Identification of Natural Regulatory T Cell Epitopes Reveals Convergence on a Dominant Autoantigen

The partitioning of TCR repertoires by thymic selection

αβ T cells are critical components of the adaptive immune system; they maintain tissue and immune homeostasis during health, provide sterilizing immunity after pathogen infection, and are capable of eliminating transformed tumor cells. Fundamental to these distinct functions is the ligand specificity of the unique antigen receptor expressed on each mature T cell (TCR), which endows lymphocytes with the ability to behave in a cell-autonomous, disease context-specific manner. Clone-specific behavioral properties are initially established during T cell development when thymocytes use TCR recognition of major histocompatibility complex (MHC) and MHC-like ligands to instruct survival versus death and to differentiate into a plethora of inflammatory and regulatory T cell lineages. Here, we review the ligand specificity of the preselection thymocyte repertoire and argue that developmental stage-specific alterations in TCR signaling control cross-reactivity and foreign versus self-specificity of T cell sublineages.

The Expression of Forkhead Box P3 T Regulatory Lymphocytes as a Prognostic Factor in Malignant Melanomas

Since transcription factor Forkhead Box P3 (FoxP3) was identified as a specific regulatory T cell (Treg) marker, researchers have scrutinized its value as a potential novel therapeutic target or a prognostic factor in various types of cancer with inconsistent results. The present analysis was performed to assess the influence of Treg FoxP3 expression on the prognosis of primary melanoma and to evaluate the correlations with various clinicopathological prognostic factors. We analyzed all eligible patients with stage pT3 primary malignant melanomas treated in a tertiary cancer center. Immunohistochemical staining for Treg FoxP3 expression was performed on retrospectively identified paraffin blocks and subsequently correlated with the outcomes of the patients. A total of 81% of the patients presented a positive Treg FoxP3 expression, being correlated with a higher risk of lymph node metastasis, tumor relapse, and death. Moreover, positive expression was statistically associated with a shorter OS. The tumor relapse rate was estimated at 36.7%. A positive expression of Treg FoxP3 and lymph node metastasis were associated with a higher risk of death based on multivariate analysis. Treg FoxP3 expression may be used as an independent prognostic factor in patients with malignant melanoma to evaluate tumor progression and survival.

Exploring Chronic Hypocalcemia: Insights into Autoimmune Polyglandular Syndrome Type 1-A Case Study and Literature Review

Hypocalcemia is a common occurrence in pediatric patients, attributed to various causes and presenting with diverse clinical manifestations. A prompt evaluation is necessary to determine its underlying cause, whether it presents acutely or chronically, and to tailor treatment based on its severity. Among the potential causes of chronic hypocalcemia, primary hypoparathyroidism stands out. The case of a seven-year-old male patient with hypocalcemia reported in this article serves as an illustration, wherein targeted next-generation sequencing revealed a homozygous p.R257X mutation in the AIRE gene, indicative of autoimmune polyendocrine syndrome type 1 (APS-1). It poses challenges due to its multisystemic nature and involvement of specific autoantibodies, often leading to underdiagnosis, owing to its rarity, varied manifestations, and incomplete penetrance. A comprehensive review of the APS-1 literature was conducted to provide insights into the clinical manifestations, genetic spectrum, potential immunological mechanisms, and current medical strategies. Additionally, the recognition of AIRE gene mutations is crucial for facilitating genetic diagnosis, prognosis, and potential treatment strategies for APS-1. The management of such cases involves individualized approaches to treatment, regular monitoring, medication adjustments, and the early identification of associated conditions.

Tracking the role of Aire in immune tolerance to the eye with a TCR transgenic mouse model

Roughly one-half of mice with partial defects in two immune tolerance pathways (AireGW/+Lyn-/- mice) spontaneously develop severe damage to their retinas due to T cell reactivity to Aire-regulated interphotoreceptor retinoid-binding protein (IRBP). Single-cell T cell receptor (TCR) sequencing of CD4+ T cells specific for a predominate epitope of IRBP showed a remarkable diversity of autoantigen-specific TCRs with greater clonal expansions in mice with disease. TCR transgenic mice made with an expanded IRBP-specific TCR (P2.U2) of intermediate affinity exhibited strong but incomplete negative selection of thymocytes. This negative selection was absent in IRBP-/- mice and greatly defective in AireGW/+ mice. Most P2.U2+/- mice and all P2.U.2+/-AireGW/+ mice rapidly developed inflammation of the retina and adjacent uvea (uveitis). Aire-dependent IRBP expression in the thymus also promoted Treg differentiation, but the niche for this fate determination was small, suggesting differences in antigen presentation leading to negative selection vs. thymic Treg differentiation and a stronger role for negative selection in preventing autoimmune disease in the retina.

Lung tumor-infiltrating Treg have divergent transcriptional profiles and function linked to checkpoint blockade response

Regulatory T cells (Treg) are conventionally viewed as suppressors of endogenous and therapy-induced antitumor immunity; however, their role in modulating responses to immune checkpoint blockade (ICB) is unclear. In this study, we integrated single-cell RNA-seq/T cell receptor sequencing (TCRseq) of >73,000 tumor-infiltrating Treg (TIL-Treg) from anti-PD-1-treated and treatment-naive non-small cell lung cancers (NSCLC) with single-cell analysis of tumor-associated antigen (TAA)-specific Treg derived from a murine tumor model. We identified 10 subsets of human TIL-Treg, most of which have high concordance with murine TIL-Treg subsets. Only one subset selectively expresses high levels of TNFRSF4 (OX40) and TNFRSF18 (GITR), whose engangement by cognate ligand mediated proliferative programs and NF-κB activation, as well as multiple genes involved in Treg suppression, including LAG3. Functionally, the OX40hiGITRhi subset is the most highly suppressive ex vivo, and its higher representation among total TIL-Treg correlated with resistance to PD-1 blockade. Unexpectedly, in the murine tumor model, we found that virtually all TIL-Treg-expressing T cell receptors that are specific for TAA fully develop a distinct TH1-like signature over a 2-week period after entry into the tumor, down-regulating FoxP3 and up-regulating expression of TBX21 (Tbet), IFNG, and certain proinflammatory granzymes. Transfer learning of a gene score from the murine TAA-specific TH1-like Treg subset to the human single-cell dataset revealed a highly analogous subcluster that was enriched in anti-PD-1-responding tumors. These findings demonstrate that TIL-Treg partition into multiple distinct transcriptionally defined subsets with potentially opposing effects on ICB-induced antitumor immunity and suggest that TAA-specific TIL-Treg may positively contribute to antitumor responses.

A robust deep learning workflow to predict CD8 + T-cell epitopes

BACKGROUND

T-cells play a crucial role in the adaptive immune system by triggering responses against cancer cells and pathogens, while maintaining tolerance against self-antigens, which has sparked interest in the development of various T-cell-focused immunotherapies. However, the identification of antigens recognised by T-cells is low-throughput and laborious. To overcome some of these limitations, computational methods for predicting CD8 + T-cell epitopes have emerged. Despite recent developments, most immunogenicity algorithms struggle to learn features of peptide immunogenicity from small datasets, suffer from HLA bias and are unable to reliably predict pathology-specific CD8 + T-cell epitopes.

METHODS

We developed TRAP (T-cell recognition potential of HLA-I presented peptides), a robust deep learning workflow for predicting CD8 + T-cell epitopes from MHC-I presented pathogenic and self-peptides. TRAP uses transfer learning, deep learning architecture and MHC binding information to make context-specific predictions of CD8 + T-cell epitopes. TRAP also detects low-confidence predictions for peptides that differ significantly from those in the training datasets to abstain from making incorrect predictions. To estimate the immunogenicity of pathogenic peptides with low-confidence predictions, we further developed a novel metric, RSAT (relative similarity to autoantigens and tumour-associated antigens), as a complementary to 'dissimilarity to self' from cancer studies.

RESULTS

TRAP was used to identify epitopes from glioblastoma patients as well as SARS-CoV-2 peptides, and it outperformed other algorithms in both cancer and pathogenic settings. TRAP was especially effective at extracting immunogenicity-associated properties from restricted data of emerging pathogens and translating them onto related species, as well as minimising the loss of likely epitopes in imbalanced datasets. We also demonstrated that the novel metric termed RSAT was able to estimate immunogenic of pathogenic peptides of various lengths and species. TRAP implementation is available at: https://github.com/ChloeHJ/TRAP .

CONCLUSIONS

This study presents a novel computational workflow for accurately predicting CD8 + T-cell epitopes to foster a better understanding of antigen-specific T-cell response and the development of effective clinical therapeutics.

Germinal center B cells that acquire nuclear proteins are specifically suppressed by follicular regulatory T cells

Follicular regulatory T cells (Tfr) restrict development of autoantibodies and autoimmunity while supporting high-affinity foreign antigen-specific humoral response. However, whether Tfr can directly repress germinal center (GC) B cells that acquire autoantigens is unclear. Moreover, TCR specificity of Tfr to self-antigens is not known. Our study suggests that nuclear proteins contain antigens specific to Tfr. Targeting of these proteins to antigen-specific B cells in mice triggers rapid accumulation of Tfr with immunosuppressive characteristics. Tfr then exert negative regulation of GC B cells with predominant inhibition of the nuclear protein-acquiring GC B cells, suggesting an important role of direct cognate Tfr-GC B cells interactions for the control of effector B cell response.

The endogenous repertoire harbors self-reactive CD4+ T cell clones that adopt a follicular helper T cell-like phenotype at steady state

The T cell repertoire of healthy mice and humans harbors self-reactive CD4+ conventional T (Tconv) cells capable of inducing autoimmunity. Using T cell receptor profiling paired with in vivo clonal analysis of T cell differentiation, we identified Tconv cell clones that are recurrently enriched in non-lymphoid organs following ablation of Foxp3+ regulatory T (Treg) cells. A subset of these clones was highly proliferative in the lymphoid organs at steady state and exhibited overt reactivity to self-ligands displayed by dendritic cells, yet were not purged by clonal deletion. These clones spontaneously adopted numerous hallmarks of follicular helper T (TFH) cells, including expression of Bcl6 and PD-1, exhibited an elevated propensity to localize within B cell follicles at steady state, and produced interferon-γ in non-lymphoid organs following sustained Treg cell depletion. Our work identifies a naturally occurring population of self-reactive TFH-like cells and delineates a previously unappreciated fate for self-specific Tconv cells.

Human FOXP3 and tumour microenvironment

The tumour microenvironment (TME) is a complex system composed of cancer cells, stromal cells and immune cells. Regulatory T cells (Tregs) in the TME impede immune surveillance of tumours and suppress antitumor immune responses. Transcription factor forkhead box protein 3 (FOXP3) is the main marker of Tregs, which dominates the function of Tregs. FOXP3 was originally thought to be a Tregs-specific expression molecule, and recent studies have found that FOXP3 is expressed in a variety of tumours with inconsistent functional roles. This review summarizes the recent progress of infiltrating Treg-FOXP3 and tumour-FOXP3 in TME, discusses the communication mechanism between FOXP3⁺ cells and effector T cells in TME, the relationship between FOXP3 and clinical prognosis, and the potential of FOXP3-targeted therapy.

Cytokine and metabolic regulation of adipose tissue Tregs

Since their discovery over a decade ago, much has been learned regarding the importance and function of visceral adipose tissue (VAT)-resident regulatory T cells (Tregs). VAT Tregs play a critical role in controlling VAT inflammation and alleviating metabolic disease. However, this population is disrupted in obesity which exacerbates VAT inflammation and metabolic abnormalities. Therefore, understanding the factors governing the accumulation and maintenance of VAT Tregs, both at steady state and under disease conditions, is crucial for identifying the mechanisms underlying obesity-associated metabolic disease and developing novel therapies. Expansion and maintenance of the VAT Treg compartment is strongly influenced by factors in the local tissue microenvironment, including cytokines, T-cell receptor ligands, hormones, and various metabolites. This mini-review will primarily focus on recent advances in our understandings regarding the regulation of mouse epididymal VAT (eVAT) Tregs, which are the most thoroughly characterized VAT Treg population, by tissue microenvironmental factors and cellular metabolic processes. We will also briefly discuss the limited knowledge available regarding the regulation of mouse ovarian VAT (oVAT) Tregs and human omental VAT Tregs, highlight some lingering questions, and provide a prospective view on where the field is heading.

Thymic self-antigen expression for immune tolerance and surveillance

T cells are a group of lymphocytes that play a central role in the immune system, notably, eliminating pathogens and attacking cancer while being tolerant of the self. Elucidating how immune tolerance is ensured has become a significant research issue for understanding the pathogenesis of autoimmune diseases as well as cancer immunity. T cell immune tolerance is established mainly in the thymic medulla by the removal of self-responsive T cells and the generation of regulatory T cells, this process depends mainly on the expression of a variety of tissue restricted antigens (TRAs) by medullary thymic epithelial cells (mTECs). The expression of TRAs is known to be regulated by at least two independent factors, Fezf2 and Aire, which play non-redundant and complementary roles by different mechanisms. In this review, we introduce the molecular logic of thymic self-antigen expression that underlies T cell selection for the prevention of autoimmunity and the establishment of immune surveillance.

How Thymocyte Deletion in the Cortex May Curtail Antigen-Specific T-Regulatory Cell Development in the Medulla

CD4⁺ T cell responses to self-antigens are pivotal for immunological self-tolerance. Activation of Foxp3^– T-conventional (T-conv) cells can precipitate autoimmune disease, whereas activation of Foxp3⁺ T-regulatory (T-reg) cells is essential to prevent autoimmune disease. This distinction indicates the importance of the thymus in controlling the differentiation of self-reactive CD4⁺ T cells. Thymocytes and thymic antigen-presenting cells (APC) depend on each other for normal maturation and differentiation. In this Hypothesis and Theory article, we propose this mutual dependence dictates which self-antigens induce T-reg cell development in the thymic medulla. We postulate self-reactive CD4⁺ CD8^– thymocytes deliver signals that stabilize and amplify the presentation of their cognate self-antigen by APC in the thymic medulla, thereby seeding a niche for the development of T-reg cells specific for the same self-antigen. By limiting the number of antigen-specific CD4⁺ thymocytes in the medulla, thymocyte deletion in the cortex may impede the formation of medullary T-reg niches containing certain self-antigens. Susceptibility to autoimmune disease may arise from cortical deletion creating a “hole” in the self-antigen repertoire recognized by T-reg cells.

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.

Exposed and Sequestered Antigens in Testes and Their Protection by Regulatory T Cell-Dependent Systemic Tolerance

Continuous exposure of tissue antigen (Ag) to the autoantigen-specific regulatory T cells (Treg) is required to maintain Treg-dependent systemic tolerance. Thus, testis autoantigens, previously considered as sequestered, may not be protected by systemic tolerance. We now document that the complete testis antigen sequestration is not valid. The haploid sperm Ag lactate dehydrogenase 3 (LDH3) is continuously exposed and not sequestered. It enters the residual body (RB) to egress from the seminiferous tubules and interact with circulating antibody (Ab). Some LDH3 also remains inside the sperm cytoplasmic droplets (CD). Treg-depletion in the DEREG mice that express diphtheria toxin receptor on the Foxp3 promoter results in spontaneous experimental autoimmune orchitis (EAO) and Ab to LDH3. Unlike the wild-type male mice, mice deficient in LDH3 (wild-type female or LDH3 NULL males) respond vigorously to LDH3 immunization. However, partial Treg depletion elevated the wild-type male LDH3 responses to the level of normal females. In contrast to LDH3, zonadhesin (ZAN) in the sperm acrosome displays properties of a sequestered Ag. However, when ZAN and other sperm Ag are exposed by vasectomy, they rapidly induce testis Ag-specific tolerance, which is terminated by partial Treg-depletion, leading to bilateral EAO and ZAN Ab response. We conclude that some testis/sperm Ag are normally exposed because of the unique testicular anatomy and physiology. The exposed Ag: 1) maintain normal Treg-dependent systemic tolerance, and 2) are pathogenic and serve as target Ag to initiate EAO. Unexpectedly, the sequestered Ags, normally non-tolerogenic, can orchestrate de novo Treg-dependent, systemic tolerance when exposed in vasectomy.

CCR8-targeted specific depletion of clonally expanded Treg cells in tumor tissues evokes potent tumor immunity with long-lasting memory

Immunosuppressive Foxp3-expressing regulatory T cells (Tregs) in tumor tissues are assumed to be clonally expanding via recognizing tumor-associated antigens. By single-cell RNA sequencing, we have searched for the molecules that are specifically expressed by such multiclonal tumor Tregs, but not by tumor-infiltrating effector T cells or natural Tregs in other tissues. The search revealed the chemokine receptor CCR8 as a candidate. Treatment of tumor-bearing mice with cell-depleting anti-CCR8 antibody indeed selectively removed multiclonal tumor Tregs without affecting effector T cells or tissue Tregs, eradicating established tumors with induction of potent tumor-specific effector/memory T cells and without activating autoimmune T cells. Thus, specific depletion of clonally expanding tumor Tregs is clinically instrumental for evoking effective tumor immunity without autoimmune adverse effects.

Foxp3-expressing CD25⁺CD4⁺ regulatory T cells (Tregs) are abundant in tumor tissues. Here, hypothesizing that tumor Tregs would clonally expand after they are activated by tumor-associated antigens to suppress antitumor immune responses, we performed single-cell analysis on tumor Tregs to characterize them by T cell receptor clonotype and gene-expression profiles. We found that multiclonal Tregs present in tumor tissues predominantly expressed the chemokine receptor CCR8. In mice and humans, CCR8⁺ Tregs constituted 30 to 80% of tumor Tregs in various cancers and less than 10% of Tregs in other tissues, whereas most tumor-infiltrating conventional T cells (Tconvs) were CCR8^–. CCR8⁺ tumor Tregs were highly differentiated and functionally stable. Administration of cell-depleting anti-CCR8 monoclonal antibodies (mAbs) indeed selectively eliminated multiclonal tumor Tregs, leading to cure of established tumors in mice. The treatment resulted in the expansion of CD8⁺ effector Tconvs, including tumor antigen-specific ones, that were more activated and less exhausted than those induced by PD-1 immune checkpoint blockade. Anti-CCR8 mAb treatment also evoked strong secondary immune responses against the same tumor cell line inoculated several months after tumor eradication, indicating that elimination of tumor-reactive multiclonal Tregs was sufficient to induce memory-type tumor-specific effector Tconvs. Despite induction of such potent tumor immunity, anti-CCR8 mAb treatment elicited minimal autoimmunity in mice, contrasting with systemic Treg depletion, which eradicated tumors but induced severe autoimmune disease. Thus, specific removal of clonally expanding Tregs in tumor tissues for a limited period by cell-depleting anti-CCR8 mAb treatment can generate potent tumor immunity with long-lasting memory and without deleterious autoimmunity.

Lack of CD8+ T cell effector differentiation during priming mediates checkpoint blockade resistance in non-small cell lung cancer

In non–small cell lung cancer (NSCLC), response to immune checkpoint blockade (ICB) is associated with programmed cell death ligand 1 expression that is induced by interferon-γ–producing, tumor-infiltrating CD8+ T cells. However, not all tumors with a CD8+ T cell infiltrate respond to ICB, and little is known about the mechanisms governing ICB resistance in T cell–infiltrated NSCLC. We used an orthotopic NSCLC mouse model to study ICB-refractory CD8+ T cell responses. Single-cell RNA sequencing of the NSCLC mouse tumors revealed that lung cancer–specific tumor-infiltrating CD8+ T cells exhibited clonal expansion but lacked expression of genes associated with effector and exhausted T cell responses, indicating that they underwent a differentiation program distinct from conventional T cell exhaustion. This lung cancer–specific T cell dysfunction program was established early during priming in the mediastinal lymph node and was characterized by robust proliferation but a failed up-regulation of effector and exhausted T cell characteristics. Intriguingly, CD8+ T cells from patients with NSCLC expressed an analogous gene expression program, which appeared distinct from conventional T cell exhaustion. Administration of recombinant interleukin-2 (IL-2) and IL-12 was sufficient to restore effector T cell differentiation and induce control of KP lung tumors. These findings imply that a CD8+ T cell differentiation trajectory, activated during T cell priming in the mediastinal lymph node, limits the response of CD8+ T cells to ICB and thereby may contribute to failure of ICB in a subset T cell–infiltrated NSCLC.

The Multifaceted Role of Regulatory T Cells in Breast Cancer

The microenvironment of breast cancer hosts a dynamic cross talk between diverse players of the immune system. While cytotoxic immune cells are equipped to control tumor growth and metastasis, tumor-corrupted immunosuppressive immune cells strive to impair effective immunity and promote tumor progression. Of these, regulatory T cells (T_regs), the gatekeepers of immune homeostasis, emerge as multifaceted players involved in breast cancer. Intriguingly, clinical observations suggest that blood and intratumoral T_regs can have strong prognostic value, dictated by breast cancer subtype. Accordingly, emerging preclinical evidence shows that T_regs occupy a central role in breast cancer initiation and progression and provide critical support to metastasis formation. Here, T_regs are not only important for immune escape but also promote tumor progression independent of their immune regulatory capacity. Combining insights into T_reg biology with advances made across the rapidly growing field of immuno-oncology is expected to set the stage for the design of more effective immunotherapy strategies.

Effector T cell responses unleashed by regulatory T cell ablation exacerbate oral squamous cell carcinoma

Immune suppression by CD4+FOXP3+ regulatory T (Treg) cells and tumor infiltration by CD8+ effector T cells represent two major factors impacting response to cancer immunotherapy. Using deconvolution-based transcriptional profiling of human papilloma virus (HPV)-negative oral squamous cell carcinomas (OSCCs) and other solid cancers, we demonstrate that the density of Treg cells does not correlate with that of CD8+ T cells in many tumors, revealing polarized clusters enriched for either CD8+ T cells or CD4+ Treg and conventional T cells. In a mouse model of carcinogen-induced OSCC characterized by CD4+ T cell enrichment, late-stage Treg cell ablation triggers increased densities of both CD4+ and CD8+ effector T cells within oral lesions. Notably, this intervention does not induce tumor regression but instead induces rapid emergence of invasive OSCCs via an effector T cell-dependent process. Thus, induction of a T cell-inflamed phenotype via therapeutic manipulation of Treg cells may trigger unexpected tumor-promoting effects in OSCC.

Nfkb2 variants reveal a p100-degradation threshold that defines autoimmune susceptibility

NF-κB2/p100 (p100) is an inhibitor of κB (IκB) protein that is partially degraded to produce the NF-κB2/p52 (p52) transcription factor. Heterozygous NFKB2 mutations cause a human syndrome of immunodeficiency and autoimmunity, but whether autoimmunity arises from insufficiency of p52 or IκB function of mutated p100 is unclear. Here, we studied mice bearing mutations in the p100 degron, a domain that harbors most of the clinically recognized mutations and is required for signal-dependent p100 degradation. Distinct mutations caused graded increases in p100-degradation resistance. Severe p100-degradation resistance, due to inheritance of one highly degradation-resistant allele or two subclinical alleles, caused thymic medullary hypoplasia and autoimmune disease, whereas the absence of p100 and p52 did not. We inferred a similar mechanism occurs in humans, as the T cell receptor repertoires of affected humans and mice contained a hydrophobic signature of increased self-reactivity. Autoimmunity in autosomal dominant NFKB2 syndrome arises largely from defects in nonhematopoietic cells caused by the IκB function of degradation-resistant p100.

Tissue regulatory T cells: regulatory chameleons

The FOXP3⁺CD4⁺ regulatory T (T_reg) cells located in non-lymphoid tissues differ in phenotype and function from their lymphoid organ counterparts. Tissue T_reg cells have distinct transcriptomes, T cell receptor repertoires and growth and survival factor dependencies that arm them to survive and operate in their home tissue. Their functions extend beyond immune surveillance to tissue homeostasis, including regulation of local and systemic metabolism, promotion of tissue repair and regeneration, and control of the proliferation, differentiation and fate of non-lymphoid cell progenitors. T_reg cells in diverse tissues share a common FOXP3⁺CD4⁺ precursor located within lymphoid organs. This precursor undergoes definitive specialization once in the home tissue, following a multilayered array of common and tissue-distinct transcriptional programmes. Our deepening knowledge of tissue T_reg cell biology will inform ongoing attempts to harness T_reg cells for precision immunotherapeutics.

Expansion of tumor-associated Treg cells upon disruption of a CTLA-4-dependent feedback loop

Foxp3+ T regulatory (Treg) cells promote immunological tumor tolerance, but how their immune-suppressive function is regulated in the tumor microenvironment (TME) remains unknown. Here, we used intravital microscopy to characterize the cellular interactions that provide tumor-infiltrating Treg cells with critical activation signals. We found that the polyclonal Treg cell repertoire is pre-enriched to recognize antigens presented by tumor-associated conventional dendritic cells (cDCs). Unstable cDC contacts sufficed to sustain Treg cell function, whereas T helper cells were activated during stable interactions. Contact instability resulted from CTLA-4-dependent downregulation of co-stimulatory B7-family proteins on cDCs, mediated by Treg cells themselves. CTLA-4-blockade triggered CD28-dependent Treg cell hyper-proliferation in the TME, and concomitant Treg cell inactivation was required to achieve tumor rejection. Therefore, Treg cells self-regulate through a CTLA-4- and CD28-dependent feedback loop that adjusts their population size to the amount of local co-stimulation. Its disruption through CTLA-4-blockade may off-set therapeutic benefits in cancer patients.

Altered selection on a single self-ligand promotes susceptibility to organ-specific T cell infiltration

For the large array of self-peptide/MHC class II (pMHC-II) complexes displayed in the body, it is unclear whether CD4⁺ T cell tolerance must be imparted for each individual complex or whether pMHC-II–nonspecific bystander mechanisms are sufficient to confer tolerance by acting broadly on T cells reactive to multiple self-pMHC-II ligands. Here, via reconstitution of T cell–deficient mice, we demonstrate that altered T cell selection on a single prostate-specific self-pMHC-II ligand renders recipient mice susceptible to prostate-specific T cell infiltration. Mechanistically, this self-pMHC-II complex is required for directing antigen-specific cells into the Foxp3⁺ regulatory T cell lineage but does not induce clonal deletion to a measurable extent. Thus, our data demonstrate that polyclonal T reg cells are unable to functionally compensate for a breach in tolerance to a single self-pMHC-II complex in this setting, revealing vulnerabilities in antigen-nonspecific bystander mechanisms of immune tolerance.

Single-Cell Sequencing Reveals the Transcriptome and TCR Characteristics of pTregs and in vitro Expanded iTregs

Regulatory T cells (Tregs) play a critical role in the maintenance of immune tolerance and tumor evasion. However, the relative low proportion of these cells in peripheral blood and tissues has hindered many studies. We sought to establish a rapamycin-based in vitro Treg expansion procedure in patients diagnosed with colorectal cancer and perform single-cell sequencing to explore the characteristics of Treg cells. CD25+ cells enriched from peripheral blood mononuclear cells (PBMC) of colorectal tumor patients were cultured in X-VIVO15 medium, supplemented with 5% human AB serum, L-glutamine, rapamycin, interleukin-2 (IL-2), and Dynabeads human Treg expander for 21 days to expand Tregs. Treg cells with satisfactory phenotype and function were successfully expanded from CD4+CD25+ cells in patients with colorectal cancer. The median expansion fold was 75 (range, 20-105-fold), and >90.0% of the harvest cells were CD4+CD25+CD127^dim/- cells. The ratio of CD4+CD25+Foxp3+ cells exceeded 60%. Functional assays showed that iTregs significantly inhibited CD8+T cell proliferation in vitro. Single-cell sequencing showed that the transcriptome of pTreg (CD4+CD25+CD127^dim/- cells isolated from PBMC of colorectal cancer patients) and iTreg (CD4+CD25+CD127^dim/- cells expanded in vitro according to the above regimen) cells were interlaced. pTregs exhibited enhanced suppressive function, whereas iTregs exhibited increased proliferative capacity. TCR repertoire analysis indicated minimal overlap between pTregs and iTregs. Pseudo-time trajectory analysis of Tregs revealed that pTregs were a continuum composed of three main branches: activated/effector, resting and proliferative Tregs. In contrast, in vitro expanded iTregs were a mixture of proliferating and activated/effector cells. The expression of trafficking receptors was also different in pTregs and iTregs. Various chemokine receptors were upregulated in pTregs. Activated effector pTregs overexpressed the chemokine receptor CCR10, which was not expressed in iTregs. The chemokine CCL28 was overexpressed in colorectal cancer and associated with poor prognosis. CCR10 interacted with CCL28 to mediate the recruitment of Treg into tumors and accelerated tumor progression. Depletion of CCR10+Treg cells from tumor microenvironment (TME) could be used as an effective treatment strategy for colorectal cancer patients. Our data distinguished the transcriptomic characteristics of different subsets of Treg cells and revealed the context-dependent functions of different populations of Treg cells, which was crucial to the development of alternative therapeutic strategies for Treg cells in autoimmune disease and cancer.

Thymus and autoimmunity

The thymus prevents autoimmune diseases through mechanisms that operate in the cortex and medulla, comprising positive and negative selection and the generation of regulatory T-cells (Tregs). Egress from the thymus through the perivascular space (PVS) to the blood is another possible checkpoint, as shown by some autoimmune/immunodeficiency syndromes. In polygenic autoimmune diseases, subtle thymic dysfunctions may compound genetic, hormonal and environmental cues. Here, we cover (a) tolerance-inducing cell types, whether thymic epithelial or tuft cells, or dendritic, B- or thymic myoid cells; (b) tolerance-inducing mechanisms and their failure in relation to thymic anatomic compartments, and with special emphasis on human monogenic and polygenic autoimmune diseases and the related thymic pathologies, if known; (c) polymorphisms and mutations of tolerance-related genes with an impact on positive selection (e.g. the gene encoding the thymoproteasome-specific subunit, PSMB11), promiscuous gene expression (e.g. AIRE, PRKDC, FEZF2, CHD4), Treg development (e.g. SATB1, FOXP3), T-cell migration (e.g. TAGAP) and egress from the thymus (e.g. MTS1, CORO1A); (d) myasthenia gravis as the prototypic outcome of an inflamed or disordered neoplastic ‘sick thymus’.

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.

Discovery of surrogate agonists for visceral fat Treg cells that modulate metabolic indices in vivo

T regulatory (Treg) cells play vital roles in modulating immunity and tissue homeostasis. Their actions depend on TCR recognition of peptide-MHC molecules; yet the degree of peptide specificity of Treg-cell function, and whether Treg ligands can be used to manipulate Treg cell biology are unknown. Here, we developed an Ab-peptide library that enabled unbiased screening of peptides recognized by a bona fide murine Treg cell clone isolated from the visceral adipose tissue (VAT), and identified surrogate agonist peptides, with differing affinities and signaling potencies. The VAT-Treg cells expanded in vivo by one of the surrogate agonists preserved the typical VAT-Treg transcriptional programs. Immunization with this surrogate, especially when coupled with blockade of TNFα signaling, expanded VAT-Treg cells, resulting in protection from inflammation and improved metabolic indices, including promotion of insulin sensitivity. These studies suggest that antigen-specific targeting of VAT-localized Treg cells could eventually be a strategy for improving metabolic disease.

Regulatory T cells in tumor microenvironment: new mechanisms, potential therapeutic strategies and future prospects

Regulatory T cells (Tregs) characterized by the expression of the master transcription factor forkhead box protein p3 (Foxp3) suppress anticancer immunity, thereby hindering protective immunosurveillance of tumours and hampering effective antitumour immune responses in tumour-bearing hosts, constitute a current research hotspot in the field. However, Tregs are also essential for the maintenance of the immune tolerance of the body and share many molecular signalling pathways with conventional T cells, including cytotoxic T cells, the primary mediators of tumour immunity. Hence, the inability to specifically target and neutralize Tregs in the tumour microenvironment without globally compromising self-tolerance poses a significant challenge. Here, we review recent advances in characterizing tumour-infiltrating Tregs with a focus on the functional roles of costimulatory and inhibitory receptors in Tregs, evaluate their potential as clinical targets, and systematically summarize their roles in potential treatment strategies. Also, we propose modalities to integrate our increasing knowledge on Tregs phenotype and function for the rational design of checkpoint inhibitor-based combination therapies. Finally, we propose possible treatment strategies that can be used to develop Treg-targeted therapies.

Identification of novel, clinically correlated autoantigens in the monogenic autoimmune syndrome APS1 by proteome-wide PhIP-Seq

The identification of autoantigens remains a critical challenge for understanding and treating autoimmune diseases. Autoimmune polyendocrine syndrome type 1 (APS1), a rare monogenic form of autoimmunity, presents as widespread autoimmunity with T and B cell responses to multiple organs. Importantly, autoantibody discovery in APS1 can illuminate fundamental disease pathogenesis, and many of the antigens found in APS1 extend to more common autoimmune diseases. Here, we performed proteome-wide programmable phage-display (PhIP-Seq) on sera from a cohort of people with APS1 and discovered multiple common antibody targets. These novel APS1 autoantigens exhibit tissue-restricted expression, including expression in enteroendocrine cells, pineal gland, and dental enamel. Using detailed clinical phenotyping, we find novel associations between autoantibodies and organ-restricted autoimmunity, including a link between anti-KHDC3L autoantibodies and premature ovarian insufficiency, and between anti-RFX6 autoantibodies and diarrheal-type intestinal dysfunction. Our study highlights the utility of PhIP-Seq for extensively interrogating antigenic repertoires in human autoimmunity and the importance of antigen discovery for improved understanding of disease mechanisms.

Infection History Determines Susceptibility to Unrelated Diseases

Epidemiological data suggest that previous infections can alter an individual's susceptibility to unrelated diseases. Nevertheless, the underlying mechanisms are not completely understood. Substantial research efforts have expanded the classical concept of immune memory to also include long-lasting changes in innate immunity and antigen-independent reactivation of adaptive immunity. Collectively, these processes provide possible explanations on how acute infections might induce long-term changes that also affect immunity to unrelated diseases. Here, we review lasting changes the immune compartment undergoes upon infection and how infection experience alters the responsiveness of immune cells towards universal signals. This heightened state of alert enhances the ability of the immune system to combat even unrelated infections but may also increase susceptibility to autoimmunity. At the same time, infection-induced changes in the regulatory compartment may dampen subsequent immune responses and promote pathogen persistence. The concepts presented here outline how infection-induced changes in the immune system may affect human health.

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.

Guidance factors orchestrating regulatory T cell positioning in tissues during development, homeostasis, and response

Over their lifetime, regulatory T cells (Treg) recalibrate their expression of trafficking receptors multiple times as they progress through development, respond to immune challenges, or adapt to the requirements of functioning in various non-lymphoid tissue environments. These trafficking receptors, which include chemokine receptors and other G-protein coupled receptors, integrins, as well as selectins and their ligands, enable Treg not only to enter appropriate tissues from the bloodstream via post-capillary venules, but also to navigate these tissues to locally execute their immune-regulatory functions, and finally to seek out the right antigen-presenting cells and interact with these, in part in order to receive the signals that sustain their survival, proliferation, and functional activity, in part in order to execute their immuno-regulatory function by altering antigen presenting cell function. Here, we will review our current knowledge of when and in what ways Treg alter their trafficking properties. We will focus on the chemokine system and try to identify specialized, non-redundant roles of individual receptors as well as similarities and differences to the conventional T cell compartment.

Trial watch: chemotherapy-induced immunogenic cell death in immuno-oncology

The term ‘immunogenic cell death’ (ICD) denotes an immunologically unique type of regulated cell death that enables, rather than suppresses, T cell-driven immune responses that are specific for antigens derived from the dying cells. The ability of ICD to elicit adaptive immunity heavily relies on the immunogenicity of dying cells, implying that such cells must encode and present antigens not covered by central tolerance (antigenicity), and deliver immunostimulatory molecules such as damage-associated molecular patterns and cytokines (adjuvanticity). Moreover, the host immune system must be equipped to detect the antigenicity and adjuvanticity of dying cells. As cancer (but not normal) cells express several antigens not covered by central tolerance, they can be driven into ICD by some therapeutic agents, including (but not limited to) chemotherapeutics of the anthracycline family, oxaliplatin and bortezomib, as well as radiation therapy. In this Trial Watch, we describe current trends in the preclinical and clinical development of ICD-eliciting chemotherapy as partner for immunotherapy, with a focus on trials assessing efficacy in the context of immunomonitoring.

Next-generation regulatory T cell therapy

Regulatory T cells (Treg cells) are a small subset of immune cells that are dedicated to curbing excessive immune activation and maintaining immune homeostasis. Accordingly, deficiencies in Treg cell development or function result in uncontrolled immune responses and tissue destruction and can lead to inflammatory disorders such as graft-versus-host disease, transplant rejection and autoimmune diseases. As Treg cells deploy more than a dozen molecular mechanisms to suppress immune responses, they have potential as multifaceted adaptable smart therapeutics for treating inflammatory disorders. Indeed, early-phase clinical trials of Treg cell therapy have shown feasibility, tolerability and potential efficacy in these disease settings. In the meantime, progress in the development of chimeric antigen receptors and in genome editing (including the application of CRISPR-Cas9) over the past two decades has facilitated the genetic optimization of primary T cell therapy for cancer. These technologies are now being used to enhance the specificity and functionality of Treg cells. In this Review, we describe the key advances and prospects in designing and implementing Treg cell-based therapy in autoimmunity and transplantation.

Tissue Site and the Cancer Immunity Cycle

Checkpoint blockade immunotherapy (CBT) has revolutionized cancer treatment; however, the cellular and molecular factors that govern responsiveness to immunotherapy remain poorly understood. One emerging area of clinical importance is differential responsiveness to CBT across different tissue sites of tumor growth. Each tissue site in the body can contain unique tissue-resident immune cells from both the lymphoid and the myeloid compartment and differences in tissue-specific immune cell composition might predispose tumors in certain tissue sites to be more or less responsive to immunotherapy. Understanding the interplay between tissue-resident and systemic immune responses against tumors will help to determine how to better therapeutically target the immune system to fight cancer. This review summarizes clinical and preclinical investigations of tissue-specific antitumor immune responses and how they influence the tumor immune microenvironment and the efficacy of immunotherapy.

A temporal thymic selection switch and ligand binding kinetics constrain neonatal Foxp3+ Treg cell development

The neonatal thymus generates Foxp3⁺ regulatory T (tT_reg) cells that are critical in controlling immune homeostasis and preventing multiorgan autoimmunity. The role of antigen specificity on neonatal tT_reg cell selection is unresolved. Here we identify 17 self-peptides recognized by neonatal tT_reg cells, and reveal ligand specificity patterns that include self-antigens presented in an age- and inflammation-dependent manner. Fate-mapping studies of neonatal peptidyl arginine deiminase type IV (Padi4)-specific thymocytes reveal disparate fate choices. Neonatal thymocytes expressing T cell receptors that engage IA^b-Padi4 with moderate dwell times within a conventional docking orientation are exported as tT_reg cells. In contrast, Padi4-specific T cell receptors with short dwell times are expressed on CD4⁺ T cells, while long dwell times induce negative selection. Temporally, Padi4-specific thymocytes are subject to a developmental stage-specific change in negative selection, which precludes tT_reg cell development. Thus, a temporal switch in negative selection and ligand binding kinetics constrains the neonatal tT_reg selection window.

Prolongation of allograft survival by passenger donor regulatory T cells

Tissue resident lymphocytes are present within many organs, and are presumably transferred at transplantation, but their impact on host immunity is unclear. Here, we examine whether transferred donor natural regulatory CD4 T cells (nT-regs) inhibit host alloimmunity and prolong allograft survival. Transfer of donor-strain lymphocytes was first assessed by identifying circulating donor-derived CD4 T cells in 21 consecutive human lung transplant recipients, with 3 patterns of chimerism apparent: transient, intermediate, and persistent (detectable for up to 6 weeks, 6 months, and beyond 1 year, respectively). The potential for transfer of donor nT-regs was then confirmed by analysis of leukocyte filters recovered from ex vivo normothermic perfusion circuits of human kidneys retrieved for transplantation. Finally, in a murine model of cardiac allograft vasculopathy, depletion of donor CD4 nT-regs before organ recovery resulted in markedly accelerated heart allograft rejection and augmented host effector antibody responses. Conversely, adoptive transfer or purified donor-strain nT-regs inhibited host humoral immunity and prolonged allograft survival, and more effectively so than following administration of recipient nT-regs. In summary, following transplantation, passenger donor-strain nT-regs can inhibit host adaptive immune responses and prolong allograft survival. Isolated donor-derived nT-regs may hold potential as a cellular therapy to improve transplant outcomes.

A review on the interactions between the tumor microenvironment and androgen receptor signaling in prostate cancer

Prostate cancer growth is controlled by androgen receptor signaling via both androgen-dependent and androgen-independent pathways. Furthermore, the prostate is an immune competent organ with inflammatory changes both within the systemic and local environment contributing to the reprogramming of the prostatic epithelium with consistently elevated lymphocyte infiltration and proinflammatory cytokines being found in prostate cancer. The crosstalk between the tumor microenvironment and androgen receptor signaling is complex with both protumorigenic and antitumorigenic roles observed. However, despite an increase in immune checkpoint inhibitors and inflammatory signaling blockades available for a range of cancer types, we are yet to see substantial progress in the treatment of prostate cancer. Therefore, this review aims to summarize the tumor microenvironment and its impact on androgen receptor signaling in prostate cancer.

Negligible Role for Deletion Mediated by cDC1 in CD8+ T Cell Tolerance

Deletion of CD8⁺ T cells by dendritic cells (DCs) is recognized as a critical mechanism of immune tolerance to self-antigens. Although DC-mediated peripheral deletion of autoreactive CD8⁺ T cells has been demonstrated using T cells reactive to model Ags, its role in shaping the naturally occurring polyclonal CD8⁺ T cell repertoire has not been defined. Using Batf3^-/- mice lacking cross-presenting CD8α⁺ and CD103⁺ DCs (also known as type 1 conventional [cDC1]), we demonstrate that peripheral deletion of CD8⁺ T cells reactive to a model tissue Ag is dependent on cDC1. However, endogenous CD8⁺ T cells from the periphery of Batf3^-/- mice do not exhibit heightened self-reactivity, and deep TCR sequencing of CD8⁺ T cells from Batf3^-/- and Batf3^+/+ mice reveals that cDC1 have a minimal impact on shaping the peripheral CD8⁺ T cell repertoire. Thus, although evident in reductionist systems, deletion of polyclonal self-specific CD8⁺ T cells by cDC1 plays a negligible role in enforcing tolerance to natural self-ligands.

Regulatory T Cells in an Endogenous Mouse Lymphoma Recognize Specific Antigen Peptides and Contribute to Immune Escape

Foxp3⁺ regulatory T cells (Tregs) sustain immune homeostasis and may contribute to immune escape in malignant disease. As a prerequisite for developing immunologic approaches in cancer therapy, it is necessary to understand the ontogeny and the antigenic specificities of tumor-infiltrating Tregs. We addressed this question by using a λ-MYC transgenic mouse model of endogenously arising B-cell lymphoma, which mirrors key features of human Burkitt lymphoma. We show that Foxp3⁺ Tregs suppress antitumor responses in endogenous lymphoma. Ablation of Foxp3⁺ Tregs significantly delayed tumor development. The ratio of Treg to effector T cells was elevated in growing tumors, which could be ascribed to differential proliferation. The Tregs detected were mainly natural Tregs that apparently recognized self-antigens. We identified MHC class II-restricted nonmutated self-epitopes, which were more prevalent in lymphoma than in normal B cells and could be recognized by Tregs. These epitopes were derived from proteins that are associated with cellular processes related to malignancy and may be overexpressed in the tumor.

Foxp3+ Regulatory and Conventional CD4+ T Cells Display Similarly High Frequencies of Alloantigen-Reactive Cells

Foxp3⁺ regulatory T cells (Tregs) play a major role in acquired immune tolerance to allogenic transplants. Their suppressive activity is thought to require T cell receptor (TCR)-driven antigen recognition; little, however, is known about the fraction of Tregs able to recognize alloantigens within this T cell subset primarily educated against self-antigens. Performing transfer experiments of Tregs or conventional T cells (Tconv) into both lymphoreplete and lymphopenic mice, we observed a similarly high proportion of cells signaling through their TCR and proliferating in allogenic hosts. Furthermore, using an in vivo proliferation assay with limited T cell numbers infused into lymphopenic mice, we found that the overall frequency of alloreactive Tregs was similar if not higher to that of alloreactive Tconv. Overall our study highlights a noticeably high level of alloreactive Foxp3⁺ regulatory T cells accounting for their predominant role in transplantation tolerance.

Antigen-Specificity in the Thymic Development and Peripheral Activity of CD4+FOXP3+ T Regulatory Cells

CD4⁺Foxp3⁺ T regulatory cells (Treg) are essential for the life of the organism, in particular because they protect the host against its own autoaggressive CD4⁺Foxp3^- T lymphocytes (Tconv). Treg distinctively suppress autoaggressive immunity while permitting efficient defense against infectious diseases. This split effect indicates that Treg activity is controlled in an antigen-specific manner. This specificity is achieved first by the formation of the Treg repertoire during their development, and second by their activation in the periphery. This review presents novel information on the antigen-specificity of Treg development in the thymus, and Treg function in the periphery. These aspects have so far remained imprecisely understood due to the lack of knowledge of the actual antigens recognized by Treg during the different steps of their life, so that most previous studies have been performed using artificial antigens. However, recent studies identified some antigens mediating the positive selection of autoreactive Treg in the thymus, and the function of Treg in the periphery in autoimmune and allergic disorders. These investigations emphasized the remarkable specificity of Treg development and function. Indeed, the development of autoreactive Treg in the thymus was found to be mediated by single autoantigens, so that the absence of one antigen led to a dramatic loss of Treg reacting toward that antigen. The specificity of Treg development is important because the constitution of the Treg repertoire, and especially the presence of holes in this repertoire, was found to crucially influence human immunopathology. Indeed, it was found that the development of human immunopathology was permitted by the lack of Treg against the antigens driving the autoimmune or allergic T cell responses rather than by the impairment of Treg activation or function. The specificity of Treg suppression in the periphery is therefore intimately associated with the mechanisms shaping the formation of the Treg repertoire during their development. This novel information refines significantly our understanding of the antigen-specificity of Treg protective function, which is required to envision how these cells distinctively regulate unwanted immune responses as well as for the development of appropriate approaches to optimally harness them therapeutically in autoimmune, malignant, and infectious diseases.

Thymic tolerance as a key brake on autoimmunity

Although the thymus has long been recognized as a key organ for T cell selection, the intricate details linking these selection events to human autoimmunity have been challenging to decipher. Over the last two decades, there has been rapid progress in understanding the role of thymic tolerance mechanisms in autoimmunity through genetics. Here we review some of the recent progress in understanding key thymic tolerance processes that are critical for preventing autoimmune disease.

Emerging Functions of Regulatory T Cells in Tissue Homeostasis

CD4⁺Foxp3⁺ regulatory T-cells (Tregs) are a unique subset of helper T-cells, which regulate immune response and establish peripheral tolerance. Tregs not only maintain the tone and tenor of an immune response by dominant tolerance but, in recent years, have also been identified as key players in resolving tissue inflammation and as mediators of tissue healing. Apart from being diverse in their origin (thymic and peripheral) and location (lymphoid and tissue resident), Tregs are also phenotypically heterogeneous as per the orientation of ongoing immune response. In this review, we discuss the recent advances in the field of Treg biology in general, and non-lymphoid and tissue-resident Tregs in particular. We elaborate upon well-known visceral adipose tissue, colon, skin, and tumor-infiltrating Tregs and newly identified tissue Treg populations as in lungs, skeletal muscle, placenta, and other tissues. Our attempt is to differentiate Tregs based on distinctive properties of their location, origin, ligand specificity, chemotaxis, and specific suppressive mechanisms. Despite ever expanding roles in maintaining systemic homeostasis, Tregs are employed by large varieties of tumors to dampen antitumor immunity. Thus, a comprehensive understanding of Treg biology in the context of inflammation can be instrumental in effectively managing tissue transplantation, autoimmunity, and antitumor immune responses.

Enteric α-defensins on the verge of intestinal immune tolerance and inflammation

The gut is the biggest immune organ in the body that encloses commensal microbiota which aids in food digestion. Paneth cells, positioned at the frontline of host-microbiota interphase, can modulate the composition of microbiota. Paneth cells achieve this via the delivery of microbicidal substances, among which enteric α-defensins play the primary role. If microbiota is dysregulated, it can impact the function of the local mucosal immune system. Importantly, this system is also exposed to an enormous number of antigens which are derived from the gut-resident microbiota and processed food, and may potentially trigger undesirable local inflammatory responses. To understand the intricate regulations and liaisons between Paneth cells, microbiota and the immune system in this intestinal-specific setting, one must consider their mode of interaction in a wider context of regulatory processes which impose immune tolerance not only to self, but also to microbiota and food-derived antigens. These include, but are not limited to, tolerogenic mechanisms of central tolerance in the thymus and peripheral tolerance in the secondary lymphoid organs, and the intestine itself. Defects in these processes can compromise homeostasis in the intestinal mucosal immunity. In this review, which is focused on tolerance to intestinal antigens and its relevance for the pathogenesis of gut immune diseases, we provide an outline of such multilayered immune control mechanisms and highlight functional links that underpin their cooperative nature.

Tregs: Where We Are and What Comes Next?

Regulatory T cells are usually recognized as a specialized subset of CD4⁺ T cells functioning in establishment and maintenance of immune tolerance. Meanwhile, there is emerging evidence that regulatory T cells (Tregs) are also present in various non-lymphoid tissues, and that they have unique phenotypes credited with activities distinct from regulatory function. Their development and function have been described in plenty of manuscripts in the past two decades. However, with the deepening of research in recent years, emerging evidence revealed some novel mechanisms about how Tregs exert their activities. First, we discuss the expanding family of regulatory lymphocytes briefly and then, try to interpret how fork-head box P3 (Foxp3), a master regulator of the regulatory pathway in the development and function of regulatory T cells, functions. Subsequently, another part of our focus is varieties of tissue Tregs. Next, we primarily discuss recent research on how Tregs work and their faceted functions in terms of soluble mediators, functional proteins, and inhibitory receptors. In particular, unless otherwise noted, the term “Treg” is used here to refer specially to the “CD4⁺CD25⁺Foxp3^+” regulatory cells.

Trial watch: Immunogenic cell death induction by anticancer chemotherapeutics

The expression "immunogenic cell death" (ICD) refers to a functionally unique form of cell death that facilitates (instead of suppressing) a T cell-dependent immune response specific for dead cell-derived antigens. ICD critically relies on the activation of adaptive responses in dying cells, culminating with the exposure or secretion of immunostimulatory molecules commonly referred to as "damage-associated molecular patterns". Only a few agents can elicit bona fide ICD, including some clinically established chemotherapeutics such as doxorubicin, epirubicin, idarubicin, mitoxantrone, bleomycin, bortezomib, cyclophosphamide and oxaliplatin. In this Trial Watch, we discuss recent progress on the development of ICD-inducing chemotherapeutic regimens, focusing on studies that evaluate clinical efficacy in conjunction with immunological biomarkers.