Cutting edge: CD28 and c-Rel-dependent pathways initiate regulatory T cell development

IKK2 controls the inflammatory potential of tissue-resident regulatory T cells in a murine gain of function model

Loss-of-function mutations have provided crucial insights into the immunoregulatory actions of Foxp3+ regulatory T cells (Tregs). By contrast, we know very little about the consequences of defects that amplify aspects of Treg function or differentiation. Here we show that mice heterozygous for an Ikbkb gain-of-function mutation develop psoriasis. Doubling the gene dose (IkbkbGoF/GoF) results in dactylitis, spondylitis, and characteristic nail changes, which are features of psoriatic arthritis. IkbkbGoF mice exhibit a selective expansion of Foxp3 + CD25+ Tregs of which a subset express IL-17. These modified Tregs are enriched in both inflamed tissues, blood and spleen, and their transfer is sufficient to induce disease without conventional T cells. Single-cell transcriptional and phenotyping analyses of isolated Tregs reveal expansion of non-lymphoid tissue (tissue-resident) Tregs expressing Th17-related genes, Helios, tissue-resident markers including CD103 and CD69, and a prominent NF-κB transcriptome. Thus, IKK2 regulates tissue-resident Treg differentiation, and overactivity drives dose-dependent skin and systemic inflammation.

Regulating the regulatory T cells as cell therapies in autoimmunity and cancer

Regulatory T cells (Tregs), possess a pivotal function in the maintenance of immune homeostasis. The dysregulated activity of Tregs has been associated with the onset of autoimmune diseases and cancer. Hence, Tregs are promising targets for interventions aimed at steering the immune response toward the desired path, either by augmenting the immune system to eliminate infected and cancerous cells or by dampening it to curtail the damage to self-tissues in autoimmune disorders. The activation of Tregs has been observed to have a potent immunosuppressive effect against T cells that respond to self-antigens, thus safeguarding our body against autoimmunity. Therefore, promoting Treg cell stability presents a promising strategy for preventing or managing chronic inflammation that results from various autoimmune diseases. On the other hand, Tregs have been found to be overactivated in several forms of cancer, and their role as immune response regulators with immunosuppressive properties poses a significant impediment to the successful implementation of cancer immunotherapy. However, the targeting of Tregs in a systemic manner may lead to the onset of severe inflammation and autoimmune toxicity. It is imperative to develop more selective methods for targeting the function of Tregs in tumors. In this review, our objective is to elucidate the function of Tregs in tumors and autoimmunity while also delving into numerous therapeutic strategies for reprogramming their function. Our focus is on reprogramming Tregs in a highly activated phenotype driven by the activation of key surface receptors and metabolic reprogramming. Furthermore, we examine Treg-based therapies in autoimmunity, with a specific emphasis on Chimeric Antigen Receptor (CAR)-Treg therapy and T-cell receptor (TCR)-Treg therapy. Finally, we discuss key challenges and the future steps in reprogramming Tregs that could lead to the development of novel and effective cancer immunotherapies.

In or out of control: Modulating regulatory T cell homeostasis and function with immune checkpoint pathways

Regulatory T cells (Tregs) are the master regulators of immunity and they have been implicated in different disease states such as infection, autoimmunity and cancer. Since their discovery, many studies have focused on understanding Treg development, differentiation, and function. While there are many players in the generation and function of truly suppressive Tregs, the role of checkpoint pathways in these processes have been studied extensively. In this paper, we systematically review the role of different checkpoint pathways in Treg homeostasis and function. We describe how co-stimulatory and co-inhibitory pathways modulate Treg homeostasis and function and highlight data from mouse and human studies. Multiple checkpoint pathways are being targeted in cancer and autoimmunity; therefore, we share insights from the clinic and discuss the effect of experimental and approved therapeutics on Treg biology.

Regulatory T cell niche in the bone marrow, a new player in Haematopoietic stem cell transplantation

Challenges in haematopoietic stem cell transplantation such as low bone marrow (BM) engraftment, graft versus host disease (GvHD) and the need for long-term immunosuppression could be addressed using T regulatory cells (Tregs) resident in the tissue of interest, in this case, BM Tregs. Controlling the adverse immune response in haematopoietic stem cell transplantation (HSCT) and minimising the associated risks such as infection and secondary cancers due to long-term immunosuppression is a crucial aspect of clinical practice in this field. While systemic immunosuppressive therapy could achieve reasonable GvHD control in most patients, related side effects remain the main limiting factor. Developing more targeted immunosuppressive strategies is an unmet clinical need and is the focus of several ongoing research projects. Tregs are a non-redundant sub-population of CD4⁺ T cells essential for controlling the immune homeostasis. Tregs are known to be reduced in number and function in autoimmune conditions. There is considerable interest in these cells as cell therapy products since they can be expanded in vitro and infused into patients. These trials have found Treg therapy to be safe, well-tolerated, and with some early signs of efficacy. However, Tregs are a heterogeneous subpopulation of T cells, and several novel subpopulations have been identified in recent years beyond the conventional thymic (tTregs) and peripheral (pTregs). There is increasing evidence for the presence of resident and tissue-specific Tregs. Bone marrow (BM) Tregs are one example of tissue-resident Tregs. BM Tregs are enriched within the marrow, serving a dual function of immunosuppression and maintenance of haematopoietic stem cells (HSCs). HSCs maintenance is achieved through direct suppression of HSCs differentiation, maintaining a proliferating pool of HSCs, and promoting the development of functional stromal cells that support HSCs. In this review, we will touch upon the biology of Tregs, focusing on their development and heterogeneity. We will focus on the BM Tregs from their biology to their therapeutic potential, focusing on their use in HSCT.

Type 2 cytokines in the thymus activate Sirpα+ dendritic cells to promote clonal deletion

The thymus contains a diversity of dendritic cells (DCs) that exist in defined locations and have different antigen-processing and -presenting features. This suggests that they play nonredundant roles in mediating thymocyte selection. In an effort to eliminate SIRPα+ classic DC2 subsets, we discovered that a substantial proportion expresses the surface lectin, CD301b, in the thymus. These cells resemble the CD301b+ type 2 immune response promoting DCs that are present in the skin-draining lymph nodes. Transcriptional and phenotypic comparison to other DC subsets in the thymus revealed that thymic CD301b+ cDCs represent an activated state that exhibits enhanced antigen processing and presentation. Furthermore, a CD301b+ cDC2 subset demonstrated a type 2 cytokine signature and required steady-state interleukin-4 receptor signaling. Selective ablation of CD301b+ cDC2 subsets impaired clonal deletion without affecting regulatory T cells (Treg cells). The T cell receptor α repertoire sequencing confirmed that a cDC2 subset promotes deletion of conventional T cells with minimal effect on Treg cell selection. Together, these findings suggest that cytokine-induced activation of DCs in the thymus substantially enforces central tolerance.

The perception and response of T cells to a changing environment are based on the law of initial value

αβ T cells are critical components of the adaptive immune system and are capable of inducing sterilizing immunity after pathogen infection and eliminating transformed tumor cells. The development and function of T cells are controlled through the T cell antigen receptor, which recognizes peptides displayed on major histocompatibility complex (MHC) molecules. Here, we review how T cells generate the ability to recognize self-peptide-bound MHC molecules and use signals derived from these interactions to instruct cellular development, activation thresholds, and functional specialization in the steady state and during immune responses. We argue that the basic tenants of T cell development and function follow Weber-Fetcher's law of just noticeable differences and Wilder's law of initial value. Together, these laws argue that the ability of a system to respond and the quality of that response are scalable to the basal state of that system. Manifestation of these laws in T cells generates clone-specific activation thresholds that are based on perceivable differences between homeostasis and pathogen encounter (self versus nonself discrimination), as well as poised states for subsequent differentiation into specific effector cell lineages.

Instructive Cues of Thymic T Cell Selection

A high diversity of αβ T cell receptors (TCRs), capable of recognizing virtually any pathogen but also self-antigens, is generated during T cell development in the thymus. Nevertheless, a strict developmental program supports the selection of a self-tolerant T cell repertoire capable of responding to foreign antigens. The steps of T cell selection are controlled by cortical and medullary stromal niches, mainly composed of thymic epithelial cells and dendritic cells. The integration of important cues provided by these specialized niches, including (a) the TCR signal strength induced by the recognition of self-peptide-MHC complexes, (b) costimulatory signals, and (c) cytokine signals, critically controls T cell repertoire selection. This review discusses our current understanding of the signals that coordinate positive selection, negative selection, and agonist selection of Foxp3⁺ regulatory T cells. It also highlights recent advances that have unraveled the functional diversity of thymic antigen-presenting cell subsets implicated in T cell selection.

Gold nanorods enhance different immune cells and allow for efficient targeting of CD4+ Foxp3+ Tregulatory cells

Gold nanoparticles (AuNPs) hold great promise in nanomedicine, yet their successful clinical translation has not been realized. Some challenges include effective AuNP targeting and delivery to improve modulation of immune cells of interest while limiting potential adverse effects. In order to overcome these challenges, we must fully understand how AuNPs impact different immune cell subsets, particularly within the dendritic cell and T cell compartments. Herein, we show that polyethylene glycol coated (PEG) gold nanorods (AuNRs) and PEG AuNRs covered with a thin layer of silver (AuNR/Ag) may enhance the immune response towards immune suppression or activation. We also studied the ability to enhance CD4+ Foxp3+ Tregs in vitro using AuNRs functionalized with interleukin 2 (IL2), a cytokine that is important in Treg development and homeostasis. Our results indicate that AuNRs enhance different immune cells and that NP composition matters in immune targeting. This knowledge will help us understand how to better design AuNRs to target and enhance the immune system.

Treg: A Promising Immunotherapeutic Target in Oral Diseases

With the pandemic of COVID-19, maintenance of oral health has increasingly become the main challenge of global health. Various common oral diseases, such as periodontitis and oral cancer, are closely associated with immune disorders in the oral mucosa. Regulatory T cells (Treg) are essential for maintaining self-tolerance and immunosuppression. During the process of periodontitis and apical periodontitis, two typical chronic immune-inflammatory diseases, Treg contributes to maintain host immune homeostasis and minimize tissue damage. In contrast, in the development of oral precancerous lesions and oral cancer, Treg is expected to be depleted or down-regulated to enhance the anti-tumor immune response. Therefore, a deeper understanding of the distribution, function, and regulatory mechanisms of Treg cells may provide a prospect for the immunotherapy of oral diseases. In this review, we summarize the distribution and multiple roles of Treg in different oral diseases and discuss the possible mechanisms involved in Treg cell regulation, hope to provide a reference for future Treg-targeted immunotherapy in the treatment of oral diseases.

MHC Class I on murine hematopoietic APC selects Type A IEL precursors in the thymus

TCRαβ⁺ CD8α⁺ CD8β^- intestinal intraepithelial lymphocytes (CD8αα IEL) are gut T cells that maintain barrier surface homeostasis. Most CD8αα IEL are derived from thymic precursors (IELp) through a mechanism referred to as clonal diversion. In this model, self-reactive thymocytes undergo deletion in the presence of CD28 costimulation, but in its absence undergo diversion to the IEL fate. While previous reports showed that IELp were largely β2m dependent, the APC that drive the development of these cells are poorly defined. We found that both CD80 and CD86 restrain IELp development, and conventional DCs play a prominent role. We sought to define a CD80/86 negative, MHCI positive APC that supports the development to the IEL lineage. Chimera studies showed that MHCI needs to be expressed on hematopoietic APC for selection. As thymic hematopoietic APC are heterogeneous in their expression of MHCI and costimulatory molecules, we identified four thymic APC types that were CD80/86^neg/low and MHCI⁺ . However, selective depletion of β2m in individual APC suggested functional redundancy. Thus, while hematopoietic APC play a critical role in clonal diversion, no single APC subset is specialized to promote the CD8αα IEL fate.

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.

Myeloid-Derived Suppressor Cell Differentiation in Cancer: Transcriptional Regulators and Enhanceosome-Mediated Mechanisms

Myeloid-derived Suppressor Cells (MDSCs) are a sub-population of leukocytes that are important for carcinogenesis and cancer immunotherapy. During carcinogenesis or severe infections, inflammatory mediators induce MDSCs via aberrant differentiation of myeloid precursors. Although several transcription factors, including C/EBPβ, STAT3, c-Rel, STAT5, and IRF8, have been reported to regulate MDSC differentiation, none of them are specifically expressed in MDSCs. How these lineage-non-specific transcription factors specify MDSC differentiation in a lineage-specific manner is unclear. The recent discovery of the c-Rel−C/EBPβ enhanceosome in MDSCs may help explain these context-dependent roles. In this review, we examine several transcriptional regulators of MDSC differentiation, and discuss the concept of non-modular regulation of MDSC signature gene expression by transcription factors such as c-Rel and C/EBPß.

Dendritic cell biocompatibility of ether-based urethane films

The use of synthetic materials for biomedical applications is ever expanding. One of the major requirements for these materials is biocompatibility, which includes prevention of immune system responses. Due to the inherent complexity of their structural composition, the polyurethane (PU) family of polymers is being used in a variety of medical applications, from soft and hard tissue scaffolds to intricate coatings on implantable devices. Herein, we investigated whether two polymer materials, D3 and D7, induced an immune response, measured by their effects on a dendritic cell (DC) line, JAWS II. Using a lactate dehydrogenase cytotoxicity assay and Annexin V/PI staining, we found that the PU materials did not induce cytotoxicity in DC cells. Using confocal microscopy, we also showed that the materials did not induce activation or maturation, as compared to positive controls. This was confirmed by looking at various markers, CD80, CD86, MHC class I, and MHC class II, via flow cytometry. Overall, the results indicated that the investigated PU films are biocompatible in terms of immunotoxicology and immunogenicity and show great promise for use in regenerative medicine.

B7-CD28 co-stimulation modulates central tolerance via thymic clonal deletion and Treg generation through distinct mechanisms

The molecular and cellular mechanisms mediating thymic central tolerance and prevention of autoimmunity are not fully understood. Here we show that B7-CD28 co-stimulation and B7 expression by specific antigen-presenting cell (APC) types are required for clonal deletion and for regulatory T (Treg) cell generation from endogenous tissue-restricted antigen (TRA)-specific thymocytes. While B7-CD28 interaction is required for both clonal deletion and Treg induction, these two processes differ in their CD28 signaling requirements and in their dependence on B7-expressing dendritic cells, B cells, and thymic epithelial cells. Meanwhile, defective thymic clonal deletion due to altered B7-CD28 signaling results in the accumulation of mature, peripheral TRA-specific T cells capable of mediating destructive autoimmunity. Our findings thus reveal a function of B7-CD28 co-stimulation in shaping the T cell repertoire and limiting autoimmunity through both thymic clonal deletion and Treg cell generation.

The many-sided contributions of NF-κB to T-cell biology in health and disease

T cells (or T lymphocytes) exhibit a myriad of functions in immune responses, ranging from pathogen clearance to autoimmunity, cancer and even non-lymphoid tissue homeostasis. Therefore, deciphering the molecular mechanisms orchestrating their specification, function and gene expression pattern is critical not only for our comprehension of fundamental biology, but also for the discovery of novel therapeutic targets. Among the master regulators of T-cell identity, the functions of the NF-κB family of transcription factors have been under scrutiny for several decades. However, a more precise understanding of their pleiotropic functions is only just emerging. In this review we will provide a global overview of the roles of NF-κB in the different flavors of mature T cells. We aim at highlighting the complex and sometimes diverging roles of the five NF-κB subunits in health and disease.

Regulatory T Cell Development in the Thymus

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

Inhibition of IL-2 or NF-κB Subunit c-Rel-Dependent Signaling Inhibits Expansion of Regulatory T Cells During Acute Friend Retrovirus Infection

In retroviral infections, different immunological mechanisms are involved in the development of a chronic infection. In the Friend virus (FV) model, regulatory T cells (Tregs) were found to induce CD8⁺ T cell dysfunction before viral clearance is achieved and thus contribute to viral chronicity. Although studied for decades, the exact suppressive mechanisms of Tregs in the FV model remain elusive and an unavailable therapeutic target. However, extracellular IL-2 and intracellular NF-κB signaling were shown to be important pathways for Treg expansion and activation. Therefore, we decided to focus on these two pathways to test therapeutic approaches inhibiting Treg activation during FV infection. In this study, we show that the inhibition of either IL-2 or the NF-κB subunit c-Rel, impaired Treg expansion and activation at 2 weeks post-FV infection. Total numbers of Tregs as well as activated Tregs were reduced in FV-infected mice after treatment with anti-IL-2 antibodies or the c-Rel blocking reagent pentoxifylline. Surprisingly, this did not affect the expansion or function of virus-specific CD8⁺ T cells nor viral loads in the spleen. However, our data suggest that neutralization of IL-2 as well as blocking c-Rel efficiently inhibits virus-induced Treg expansion.

Regulatory T Cell Development

Foxp3-expressing CD4⁺ regulatory T (Treg) cells play key roles in the prevention of autoimmunity and the maintenance of immune homeostasis and represent a major barrier to the induction of robust antitumor immune responses. Thus, a clear understanding of the mechanisms coordinating Treg cell differentiation is crucial for understanding numerous facets of health and disease and for developing approaches to modulate Treg cells for clinical benefit. Here, we discuss current knowledge of the signals that coordinate Treg cell development, the antigen-presenting cell types that direct Treg cell selection, and the nature of endogenous Treg cell ligands, focusing on evidence from studies in mice. We also highlight recent advances in this area and identify key unanswered questions.

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.

Treg programming and therapeutic reprogramming in cancer

Overcoming the immunosuppressive tumour microenvironment is the major challenge impeding cancer immunotherapy today. Regulatory T-cells (Tregs) are prevalent in nearly all cancers and, as immunosuppressive regulators of immune responses, they are the principal opponents of cancer immunotherapy. However, disabling Tregs systemically causes severe autoimmune toxicity, hastening the need for more selective methods to target intratumoural Tregs. In this review, we discuss a burgeoning new modality to specifically target tumour-infiltrating Tregs (TI-Tregs) by reprogramming their functionality from immunosuppressive to immune stimulatory within tumours. As the basis for therapeutic selectivity of TI-Tregs, we will focus on the defining features of Tregs within cancer: their highly activated state controlled by the engagement of key surface receptors, their distinct metabolic programme, and their unique transcriptional programme. By identifying proteins and pathways that distinguish TI-Tregs from other Tregs in the body, as well as from the beneficial antitumour effector T-cells within tumours, we highlight mechanisms to selectively reprogramme TI-Tregs for the treatment of cancer.

Measuring Thymic Clonal Deletion at the Population Level

Clonal deletion of T cells specific for self-antigens in the thymus has been widely studied, primarily by approaches that focus on a single receptor (using TCR transgenes) or a single specificity (using peptide-MHC tetramers). However, less is known about clonal deletion at the population level. In this article, we report an assay that measures cleaved caspase 3 to define clonal deletion at the population level. This assay distinguishes clonal deletion from apoptotic events caused by neglect and approximates the anatomic site of deletion using CCR7. This approach showed that 78% of clonal deletion events occur in the cortex in mice. Medullary deletion events were detected at both the semimature and mature stages, although mature events were associated with failed regulatory T cell induction. Using this assay, we showed that bone marrow-derived APC drive approximately half of deletion events at both stages. We also found that both cortical and medullary deletion rely heavily on CD28 costimulation. These findings demonstrate a useful strategy for studying clonal deletion within the polyclonal repertoire.

miR-181a/b-1 controls thymic selection of Treg cells and tunes their suppressive capacity

The interdependence of selective cues during development of regulatory T cells (Treg cells) in the thymus and their suppressive function remains incompletely understood. Here, we analyzed this interdependence by taking advantage of highly dynamic changes in expression of microRNA 181 family members miR-181a-1 and miR-181b-1 (miR-181a/b-1) during late T-cell development with very high levels of expression during thymocyte selection, followed by massive down-regulation in the periphery. Loss of miR-181a/b-1 resulted in inefficient de novo generation of Treg cells in the thymus but simultaneously permitted homeostatic expansion in the periphery in the absence of competition. Modulation of T-cell receptor (TCR) signal strength in vivo indicated that miR-181a/b-1 controlled Treg-cell formation via establishing adequate signaling thresholds. Unexpectedly, miR-181a/b-1-deficient Treg cells displayed elevated suppressive capacity in vivo, in line with elevated levels of cytotoxic T-lymphocyte-associated 4 (CTLA-4) protein, but not mRNA, in thymic and peripheral Treg cells. Therefore, we propose that intrathymic miR-181a/b-1 controls development of Treg cells and imposes a developmental legacy on their peripheral function.

Signal Transduction Via Co-stimulatory and Co-inhibitory Receptors

T-cell receptor (TCR)-mediated antigen-specific stimulation is essential for initiating T-cell activation. However, signaling through the TCR alone is not sufficient for inducing an effective response. In addition to TCR-mediated signaling, signaling through antigen-independent co-stimulatory or co-inhibitory receptors is critically important not only for the full activation and functional differentiation of T cells but also for the termination and suppression of T-cell responses. Many studies have investigated the signaling pathways underlying the function of each molecular component. Co-stimulatory and co-inhibitory receptors have no kinase activity, but their cytoplasmic region contains unique functional motifs and potential phosphorylation sites. Engagement of co-stimulatory receptors leads to recruitment of specific binding partners, such as adaptor molecules, kinases, and phosphatases, via recognition of a specific motif. Consequently, each co-stimulatory receptor transduces a unique pattern of signaling pathways. This review focuses on our current understanding of the intracellular signaling pathways provided by co-stimulatory and co-inhibitory molecules, including B7:CD28 family members, immunoglobulin, and members of the tumor necrosis factor receptor superfamily.

Requirement for POH1 in differentiation and maintenance of regulatory T cells

Foxp3-expressing regulatory T (Treg) cells are essential for averting autoimmune diseases and maintaining immune homeostasis. However, the molecular mechanisms underlying the development and maintenance of Treg cells are still unclear. Here, we found that T cell-specific deletion of the gene encoding the deubiquitinase POH1 compromised the development of mature T cells, especially CD4⁺Foxp3⁺ Treg cells. Moreover, POH1 deficiency significantly attenuated the transition of CD25⁺ Treg cell precursors into Foxp3⁺ Treg cells accompanied by downregulation of interleukin 2 (IL-2)-STAT5 signaling. Deletion of POH1 in generated CD4⁺Foxp3⁺ Treg cells led to an early onset of fetal autoimmune disorders and a decrease in the pool size of peripheral Treg cells in mice, which were mostly due to decreased expansion of these cells. Thus, these results revealed that POH1 has a pivotal role in the development and maintenance of CD4⁺Foxp3⁺ Treg cells and contributes to immune tolerance.

Regulatory T cell subsets are differentially dependent on CD28 for their proliferation

It is thought that CD28 plays a crucial role in the maintenance of regulatory T cell (Treg) pool size through promoting the development and proliferation of these cells. However, recently we found that the dependency on CD28 co-stimulation for their development is different between Treg subsets, thymus-derived Tregs (tTregs, CD28-dependent) and peripherally-derived Tregs (pTregs, CD28-independent), suggesting that CD28 may also have differential influences on the homeostasis of each Treg subset. Here, we demonstrated that both Treg subsets were reduced in secondary lymphoid organs of CD28 deficient mice, and that this reduction was due to impaired proliferation in both Treg subsets by the intrinsic CD28 defect. However, we found that the massive proliferation of both Treg subsets under lymphopenic condition was regulated by CD28, whereas the proliferative activity of tTregs but not pTregs in the steady state was dependent on CD28. Also, experiments using mutant CD28 knock-in mice revealed that proliferation of pTregs under lymphopenic condition required only the Lck-NFκB pathway of CD28, whereas tTregs required an additional unknown pathway. These findings indicate that the dependency on CD28 for proliferation in each Treg subset differs depending on the environment.

The Alternative NF-κB Pathway in Regulatory T Cell Homeostasis and Suppressive Function

CD4⁺Foxp3⁺ regulatory T cells (Tregs) are essential regulators of immune responses. Perturbation of Treg homeostasis or function can lead to uncontrolled inflammation and autoimmunity. Therefore, understanding the molecular mechanisms involved in Treg biology remains an active area of investigation. It has been shown previously that the NF-κB family of transcription factors, in particular, the canonical pathway subunits, c-Rel and p65, are crucial for the development, maintenance, and function of Tregs. However, the role of the alternative NF-κB pathway components, p100 and RelB, in Treg biology remains unclear. In this article, we show that conditional deletion of the p100 gene, nfkb2, in Tregs, resulted in massive inflammation because of impaired suppressive function of nfkb2-deficient Tregs. Surprisingly, mice lacking RelB in Tregs did not exhibit the same phenotype. Instead, deletion of both relb and nfkb2 rescued the inflammatory phenotype, demonstrating an essential role for p100 as an inhibitor of RelB in Tregs. Our data therefore illustrate a new role for the alternative NF-κB signaling pathway in Tregs that has implications for the understanding of molecular pathways driving tolerance and immunity.

SGK1 Governs the Reciprocal Development of Th17 and Regulatory T Cells

A balance between Th17 and regulatory T (Treg) cells is critical for immune homeostasis and tolerance. Our previous work has shown Serum- and glucocorticoid-induced kinase 1 (SGK1) is critical for the development and function of Th17 cells. Here, we show that SGK1 restrains the function of Treg cells and reciprocally regulates development of Th17/Treg balance. SGK1 deficiency leads to protection against autoimmunity and enhances self-tolerance by promoting Treg cell development and disarming Th17 cells. Treg cell-specific deletion of SGK1 results in enhanced Treg cell-suppressive function through preventing Foxo1 out of the nucleus, thereby promoting Foxp3 expression by binding to Foxp3 CNS1 region. Furthermore, our data suggest that SGK1 also plays a critical role in IL-23R-mediated inhibition of Treg and development of Th17 cells. Therefore, we demonstrate that SGK1 functions as a pivotal node in regulating the reciprocal development of pro-inflammatory Th17 and Foxp3+ Treg cells during autoimmune tissue inflammation.

Nuclear Factor-kappaB in Autoimmunity: Man and Mouse

NF-κB (nuclear factor-kappa B) is a transcription complex crucial for host defense mediated by innate and adaptive immunity, where canonical NF-κB signaling, mediated by nuclear translocation of RelA, c-Rel, and p50, is important for immune cell activation, differentiation, and survival. Non-canonical signaling mediated by nuclear translocation of p52 and RelB contributes to lymphocyte maturation and survival and is also crucial for lymphoid organogenesis. We outline NF-κB signaling and regulation, then summarize important molecular contributions of NF-κB to mechanisms of self-tolerance. We relate these mechanisms to autoimmune phenotypes described in what is now a substantial catalog of immune defects conferred by mutations in NF-κB pathways in mouse models. Finally, we describe Mendelian autoimmune syndromes arising from human NF-κB mutations, and speculate on implications for understanding sporadic autoimmune disease.

Modifying Dendritic Cell Activation with Plasmonic Nano Vectors

Dendritic cells (DCs) can acquire, process, and present antigens to T-cells to induce an immune response. For this reason, targeting cancer antigens to DCs in order to cause an immune response against cancer is an emerging area of nanomedicine that has the potential to redefine the way certain cancers are treated. The use of plasmonically active silver-coated gold nanorods (henceforth referred to as plasmonic nano vectors (PNVs)) as potential carriers for DC tumor vaccines has not been presented before. Effective carriers must be able to be phagocytized by DCs, present low toxicity, and induce the maturation of DCs—an early indication of an immune response. When we treated DCs with the PNVs, we found that the cell viability of DCs was unaffected, up to 200 μg/ml. Additionally, the PNVs associated with the DCs as they were phagocytized and they were found to reside within intracellular compartments such as endosomes. More importantly, the PNVs were able to induce expression of surface markers indicative of DC activation and maturation, i.e. CD40, CD86, and MHC class II. These results provide the first evidence that PNVs are promising carriers for DC-based vaccines and warrant further investigating for clinical use.

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

Foxp3-expressing regulatory T cells (Tregs) are essential regulators of immune homeostasis and, thus, are prime targets for therapeutic interventions of diseases such as cancer and autoimmunity. c-REL and IκB_NS are important regulators of Foxp3 induction in Treg precursors upon γ-chain cytokine stimulation. In c-REL/IκB_NS double-deficient mice, Treg numbers were dramatically reduced, indicating that together, c-REL and IκB_NS are pivotal for Treg development. However, despite the highly reduced Treg compartment, double-deficient mice did not develop autoimmunity even when aged to more than 1 y, suggesting that c-REL and IκB_NS are required for T cell effector function as well. Analyzing Treg development in more detail, we identified a CD122⁺ subset within the CD25^-Foxp3^- precursor population, which gave rise to classical CD25⁺Foxp3^- Treg precursors. Importantly, c-REL, but not IκB_NS, controlled the generation of classical CD25⁺Foxp3^- precursors via direct binding to the Cd25 locus. Thus, we propose that CD4⁺GITR⁺CD122⁺CD25^-Foxp3^- cells represent a Treg pre-precursor population, whose transition into Treg precursors is mediated via c-REL.

The Lysine Methyltransferase G9a in Immune Cell Differentiation and Function

G9a (KMT1C, EHMT2) is a lysine methyltransferase (KMT) whose primary function is to di-methylate lysine 9 of histone H3 (H3K9me2). G9a-dependent H3K9me2 is associated with gene silencing and acts primarily through the recruitment of H3K9me2-binding proteins that prevent transcriptional activation. Gene repression via G9a-dependent H3K9me2 is critically required in embryonic stem (ES) cells for the development of cellular lineages by repressing expression of pluripotency factors. In the immune system, lymphoid cells such as T cells and innate lymphoid cells (ILCs) can differentiate from a naïve state into one of several effector lineages that require both activating and repressive mechanisms to maintain the correct gene expression program. Furthermore, the long-term immunity to re-infection is mediated by memory T cells, which also require specific gene expression and repression to maintain a quiescent state. In this review, we examine the molecular machinery of G9a-dependent functions, address the role of G9a in lymphoid cell differentiation and function, and identify potential functions of T cells and ILCs that may be controlled by G9a. Together, this review will highlight the dynamic nature of G9a-dependent H3K9me2 in the immune system and shed light on the nature of repressive epigenetic modifications in cellular lineage choice.

The scaffolding function of the RLTPR protein explains its essential role for CD28 co-stimulation in mouse and human T cells

In two complementary papers, Casanova, Malissen, and collaborators report the discovery of human RLTPR deficiency, the first primary immunodeficiency of the human CD28 pathway in T cells. Together, the two studies elucidate the largely (but not completely) overlapping roles of RLTPR in CD28 signaling in T and B cells of humans and mice.

The RLTPR cytosolic protein, also known as CARMIL2, is essential for CD28 co-stimulation in mice, but its importance in human T cells and mode of action remain elusive. Here, using affinity purification followed by mass spectrometry analysis, we showed that RLTPR acts as a scaffold, bridging CD28 to the CARD11/CARMA1 cytosolic adaptor and to the NF-κB signaling pathway, and identified proteins not found before within the CD28 signaling pathway. We further demonstrated that RLTPR is essential for CD28 co-stimulation in human T cells and that its noncanonical pleckstrin-homology domain, leucine-rich repeat domain, and proline-rich region were mandatory for that task. Although RLTPR is thought to function as an actin-uncapping protein, this property was dispensable for CD28 co-stimulation in both mouse and human. Our findings suggest that the scaffolding role of RLTPR predominates during CD28 co-stimulation and underpins the similar function of RLTPR in human and mouse T cells. Along that line, the lack of functional RLTPR molecules impeded the differentiation toward Th1 and Th17 fates of both human and mouse CD4⁺ T cells. RLTPR was also expressed in both human and mouse B cells. In the mouse, RLTPR did not play, however, any detectable role in BCR-mediated signaling and T cell-independent B cell responses.

Ubiquitin-dependent regulation of Foxp3 and Treg function

Regulatory T (Treg) cells are crucial enforcers of immune homeostasis. Their characteristic suppressive function largely arises from an equally unique pattern of gene expression. A complex network of factors and processes contribute to this 'signature' Treg gene expression landscape. Many of these alter the level and activity of the Treg-defining transcription factor Foxp3. As stable expression of Foxp3 is important for the ability of Treg cells to successfully prevent excessive or inappropriate immune activation, uncovering the mechanisms regulating Foxp3 level is required for the understanding and therapeutic exploitation of Tregs. While transcriptional regulation of the Foxp3 gene has been studied in depth, additional regulatory layers exist controlling the expression and activity of this key transcription factor. These include less-defined mechanisms active at the post-translational level. These pathways are just beginning to be elucidated. Here, we summarize emerging evidence for distinct, post-translationally active, ubiquitin-dependent pathways capable of controlling the activation and expression of Foxp3 and the function of Tregs. These pathways offer untapped opportunities for therapeutic fine-tuning of Tregs and their all-important restraint of the immune system.

The Molecular Control of Regulatory T Cell Induction

Regulatory T cells (Tregs) are characterized by the expression of the master transcription factor forkhead box P3 (Foxp3). Although Foxp3 expression is widely used as a marker of the Treg lineage, recent data show that the Treg fate is determined by a multifactorial signaling pathway, involving cytokines, nuclear factors, and epigenetic modifications. Foxp3 expression and the Treg phenotype can be acquired by T cells in the periphery, illustrating that the Treg fate is not necessarily conferred during thymic development. The two main Treg populations in vivo, thymic Tregs and peripheral Tregs, differ in the pathways followed for their maturation. This chapter discusses the molecular control of Treg induction, in the thymus as well as the periphery.

Treg Cell Differentiation: From Thymus to Peripheral Tissue

Regulatory T cells (Tregs) are crucial mediators of self-tolerance in the periphery. They differentiate in the thymus, where interactions with thymus-resident antigen-presenting cells, an instructive cytokine milieu, and stimulation of the T cell receptor lead to the selection into the Treg lineage and the induction of Foxp3 gene expression. Once mature, Treg cells leave the thymus and migrate into either the secondary lymphoid tissues, e.g., lymph nodes and spleen, or peripheral nonlymphoid tissues. There is growing evidence that Treg cells go beyond the classical modulation of immune responses and also play important functional roles in nonlymphoid peripheral tissues. In this review, we summarize recent findings about the thymic Treg lineage differentiation as well as the further specialization of Treg cells in the secondary lymphoid and in the peripheral nonlymphoid organs.

CD28-CD80/86 and CD40-CD40L Interactions Promote Thymic Tolerance by Regulating Medullary Epithelial Cell and Thymocyte Development

Development and central tolerance of T lymphocytes in the thymus requires both TCR signals and collaboration with signals generated through costimulatory molecule interactions. In this review, we discuss the importance of CD28-CD80/86 and CD40-CD40L costimulatory interactions in promoting normal thymic development. This discussion includes roles in the generation of a normal thymic medulla, in the development of specific T-cells subsets, including iNKT and T regulatory cells, and in the generation of a tolerant mature T-cell repertoire. We discuss recent contributions to the understanding of CD28-CD80/86 and CD40-CD40L costimulatory interactions in thymic development, and we highlight the ways in which the many important roles mediated by these interactions collaborate to promote normal thymic development.

Signals and pathways controlling regulatory T cells

Induction of specific immune tolerance to grafts remains the sought-after standard following transplantation. Defined by expression of the Foxp3 (forkhead box protein 3) transcription factor, the regulatory T-cell (Treg) lineage has been noted to exert potent immunoregulatory functions that contribute to specific graft tolerance. In this review, we discuss the known signals and pathways which govern Treg development, both in the thymus and in peripheral sites, as well as lineage maintenance and homeostasis. In particular, we highlight the roles of T-cell receptor signaling, CD28 costimulation, and signals through phosphatidyl inositol 3-kinase (PI3K) and related metabolic pathways in multiple aspects of Treg biology.

Dynamic changes in E-protein activity regulate T reg cell development

Gao et al. show that E-box proteins dampen the generation and function of Foxp3⁺ regulatory T cells in part by inhibiting IL-2Rα expression and IL-2 responsiveness.

E-proteins are TCR-sensitive transcription factors essential for intrathymic T cell transitions. Here, we show that deletion of E-proteins leads to both enhanced peripheral TGF-β–induced regulatory T (iT reg) cell and thymic naturally arising T reg cell (nT reg cell) differentiation. In contrast, deletion of Id proteins results in reduced nT reg cell differentiation. Mechanistic analysis indicated that decreased E-protein activity leads to de-repression of signaling pathways that are essential to Foxp3 expression. Decreased E-protein binding to an IL-2Rα enhancer locus facilitated TCR-induced IL-2Rα expression. Similarly, decreased E-protein activity facilitated TCR-induced NF-κB activation and generation of c-Rel. Consistent with this, microarray analysis indicated that cells with E-protein depletion that are not yet expressing Foxp3 exhibit activation of the IL-2 and NF-κB signaling pathways as well as enhanced expression of many of the genes associated with Foxp3 induction. Finally, studies using Nur77-GFP mice to monitor TCR signaling showed that TCR signaling strength sufficient to induce Foxp3 differentiation is accompanied by down-regulation of E-protein levels. Collectively, these data suggest that TCR stimulation acts in part through down-regulation of E-protein activity to induce T reg cell lineage development.

Differential requirement for IL-2 and IL-15 during bifurcated development of thymic regulatory T cells

The developmental pathways of regulatory T cells (T(reg)) generation in the thymus are not fully understood. In this study, we reconstituted thymic development of Zap70-deficient thymocytes with a tetracycline-inducible Zap70 transgene to allow temporal dissection of T(reg) development. We find that T(reg) develop with distinctive kinetics, first appearing by day 4 among CD4 single-positive (SP) thymocytes. Accepted models of CD25(+)Foxp3(+) T(reg) selection suggest development via CD25(+)Foxp3(-) CD4 SP precursors. In contrast, our kinetic analysis revealed the presence of abundant CD25(-)Foxp3(+) cells that are highly efficient at maturing to CD25(+)Foxp3(+) cells in response to IL-2. CD25(-)Foxp3(+) cells more closely resembled mature T(reg) both with respect to kinetics of development and avidity for self-peptide MHC. These population also exhibited distinct requirements for cytokines during their development. CD25(-)Foxp3(+) cells were IL-15 dependent, whereas generation of CD25(+)Foxp3(+) specifically required IL-2. Finally, we found that IL-2 and IL-15 arose from distinct sources in vivo. IL-15 was of stromal origin, whereas IL-2 was of exclusively from hemopoetic cells that depended on intact CD4 lineage development but not either Ag-experienced or NKT cells.

Interleukin-2 and STAT5 in regulatory T cell development and function

Interleukin-2 and its downstream target STAT5 have effects on many aspects of immune function. This has been perhaps best documented in regulatory T cells. In this review we summarize the initial findings supporting a role for IL2 and STAT5 in regulatory T cell development and outline more recent studies describing how this critical signaling pathway entrains regulatory T cell differentiation and affects regulatory T cell function.

Id1 expression promotes T regulatory cell differentiation by facilitating TCR costimulation

T regulatory (Treg) cells play crucial roles in the regulation of cellular immunity. The development of Treg cells depends on signals from TCRs and IL-2Rs and is influenced by a variety of transcription factors. The basic helix-loop-helix proteins are known to influence TCR signaling thresholds. Whether this property impacts Treg differentiation is not understood. In this study, we interrogated the role of basic helix-loop-helix proteins in the production of Treg cells using the CD4 promoter-driven Id1 transgene. We found that Treg cells continued to accumulate as Id1 transgenic mice aged, resulting in a significant increase in Treg cell counts in the thymus as well as in the periphery compared with wild-type controls. Data from mixed bone marrow assays suggest that Id1 acts intrinsically on developing Treg cells. We made a connection between Id1 expression and CD28 costimulatory signaling because Id1 transgene expression facilitated the formation of Treg precursors in CD28(-/-) mice and the in vitro differentiation of Treg cells on thymic dendritic cells despite the blockade of costimulation by anti-CD80/CD86. Id1 expression also allowed in vitro Treg differentiation without anti-CD28 costimulation, which was at least in part due to enhanced production of IL-2. Notably, with full strength of costimulatory signals, however, Id1 expression caused modest but significant suppression of Treg induction. Finally, we demonstrate that Id1 transgenic mice were less susceptible to the induction of experimental autoimmune encephalomyelitis, thus illustrating the impact of Id1-mediated augmentation of Treg cell levels on cellular immunity.

Ubiquitous points of control over regulatory T cells

Posttranslational modification by ubiquitin tagging is crucial for regulating the stability, activity and cellular localization of many target proteins involved in processes including DNA repair, cell cycle progression, protein quality control, and signal transduction. It has long been appreciated that ubiquitin-mediated events are important for certain signaling pathways leading to leukocyte activation and the stimulation of effector function. Now it is clear that the activities of molecules and pathways central to immune regulation are also modified and controlled by ubiquitin tagging. Among the mechanisms of immune control, regulatory T cells (or Tregs) are themselves particularly sensitive to such regulation. E3 ligases and deubiquitinases both influence Tregs through their effects on the signaling pathways pertinent to these cells or through the direct, posttranslational regulation of Foxp3. In this review, we will summarize and discuss several examples of ubiquitin-mediated control over multiple aspects of Treg biology including the generation, function and phenotypic fidelity of these cells. Fully explored and exploited, these potential opportunities for Treg modulation may lead to novel immunotherapies for both positive and negative fine-tuning of immune restraint.

Costimulation via the tumor-necrosis factor receptor superfamily couples TCR signal strength to the thymic differentiation of regulatory T cells

Regulatory T (T_reg) cells express tumor necrosis factor receptor superfamily (TNFRSF) members, but their role in thymic T_reg development is undefined. We demonstrate that T_reg progenitors highly express the TNFRSF members GITR, OX40, and TNFR2. Expression of these receptors correlates directly with T cell receptor (TCR) signal strength, and requires CD28 and the kinase TAK1. Neutralizing TNFSF ligands markedly reduced T_reg development. Conversely, TNFRSF agonists enhanced T_reg differentiation by augmenting IL-2R/STAT5 responsiveness. GITR-ligand costimulation elicited a dose-dependent enrichment of lower-affinity cells within the T_reg repertoire. In vivo, combined inhibition of GITR, OX40 and TNFR2 abrogated T_reg development. Thus TNFRSF expression on T_reg progenitors translates strong TCR signals into molecular parameters that specifically promote T_reg differentiation and shape the T_reg repertoire.

Gene expression in the Gitr locus is regulated by NF-κB and Foxp3 through an enhancer

Glucocorticoid-induced TNFR (Gitr) and Ox40, two members of the TNFR superfamily, play important roles in regulating activities of effector and regulatory T cells (Treg). Their gene expression is induced by T cell activation and further upregulated in Foxp3+ Treg. Although the role of Foxp3 as a transcriptional repressor in Treg is well established, the mechanisms underlying Foxp3-mediated transcriptional upregulation remain poorly understood. This transcription factor seems to upregulate expression not only of Gitr and Ox40, but also other genes, including Ctla4, Il35, Cd25, all critical to Treg function. To investigate how Foxp3 achieves such upregulation, we analyzed its activity on Gitr and Ox40 genes located within a 15.1-kb region. We identified an enhancer located downstream of the Gitr gene, and both Gitr and Ox40 promoter activities were shown to be upregulated by the NF-κB-mediated enhancer activity. We also show, using the Gitr promoter, that the enhancer activity was further upregulated in conjunction with Foxp3. Foxp3 appears to stabilize NF-κB p50 binding by anchoring it to the enhancer, thereby enabling local accumulation of transcriptional complexes containing other members of the NF-κB and IκB families. These findings may explain how Foxp3 can activate expression of certain genes while suppressing others.