treg cell differentiation early in postnatal life and cooperatively with other tolerance mechanisms minimizes autoimmunity.

Influenza A virus-induced thymus atrophy differentially affects dynamics of conventional and regulatory T-cell development in mice

Foxp3⁺ Treg cells, which are crucial for maintenance of self-tolerance, mainly develop within the thymus, where they arise from CD25⁺ Foxp3^- or CD25^- Foxp3⁺ Treg cell precursors. Although it is known that infections can cause transient thymic involution, the impact of infection-induced thymus atrophy on thymic Treg (tTreg) cell development is unknown. Here, we infected mice with influenza A virus (IAV) and studied thymocyte population dynamics post infection. IAV infection caused a massive, but transient thymic involution, dominated by a loss of CD4⁺ CD8⁺ double-positive (DP) thymocytes, which was accompanied by a significant increase in the frequency of CD25⁺ Foxp3⁺ tTreg cells. Differential apoptosis susceptibility could be experimentally excluded as a reason for the relative tTreg cell increase, and mathematical modeling suggested that enhanced tTreg cell generation cannot explain the increased frequency of tTreg cells. Yet, an increased death of DP thymocytes and augmented exit of single-positive (SP) thymocytes was suggested to be causative. Interestingly, IAV-induced thymus atrophy resulted in a significantly reduced T-cell receptor (TCR) repertoire diversity of newly produced tTreg cells. Taken together, IAV-induced thymus atrophy is substantially altering the dynamics of major thymocyte populations, finally resulting in a relative increase of tTreg cells with an altered TCR repertoire.

Regulatory T Cell Heterogeneity in the Thymus: Impact on Their Functional Activities

Foxp3⁺ regulatory T cells (Treg) maintain the integrity of the organism by preventing excessive immune responses. These cells protect against autoimmune diseases but are also important regulators of other immune responses including inflammation, allergy, infection, and tumors. Furthermore, they exert non-immune functions such as tissue repair and regeneration. In the periphery, Foxp3⁺ Treg have emerged as a highly heterogeneous cell population with distinct molecular and functional properties. Foxp3⁺ Treg mainly develop within the thymus where they receive instructive signals for their differentiation. Recent studies have revealed that thymic Treg are also heterogeneous with two distinct precursors that give rise to mature Foxp3⁺ Treg exhibiting non-overlapping regulatory activities characterized by a differential ability to control different types of autoimmune reactions. Furthermore, the thymic Treg cell pool is not only composed of newly developing Treg, but also contain a large fraction of recirculating peripheral cells. Here, we review the two pathways of thymic Treg cell differentiation and their potential impact on Treg activity in the periphery. We also summarize our current knowledge on recirculating peripheral Treg in the thymus.

Ubiquitination of MHC Class II Is Required for Development of Regulatory but Not Conventional CD4+ T Cells

MHC class II (MHC II) displays peptides at the cell surface, a process critical for CD4⁺ T cell development and priming. Ubiquitination is a mechanism that dictates surface MHC II with the attachment of a polyubiquitin chain to peptide-loaded MHC II, promoting its traffic away from the plasma membrane. In this study, we have examined how MHC II ubiquitination impacts the composition and function of both conventional CD4⁺ T cell and regulatory T cell (T_reg) compartments. Responses were examined in two models of altered MHC II ubiquitination: MHCIIKR^{KI /KI} mice that express a mutant MHC II unable to be ubiquitinated or mice that lack membrane-associated RING-CH 8 (MARCH8), the E3 ubiquitin ligase responsible for MHC II ubiquitination specifically in thymic epithelial cells. Conventional CD4⁺ T cell populations in thymus, blood, and spleen of MHCIIKR^KI/KI and March8 ^-/- mice were largely unaltered. In MLRs, March8 ^-/-, but not MHCIIKR^KI/KI, CD4⁺ T cells had reduced reactivity to both self- and allogeneic MHC II. Thymic T_reg were significantly reduced in MHCIIKR^KI/KI mice, but not March8 ^-/- mice, whereas splenic T_reg were unaffected. Neither scenario provoked autoimmunity, with no evidence of immunohistopathology and normal levels of autoantibody. In summary, MHC II ubiquitination in specific APC types does not have a major impact on the conventional CD4⁺ T cell compartment but is important for T_reg development.

Regulatory T Cell Development in the Thymus

Development of a comprehensive regulatory T (Treg) cell compartment in the thymus is required to maintain immune homeostasis and prevent autoimmunity. In this study, we review cellular and molecular determinants of Treg cell development in the thymus. We focus on the evidence for a self-antigen-focused Treg cell repertoire as well as the APCs responsible for presenting self-antigens to developing thymocytes. We also cover the contribution of different cytokines to thymic Treg development and the cellular populations that produce these cytokines. Finally, we update the originally proposed "two-step" model of thymic Treg differentiation by incorporating new evidence demonstrating that Treg cells develop from two Treg progenitor populations and discuss the functional importance of Treg cells generated via either progenitor pathway.

Dynamic Imprinting of the Treg Cell-Specific Epigenetic Signature in Developing Thymic Regulatory T Cells

Regulatory T (Treg) cells mainly develop within the thymus and arise from CD25⁺Foxp3⁻ (CD25⁺ TregP) or CD25⁻Foxp3⁺ (Foxp3⁺ TregP) Treg cell precursors resulting in Treg cells harboring distinct transcriptomic profiles and complementary T cell receptor repertoires. The stable and long-term expression of Foxp3 in Treg cells and their stable suppressive phenotype are controlled by the demethylation of Treg cell-specific epigenetic signature genes including an evolutionarily conserved CpG-rich element within the Foxp3 locus, the Treg-specific demethylated region (TSDR). Here we analyzed the dynamics of the imprinting of the Treg cell-specific epigenetic signature genes in thymic Treg cells. We could demonstrate that CD25⁺Foxp3⁺ Treg cells show a progressive demethylation of most signature genes during maturation within the thymus. Interestingly, a partial demethylation of several Treg cell-specific epigenetic signature genes was already observed in Foxp3⁺ TregP but not in CD25⁺ TregP. Furthermore, Foxp3⁺ TregP were very transient in nature and arose at a more mature developmental stage when compared to CD25⁺ TregP. When the two Treg cell precursors were cultured in presence of IL-2, a factor known to be critical for thymic Treg cell development, we observed a major impact of IL-2 on the demethylation of the TSDR with a more pronounced effect on Foxp3⁺ TregP. Together, these results suggest that the establishment of the Treg cell-specific hypomethylation pattern is a continuous process throughout thymic Treg cell development and that the two known Treg cell precursors display distinct dynamics for the imprinting of the Treg cell-specific epigenetic signature genes.

Generation of Foxp3+CD25- Regulatory T-Cell Precursors Requires c-Rel and IκBNS

Next to the classical developmental route, in which first CD25 and subsequently Foxp3 are induced to generate thymic regulatory T (Treg) cells, an alternative route has been described. This alternative route is characterized by reciprocal induction of Foxp3 and CD25, with CD25 induction being required to rescue developing Treg cells from Foxp3-induced apoptosis. NF-κB has been demonstrated to be crucial for the development of thymic Treg cells via the classical route. However, its impact on the alternative route is poorly characterized. Using single and double deficient mice for key regulators of the classical route, c-Rel and IκB_NS, we here demonstrate that NF-κB is essential for the generation of alternative CD25⁻Foxp3⁺ precursors, as well. Thus, c-Rel and IκB_NS govern both routes of thymic Treg cell development.

Essential role of IκBNS for in vivo CD4+ T-cell activation, proliferation, and Th1-cell differentiation during Listeria monocytogenes infection in mice

Acquisition of effector functions in T cells is guided by transcription factors, including NF‐κB, that itself is tightly controlled by inhibitory proteins. The atypical NF‐κB inhibitor, IκB_NS, is involved in the development of Th1, Th17, and regulatory T (Treg) cells. However, it remained unclear to which extend IκB_NS contributed to the acquisition of effector function in T cells specifically responding to a pathogen during in vivo infection. Tracking of adoptively transferred T cells in Listeria monocytogenes infected mice antigen‐specific activation of CD4⁺ T cells following in vivo pathogen encounter to strongly rely on IκB_NS. While IκB_NS was largely dispensable for the acquisition of cytotoxic effector function in CD8⁺ T cells, IκB_NS‐deficient Th1 effector cells exhibited significantly reduced proliferation, marked changes in the pattern of activation marker expression, and reduced production of the Th1‐cell cytokines IFN‐γ, IL‐2, and TNF‐α. Complementary in vitro analyses using cells from novel reporter and inducible knockout mice revealed that IκB_NS predominantly affects the early phase of Th1‐cell differentiation while its function in terminally differentiated cells appears to be negligible. Our data suggest IκB_NS as a potential target to modulate specifically CD4⁺ T‐cell responses.

STIM- and Orai-mediated calcium entry controls NF-κB activity and function in lymphocytes

The central role of Ca²⁺ signaling in the development of functional immunity and tolerance is well established. These signals are initiated by antigen binding to cognate receptors on lymphocytes that trigger store operated Ca²⁺ entry (SOCE). The underlying mechanism of SOCE in lymphocytes involves TCR and BCR mediated activation of Stromal Interaction Molecule 1 and 2 (STIM1/2) molecules embedded in the ER membrane leading to their activation of Orai channels in the plasma membrane. STIM/Orai dependent Ca²⁺ signals guide key antigen induced lymphocyte development and function principally through direct regulation of Ca²⁺ dependent transcription factors. The role of Ca²⁺ signaling in NFAT activation and signaling is well known and has been studied extensively, but a wide appreciation and mechanistic understanding of how Ca²⁺ signals also shape the activation and specificity of NF-κB dependent gene expression has lagged. Here we discuss and interpret what is known about Ca²⁺ dependent mechanisms of NF-kB activation, including what is known and the gaps in our understanding of how these signals control lymphocyte development and function.

Functional heterogeneity of gut-resident regulatory T cells

Regulatory T cells (Treg cells) have a central role in the maintenance of intestinal homeostasis by restraining inappropriate immune responses in the healthy gut. Although distinct intestinal immune cell populations have been described to exhibit regulatory activity, several genetic and functional studies provided a strong evidence for a pivotal role of forkhead box P3 (Foxp3)⁺CD4⁺ Treg cells in prevention of dysregulated mucosal immune reactions and development of chronic immunological disorders such as celiac disease, food allergies and inflammatory bowel disease. Treg cells provide an important layer of intestinal defense by suppressing immune responses against innocuous food and commensal-derived antigens. Recent functional studies suggest that Treg cells are also involved in several other processes such as controlling microbial diversity in the gut, immunoglobulin A selection and supporting tissue repair in response to intestinal tissue damage. A better understanding of the functional heterogeneity as well as of the molecular signals, which regulate distinct intestinal Treg cell subsets, will encourage strategies aimed at transplanting the optimal Treg cell subset for cellular therapy in humans.