Peripheral T cell activation, not thymic selection, expands the T follicular helper repertoire in a lupus-prone murine model

Many autoimmune diseases are characterized by the activation of autoreactive T cells. The T cell repertoire is established in the thymus; it remains uncertain whether the presence of disease-associated autoreactive T cells reflects abnormal T cell selection in the thymus or aberrant T cell activation in the periphery. Here, we describe T cell selection, activation, and T cell repertoire diversity in female mice deficient for B lymphocyte-induced maturation protein (BLIMP)-1 in dendritic cells (DCs) (Prdm1 CKO). These mice exhibit a lupus-like phenotype with an expanded population of T follicular helper (Tfh) cells having a more diverse T cell receptor (TCR) repertoire than wild-type mice and, in turn, develop a lupus-like pathology. To understand the origin of the aberrant Tfh population, we analyzed the TCR repertoire of thymocytes and naive CD4 T cells from Prdm1 CKO mice. We show that early development and selection of T cells in the thymus are not affected. Importantly, however, we observed increased TCR signal strength and increased proliferation of naive T cells cultured in vitro with antigen and BLIMP1-deficient DCs compared to control DCs. Moreover, there was increased diversity in the TCR repertoire in naive CD4+ T cells stimulated in vitro with BLIMP1-deficient DCs. Collectively, our data indicate that lowering the threshold for peripheral T cell activation without altering thymic selection and naive T cell TCR repertoire leads to an expanded repertoire of antigen-activated T cells and impairs peripheral T cell tolerance.

T cell deletional tolerance restricts AQP4 but not MOG CNS autoimmunity

Aquaporin-4 (AQP4)-specific Th17 cells are thought to have a central role in neuromyelitis optica (NMO) pathogenesis. When modeling NMO, only AQP4-reactive Th17 cells from AQP4-deficient (AQP4-/-), but not wild-type (WT) mice, caused CNS autoimmunity in recipient WT mice, indicating that a tightly regulated mechanism normally ensures tolerance to AQP4. Here, we found that pathogenic AQP4 T cell epitopes bind MHC II with exceptionally high affinity. Examination of T cell receptor (TCR) α/β usage revealed that AQP4-specific T cells from AQP4-/- mice employed a distinct TCR repertoire and exhibited clonal expansion. Selective thymic AQP4 deficiency did not fully restore AQP4-reactive T cells, demonstrating that thymic negative selection alone did not account for AQP4-specific tolerance in WT mice. Indeed, AQP4-specific Th17 cells caused paralysis in recipient WT or B cell-deficient mice, which was followed by complete recovery that was associated with apoptosis of donor T cells. However, donor AQP4-reactive T cells survived and caused persistent paralysis in recipient mice deficient in both T and B cells or mice lacking T cells only. Thus, AQP4 CNS autoimmunity was limited by T cell-dependent deletion of AQP4-reactive T cells. In contrast, myelin oligodendrocyte glycoprotein (MOG)-specific T cells survived and caused sustained disease in WT mice. These findings underscore the importance of peripheral T cell deletional tolerance to AQP4, which may be relevant to understanding the balance of AQP4-reactive T cells in health and in NMO. T cell tolerance to AQP4, expressed in multiple tissues, is distinct from tolerance to MOG, an autoantigen restricted in its expression.

Single-cell fate mapping reveals widespread clonal ignorance of low-affinity T cells exposed to systemic infection

T cell ignorance is a specific form of immunological tolerance. It describes the maintenance of naivety in antigen-specific T cells in vivo despite the presence of their target antigen. It is thought to mainly play a role during the steady state, when self-antigens are presented in absence of costimulatory signals and at low density or to T cells of low affinity. In how far antigen-specific T cells can also remain clonally ignorant to foreign antigens, presented in the inflammatory context of systemic infection, remains unclear. Using single-cell in vivo fate mapping and high throughput flow cytometric enrichment, we find that high-affinity antigen-specific CD8⁺ T cells are efficiently recruited upon systemic infection. In contrast, most low-affinity antigen-specific T cells ignore the priming antigen and persist in the naïve state while remaining fully responsive to subsequent immunization with a high-affinity ligand. These data establish the widespread clonal ignorance of low-affinity T cells as a major factor shaping the composition of antigen-specific CD8⁺ T cell responses to systemic infection.

Immune checkpoint receptors in autoimmunity

Immune checkpoint receptors such as programmed cell death protein 1 (PD-1), cytotoxic T-lymphocyte associated protein 4 (CTLA-4), lymphocyte-activation gene 3 (LAG-3), and T cell immunoglobulin and ITIM domain (TIGIT) have distinct and overlapping inhibitory functions that regulate Tcell activation, differentiation, and function. These inhibitory receptors also mediate tolerance, and dysregulation of these receptors can result in a breach of tolerance and the development of autoimmune syndromes. Similarly, antibody blockade of immune checkpoint receptors or their ligands for cancer immunotherapy may trigger a spectrum of organ inflammation that resembles autoimmunity, termed immune-related adverse events (irAE). In this review, we discuss recent advances in the regulation of autoimmunity by immune checkpoint receptors. We highlight coordinated gene expression programs linking checkpoint receptors, heterogeneity within autoreactive T-cell populations, parallels between irAE and autoimmunity, and bidirectional functional interactions between immune checkpoint receptors and their ligands.

Machine learning integrations for development of a T-cell-tolerance derived signature to improve the clinical outcomes and precision treatment of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is characterized by high rates of recurrence and metastasis and poor prognosis. A recently discovered concept of T cell tolerance (TCT) has become an entirely new target of cancer immunotherapy. Unfortunately, the effect of TCT on the outcomes of HCC has not been explored. In this study, 7 public datasets and one external clinical cohort, including 1716 HCC patients were explored. Through WGCNA analysis and differential analysis, we explored the key TCT-related modulates. A total of 95 machine learning integrations across all validation cohorts were compared and the optimal method with the highest average C-index value was selected to construct the TCT derived signature (TCTS). In all independent clinical cohorts, TCTS showed accurate prediction of the prognosis, and was significantly correlated with clinical indicators and molecular features. Compared with 77 published gene signatures, the TCTS exhibited superior predictive performance. In the external clinical cohort, a novel nomogram (comprising TNM stage, Hepatitis B, Vascular invasion, Perineural invasion, AFP and TCTS) was constructed to test the clinical performance of TCTS. The results showed that the high TCTS scoring group showed dismal prognosis, improved sensitivity to oxaliplatin and good response to anti-PD-1/PD-L1 immunotherapy. Moreover, the low TCTS score group had few genomic alterations, low immune activation and low PD-1/PD-L1 expression levels. In conclusion, TCTS is an ideal biomarker for predicting the clinical outcomes and improving precision treatment of HCC.

Restoring tolerance to β-cells in Type 1 diabetes: Current and emerging strategies

Type 1 diabetes (T1D) results from insulin insufficiency due to islet death and dysfunction following T cell-mediated autoimmune attack. The technical feasibility of durable, functional autologous islet restoration is progressing such that it presents the most likely long-term cure for T1D but cannot succeed without the necessary counterpart of clinically effective therapeutic strategies that prevent grafted islets' destruction by pre-existing anti-islet T cells. While advances have been made in broad immunosuppression to lower off-target effects, the risk of opportunistic infections and cancers remains a concern, especially for well-managed T1D patients. Current immunomodulatory strategies in development focus on autologous Treg expansion, treatments to decrease antigen presentation and T effector (Teff) activation, and broad depletion of T cells with or without hematopoietic stem cell transplants. Emerging strategies harnessing the intensified DNA damage response present in expanding T cells, exacerbating their already high sensitivity to apoptosis to abate autoreactive Teff cells.

Thymic mimetic cells: tolerogenic masqueraders

Medullary thymic epithelial cells (mTECs) clonally delete or divert autoreactive T cells by ectopically expressing a diverse array of peripheral-tissue antigens (PTAs) within the thymus. Although thymic stromal cells with histological features of extra-thymic cell types, like myocytes or neurons, have been observed by light microscopy since the mid-1800s, most modern work on PTA expression has focused on the transcription factor Aire. Here, we highlight recent work that has refocused attention on such 'misplaced' thymic cells, referred to collectively as thymic mimetic cells. We review the molecular underpinnings of mimetic cells and their roles in establishing T cell tolerance, and we propose that mimetic cells play important roles in autoimmunity. Finally, we suggest future directions for this emerging area.

Multiomics Analysis Identifies SOCS1 as Restraining T Cell Activation and Preventing Graft-Versus-Host Disease

Graft-versus-host disease (GVHD) is a major life-threatening complication of allogeneic hematopoietic stem cell transplantation (allo-HSCT). Inflammatory signaling pathways promote T-cell activation and are involved in the pathogenesis of GVHD. Suppressor of cytokine signaling 1 (SOCS1) is a critical negative regulator for several inflammatory cytokines. However, its regulatory role in T-cell activation and GVHD has not been elucidated. Multiomics analysis of the transcriptome and chromatin structure of granulocyte-colony-stimulating-factor (G-CSF)-administered hyporesponsive T cells from healthy donors reveal that G-CSF upregulates SOCS1 by reorganizing the chromatin structure around the SOCS1 locus. Parallel in vitro and in vivo analyses demonstrate that SOCS1 is critical for restraining T cell activation. Loss of Socs1 in T cells exacerbates GVHD pathogenesis and diminishes the protective role of G-CSF in GVHD mouse models. Further analysis shows that SOCS1 inhibits T cell activation not only by inhibiting the colony-stimulating-factor 3 receptor (CSF3R)/Janus kinase 2 (JAK2)/signal transducer and activator of transcription 3 (STAT3) pathway, but also by restraining activation of the inflammasome signaling pathway. Moreover, high expression of SOCS1 in T cells from patients correlates with low acute GVHD occurrence after HSCT. Overall, these findings identify that SOCS1 is critical for inhibiting T cell activation and represents a potential target for the attenuation of GVHD.

NR4A nuclear receptors in T and B lymphocytes: Gatekeepers of immune tolerance. Immunol Rev 2022;307:116-133. [PMID: 35174510 DOI: 10.1111/imr.13072]

Random VDJ recombination early in T and B cell development enables the adaptive immune system to recognize a vast array of evolving pathogens via antigen receptors. However, the potential of such randomly generated TCRs and BCRs to recognize and respond to self-antigens requires layers of tolerance mechanisms to mitigate the risk of life-threatening autoimmunity. Since they were originally cloned more than three decades ago, the NR4A family of nuclear hormone receptors have been implicated in many critical aspects of immune tolerance, including negative selection of thymocytes, peripheral T cell tolerance, regulatory T cells (Treg), and most recently in peripheral B cell tolerance. In this review, we discuss important insights from many laboratories as well as our own group into the function and mechanisms by which this small class of primary response genes promotes self-tolerance and immune homeostasis to balance the need for host defense against the inherent risks posed by the adaptive immune system.

Activation of T cell checkpoint pathways during β-cell antigen presentation by engineered dendritic cells promotes protection from type 1 diabetes

Defective immune regulation has been recognized in type 1 diabetes (T1D). Immune regulatory T cell check-point receptors, which are generally upregulated on activated T cells, have been the molecules of attention as therapeutic targets for enhancing immune response in tumor therapy. Here, we show that pancreatic β-cell antigen (BcAg) presentation by engineered tolerogenic dendritic cells (tDCs) that express CTLA4 selective ligand (B7.1wa) or a combination of CTLA4, PD1 and BTLA selective ligands (B7.1wa, PD-L1, and HVEM-CRD1 respectively; multiligand-DCs) causes an increase in regulatory cytokine and T cell (Treg) responses and suppression of the effector T cell function as compared to engineered control-DCs. Non-obese diabetic (NOD) mice treated with BcAg-pulsed CTLA4-ligand-DCs and multiligand-DCs at pre-diabetic and early-hyperglycemic stages showed significantly lower degree of insulitis, higher frequencies of insulin-positive islets, profound delay in, and reversal of, hyperglycemia for a significant duration. Immune cells from the tDC treated mice not only produced lower amounts of IFNγ and higher amounts of IL10 and TGFβ1 upon BcAg challenge, but also failed to induce hyperglycemia upon adoptive transfer. While both CTLA4-ligand-DCs and multiligand-DCs were effective in inducing tolerance, multiligand-DC treatment produced an overall higher suppressive effect on effector T cell function and disease outcome. These studies show that enhanced engagement of T cell checkpoint receptors during BcAg presentation can modulate T cell function and suppress autoimmunity and progression of the disease in T1D.

Autoimmunity in motion: Mechanisms of immune regulation and destruction revealed by in vivo imaging

Autoimmunity arises when mechanisms of immune tolerance fail. Here we discuss mechanisms of T cell activation and tolerance and the dynamics of the autoimmune response at the site of disease. Live imaging of autoimmunity provides the ability to analyze immune cell dynamics at the single-cell level within the complex intact environment where disease occurs. These analyses have revealed mechanisms of T cell activation and tolerance in the lymph nodes, mechanisms of T cell entry into sites of autoimmune disease, and mechanisms leading to pathogenesis or protection in the autoimmune lesions. The overarching conclusions point to stable versus transient T cell antigen presenting cell interactions dictating the balance between T cell activation and tolerance, and T cell restimulation as a driver of pathogenesis at the site of autoimmunity. Findings from models of multiple sclerosis and type 1 diabetes are highlighted, however, the results have implications for basic mechanisms of T cell regulation during immune responses, tumor immunity, and autoimmunity.

T cell Tolerance in Early Life

T cell-mediated immune tolerance is a state of unresponsiveness of T cells towards specific self or non-self antigens. This is particularly essential during prenatal/neonatal period when T cells are exposed to dramatically changing environment and required to avoid rejection of maternal antigens, limit autoimmune responses, tolerate inert environmental and food antigens and antigens from non-harmful commensal microorganisms, promote maturation of mucosal barrier function, yet mount an appropriate response to pathogenic microorganisms. The cell-intrinsic and cell extrinsic mechanisms promote the generation of prenatal/neonatal T cells with distinct features to meet the complex and dynamic need of tolerance during this period. Reduced exposure or impaired tolerance in early life may have significant impact on allergic or autoimmune diseases in adult life. The uniqueness of conventional and regulatory T cells in human umbilical cord blood (UCB) may also provide certain advantages in UCB transplantation for hematological disorders.

A Question of Tolerance-Antigen-Specific Immunotherapy for Type 1 Diabetes

PURPOSE OF REVIEW

Antigen-specific immunotherapy (ASI) is a long sought-after goal for type 1 diabetes (T1D), with the potential of greater long-term safety than non-specific immunotherapy. We review the most recent advances in identification of target islet epitopes, delivery platforms and the ongoing challenges.

RECENT FINDINGS

It is now recognised that human proinsulin contains a hotspot of epitopes targeted in people with T1D. Beta-cell neoantigens are also under investigation as ASI target epitopes. Consideration of the predicted HLA-specificity of the target antigen for subject selection is now being incorporated into trial design. Cell-free ASI approaches delivering antigen with or without additional immunomodulatory agents can induce antigen-specific regulatory T cell responses, including in patients and many novel nanoparticle-based platforms are under development. ASI for T1D is rapidly advancing with a number of modalities currently being trialled in patients and many more under development in preclinical models.

'Nur'turing tumor T cell tolerance and exhaustion: novel function for Nuclear Receptor Nur77 in immunity

The nuclear receptor Nur77 is expressed in a multitude of tissues, regulating cell differentiation and homeostasis. Dysregulation of Nur77 signaling is associated with cancer, cardiovascular disease, and disorders of the CNS. The role of Nur77 in T cells has been studied for almost 30 years now. There is a clear appreciation that Nur77 is crucial for apoptosis of self‐reactive T cells. However, the regulation and function of Nur77 in mature T cells remains largely unclear. In an exciting development, Nur77 has been recently demonstrated to impinge on cancer immunotherapy involving chimeric antigen receptor (CAR) T cells and tumor infiltrating lymphocytes (TILs). These studies indicated that Nur77 deficiency reduced T cell tolerance and exhaustion, thus raising the effectiveness of immune therapy in mice. Based on these novel insights, it may be proposed that regulation of Nur77 activity holds promise for innovative drug development in the field of cellular immunotherapy in cancer. In this review, we therefore summarize the role of Nur77 in T cell selection and maturation; and further develop the idea of targeting its activity in these cells as a potential strategy to augment current cancer immunotherapy treatments.

Inventories of naive and tolerant mouse CD4 T cell repertoires reveal a hierarchy of deleted and diverted T cell receptors

Deletion or T_reg cell differentiation are alternative fates of autoreactive MHCII-restricted thymocytes. How these different modes of tolerance determine the size and composition of polyclonal cohorts of autoreactive T cells with shared specificity is poorly understood. We addressed how tolerance to a naturally expressed autoantigen of the central nervous system shapes the CD4 T cell repertoire. Specific cells in the tolerant peripheral repertoire either were Foxp3⁺ or displayed anergy hallmarks and, surprisingly, were at least as frequent as in the nontolerant repertoire. Despite this apparent lack of deletional tolerance, repertoire inventories uncovered that some T cell receptors (TCRs) were lost from the CD4 T cell pool, whereas others mediated T_reg cell differentiation. The antigen responsiveness of these TCRs supported an affinity model of central tolerance. Importantly, the contribution of different diverter TCRs to the nascent thymic T_reg cell population reflected their antigen reactivity rather than their frequency among precursors. This reveals a multilayered TCR hierarchy in CD4 T cell tolerance that separates deleted and diverted TCRs and assures that the T_reg cell compartment is filled with cells of maximal permissive antigen reactivity.

In vitro Differentiation of Thymic Treg Cell Progenitors to Mature Thymic Treg Cells

Thymic T_reg cell differentiation occurs via a two-step process. Step one generates T_reg cell progenitors (T_regP) and is driven by strong TCR interactions with antigens presented in the thymus. Step two is initiated by activation of STAT5 via IL-2, or IL-15, leading to mature T_reg cells capable of emigrating from the thymus and mediating immune tolerance and homeostasis in peripheral tissues. Herein we describe an in vitro T_regP cell differentiation assay that models the second, cytokine dependent, step of thymic T_reg cell development. It can be utilized with relative ease to determine if a population of thymocytes represents a potential progenitor population for T_reg cells as well as test how different cytokines or chemical inhibitors modulate the differentiation of known T_regP cell populations into mature T_reg cells.

Regulation of immune responses by E3 ubiquitin ligase Cbl-b

Casitas B lymphoma-b (Cbl-b), a RING finger E3 ubiquitin ligase, has been identified as a critical regulator of adaptive immune responses. Cbl-b is essential for establishing the threshold for T cell activation and regulating peripheral T cell tolerance through various mechanisms. Intriguingly, recent studies indicate that Cbl-b also modulates innate immune responses, and plays a key role in host defense to pathogens and anti-tumor immunity. These studies suggest that targeting Cbl-b may represent a potential therapeutic strategy for the management of human immune-related disorders such as autoimmune diseases, infections, tumors, and allergic airway inflammation. In this review, we summarize the latest developments regarding the roles of Cbl-b in innate and adaptive immunity, and immune-mediated diseases.

Deubiquitinases as pivotal regulators of T cell functions

T cells efficiently respond to foreign antigens to mediate immune responses against infections but are tolerant to self-tissues. Defect in T cell activation is associated with severe immune deficiencies, whereas aberrant T cell activation contributes to the pathogenesis of diverse autoimmune and inflammatory diseases. An emerging mechanism that regulates T cell activation and tolerance is ubiquitination, a reversible process of protein modification that is counter-regulated by ubiquitinating enzymes and deubiquitinases (DUBs). DUBs are isopeptidases that cleave polyubiquitin chains and remove ubiquitin from target proteins, thereby controlling the magnitude and duration of ubiquitin signaling. It is now well recognized that DUBs are crucial regulators of T cell responses and serve as potential therapeutic targets for manipulating immune responses in the treatment of immunological disorders and cancer. This review will discuss the recent progresses regarding the functions of DUBs in T cells.

A novel mimovirus encoding ChgA10-19 peptide with PD-L1 induces T cell tolerance and ameliorates the severity of diabetes

Related studies demonstrate that type 1 diabetes (T1D) is caused by β-cell antigen specific autoreactive CD8+ T cells. ChgA has recently been identified as the autoantigen in NOD mice and T1D patients. Therefore, attenuating the activation of ChgA specific CD8+ T cells might be a promising target for T1D therapy. The negative co-stimulatory PD-L1 inhibits T cell mediated alloimmunity and induces tolerance. In this experiment, a novel mimovirus encoding ChgA_10-19 peptide with PD-L1 was constructed. The NOD.β2m null HHD mice were administrated with mimovirus transduced DCs. After immunization, the activation and proliferation of CD8+ T cells were detected, diabetes incidence and pancreatic tissue destruction were also analyzed. The results demonstrated that transduced DCs attenuated CD8+ T cell activation and proliferation. In addition, transduced DCs inhibited CD8+ T response to ChgA stimulation, and ameliorated the severity of diabetes. These data suggested that mimovirus transduced DCs might provide novel clues for T1D therapy.

Allorecognition by T Lymphocytes and Allograft Rejection

Recognition of donor antigens by recipient T cells in secondary lymphoid organs initiates the adaptive inflammatory immune response leading to the rejection of allogeneic transplants. Allospecific T cells become activated through interaction of their T cell receptors with intact allogeneic major histocompatibility complex (MHC) molecules on donor cells (direct pathway) and/or donor peptides presented by self-MHC molecules on recipient antigen-presenting cells (APCs) (indirect pathway). In addition, recent studies show that alloreactive T cells can also be stimulated through recognition of allogeneic MHC molecules displayed on recipient APCs (MHC cross-dressing) after their transfer via cell-cell contact or through extracellular vesicles (semi-direct pathway). The specific allorecognition pathway used by T cells is dictated by intrinsic and extrinsic factors to the allograft and can influence the nature and magnitude of the alloresponse and rejection process. Consequently, various organs and tissues such as skin, cornea, and solid organ transplants are recognized differently by pro-inflammatory T cells through these distinct pathways, which may explain why these grafts are rejected in a different fashion. On the other hand, the mechanisms by which anti-inflammatory regulatory T cells (Tregs) recognize alloantigen and promote transplantation tolerance are still unclear. It is likely that thymic Tregs are activated through indirect allorecognition, while peripheral Tregs recognize alloantigens in a direct fashion. As we gain insights into the mechanisms underlying allorecognition by pro-inflammatory and Treg cells, novel strategies are being designed to prevent allograft rejection in the absence of ongoing immunosuppressive drug treatment in patients.

The PD1:PD-L1/2 Pathway from Discovery to Clinical Implementation

The immune system maintains a critically organized network to defend against foreign particles, while evading self-reactivity simultaneously. T lymphocytes function as effectors and play an important regulatory role to orchestrate the immune signals. Although central tolerance mechanism results in the removal of the most of the autoreactive T cells during thymic selection, a fraction of self-reactive lymphocytes escapes to the periphery and pose a threat to cause autoimmunity. The immune system evolved various mechanisms to constrain such autoreactive T cells and maintain peripheral tolerance, including T cell anergy, deletion, and suppression by regulatory T cells (TRegs). These effects are regulated by a complex network of stimulatory and inhibitory receptors expressed on T cells and their ligands, which deliver cell-to-cell signals that dictate the outcome of T cell encountering with cognate antigens. Among the inhibitory immune mediators, the pathway consisting of the programed cell death 1 (PD-1) receptor (CD279) and its ligands PD-L1 (B7-H1, CD274) and PD-L2 (B7-DC, CD273) plays an important role in the induction and maintenance of peripheral tolerance and for the maintenance of the stability and the integrity of T cells. However, the PD-1:PD-L1/L2 pathway also mediates potent inhibitory signals to hinder the proliferation and function of T effector cells and have inimical effects on antiviral and antitumor immunity. Therapeutic targeting of this pathway has resulted in successful enhancement of T cell immunity against viral pathogens and tumors. Here, we will provide a brief overview on the properties of the components of the PD-1 pathway, the signaling events regulated by PD-1 engagement, and their consequences on the function of T effector cells.

The Role of Dendritic Cell Subsets and Innate Immunity in the Pathogenesis of Type 1 Diabetes and Other Autoimmune Diseases

Dendritic cells (DCs) are key antigen-presenting cells that have an important role in autoimmune pathogenesis. DCs control both steady-state T cell tolerance and activation of pathogenic responses. The balance between these two outcomes depends on several factors, including genetic susceptibility, environmental signals that stimulate varied innate responses, and which DC subset is presenting antigen. Although the specific DC phenotype can diverge depending on the tissue location and context, there are four main subsets identified in both mouse and human: conventional cDC1 and cDC2, plasmacytoid DCs, and monocyte-derived DCs. In this review, we will discuss the role of these subsets in autoimmune pathogenesis and regulation, as well as the genetic and environmental signals that influence their function. Specific topics to be addressed include impact of susceptibility loci on DC subsets, alterations in DC subset development, the role of infection- and host-derived innate inflammatory signals, and the role of the intestinal microbiota on DC phenotype. The effects of these various signals on disease progression and the relative effects of DC subset composition and maturation level of DCs will be examined. These areas will be explored using examples from several autoimmune diseases but will focus mainly on type 1 diabetes.

Loss of Nrf2 in bone marrow-derived macrophages impairs antigen-driven CD8(+) T cell function by limiting GSH and Cys availability

NF-E2-related factor 2 (Nrf2), known to protect against reactive oxygen species, has recently been reported to resolve acute inflammatory responses in activated macrophages. Consequently, disruption of Nrf2 promotes a proinflammatory macrophage phenotype. In the current study, we addressed the impact of this macrophage phenotype on CD8(+) T cell activation by using an antigen-driven coculture model consisting of Nrf2(-/-) and Nrf2(+/+) bone marrow-derived macrophages (BMDMΦ) and transgenic OT-1 CD8(+) T cells. OT-1 CD8(+) T cells encode a T cell receptor that specifically recognizes MHC class I-presented ovalbumin OVA(257-264) peptide, thereby causing a downstream T cell activation. Interestingly, coculture of OVA(257-264)-pulsed Nrf2(-/-) BMDMΦ with transgenic OT-1 CD8(+) T cells attenuated CD8(+) T cell activation, proliferation, and cytotoxic function. Since the provision of low-molecular-weight thiols such as glutathione (GSH) or cysteine (Cys) by macrophages limits antigen-driven CD8(+) T cell activation, we quantified the amounts of intracellular and extracellular GSH and Cys in both cocultures. Indeed, GSH levels were strongly decreased in Nrf2(-/-) cocultures compared to wild-type counterparts. Supplementation of thiols in Nrf2(-/-) cocultures via addition of glutathione ester, N-acetylcysteine, β-mercaptoethanol, or cysteine itself restored T cell proliferation as well as cytotoxicity by increasing intracellular GSH. Mechanistically, we identified two potential Nrf2-regulated genes involved in thiol synthesis in BMDMΦ: the cystine transporter subunit xCT and the modulatory subunit of the GSH-synthesizing enzyme γ-GCS (GCLM). Pharmacological inhibition of γ-GCS-dependent GSH synthesis as well as knockdown of the cystine antiporter xCT in Nrf2(+/+) BMDMΦ mimicked the effect of Nrf2(-/-) BMDMΦ on CD8(+) T cell function. Our findings demonstrate that reduced levels of GCLM as well as xCT in Nrf2(-/-) BMDMΦ limit GSH availability, thereby inhibiting antigen-induced CD8(+) T cell function.

T-bet regulates differentiation of forkhead box protein 3+ regulatory T cells in programmed cell death-1-deficient mice

Programmed cell death-1 (PD-1) plays an important role in peripheral T cell tolerance, but whether or not it affects the differentiation of helper T cell subsets remains elusive. Here we describe the importance of PD-1 in the control of T helper type 1 (Th1) cell activation and development of forkhead box protein 3 (FoxP3(+)) regulatory T cells (Tr(egs)). PD-1-deficient T cell-specific T-bet transgenic (P/T) mice showed growth retardation, and the majority died within 10 weeks. P/T mice showed T-bet over-expression, increased interferon (IFN)-γ production by CD4(+) T cells and significantly low FoxP3(+) T(reg) cell percentage. P/T mice developed systemic inflammation, which was probably induced by augmented Th1 response and low FoxP3(+) T(reg) count. The study identified a unique, previously undescribed role for PD-1 in Th1 and T(reg) differentiation, with potential implication in the development of Th1 cell-targeted therapy.

Confusing signals: recent progress in CTLA-4 biology

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Cell extrinsic and cell intrinsic mechanisms of action of CTLA-4 are unclear.

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Data suggest that the extracellular domain of CTLA4 elicits regulatory function.

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The function of CTLA-4 tail may lie in regulating localization rather than signaling.

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Membrane levels of CTLA-4 directly impact access of CD28 to shared ligands.

The mechanism of action of cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) remains surprisingly unclear. Regulatory T (Treg) cells can use CTLA-4 to elicit suppression; however, CTLA-4 also operates in conventional T cells, reputedly by triggering inhibitory signals. Recently, interactions mediated via the CTLA-4 cytoplasmic domain have been shown to preferentially affect Treg cells, yet other evidence suggests that the extracellular domain of CTLA-4 is sufficient to elicit suppression. Here, we discuss these paradoxical findings in the context of CTLA-4-mediated ligand regulation. We propose that the function of CTLA-4 cytoplasmic domain is not to transmit inhibitory signals but to precisely control the turnover, cellular location, and membrane delivery of CTLA-4 to facilitate its central function: regulating the access of CD28 to their shared ligands.

Cbl-b: Roles in T Cell Tolerance, Proallergic T Cell Development, and Cancer Immunity

Cbl-b is a member of the Cbl family of RING finger E3 ubiquitin ligases and polymorphisms and mutations in Cbl-b are associated with several autoimmune/inflammatory diseases in humans. Furthermore, gene targeting experiments in mice have provided proof of the in vivo effects of Cbl-b on T cell function and its involvement with these diseases. This brief review updates our understanding of Cbl-b in T cell tolerance, proallergic T cell development, and cancer immunity in light of the most recent advances, and their impact on autoimmune-/inflammatory diseases and cancer immunotherapy.

T cell-specific inhibition of multiple apoptotic pathways blocks negative selection and causes autoimmunity

T cell self-tolerance is thought to involve peripheral tolerance and negative selection, involving apoptosis of autoreactive thymocytes. However, evidence supporting an essential role for negative selection is limited. Loss of Bim, a Bcl-2 BH3-only protein essential for thymocyte apoptosis, rarely results in autoimmunity on the C57BL/6 background. Mice with T cell-specific over-expression of Bcl-2, that blocks multiple BH3-only proteins, are also largely normal. The nuclear receptor Nur77, also implicated in negative selection, might function redundantly to promote apoptosis by associating with Bcl-2 and exposing its potentially pro-apoptotic BH3 domain. Here, we report that T cell-specific expression of a Bcl2 BH3 mutant transgene results in enhanced rescue of thymocytes from negative selection. Concomitantly, Treg development is increased. However, aged BH3 mutant mice progressively accumulate activated, autoreactive T cells, culminating in development of multi-organ autoimmunity and lethality. These data provide strong evidence that negative selection is crucial for establishing T cell tolerance.

Allergy and risk of hematologic malignancies: associations and mechanisms

Increasing evidence indicates that a dysregulated immune system, as the one found in allergic disorders, can affect survival of tumor cells. A possible association between allergies and risk of hematologic malignancies has been examined in several epidemiological studies; however, results were not always consistent. The aim of this review is to report the preclinical and clinical data, which support a correlation between allergy and hematologic neoplasms. Immune system modulation could represent a powerful tool in the prevention and treatment of hematologic malignancies.

In trans T cell tolerance exacerbates experimental allergic encephalomyelitis by interfering with protective antibody responses

F1 (SJL/J×C57BL/6) mice with MOG35-55-induced EAE recover from disease when treated with Ig-MOG carrying MOG35-55 peptide. However, Ig-PLP1, carrying PLP139-151, induced reduction of anti-MOG antibodies and exacerbated EAE. Herein, we show that Ig-PLP1 specifically reduces the frequency of B cells producing protective IgG2a/b anti-MOG antibodies. Surprisingly, these cells were marginal zone (MZ), rather than follicular (FO) or newly formed (NF), B cells and transfer of MZ B cells into sick mice nullified disease exacerbation by Ig-PLP1 in a complement dependent manner. These findings reveal a potential self-limiting regulatory mechanism involving auto-antibodies in MOG EAE.

Regulation of Lipid Signaling by Diacylglycerol Kinases during T Cell Development and Function

Diacylglycerol (DAG) and phosphatidic acid (PA) are bioactive lipids synthesized when the T cell receptor binds to a cognate peptide-MHC complex. DAG triggers signaling by recruiting Ras guanyl-releasing protein 1, PKCθ, and other effectors, whereas PA binds to effector molecules that include mechanistic target of rapamycin, Src homology region 2 domain-containing phosphatase 1, and Raf1. While DAG-mediated pathways have been shown to play vital roles in T cell development and function, the importance of PA-mediated signals remains less clear. The diacylglycerol kinase (DGK) family of enzymes phosphorylates DAG to produce PA, serving as a molecular switch that regulates the relative levels of these critical second messengers. Two DGK isoforms, α and ζ, are predominantly expressed in T lineage cells and play an important role in conventional αβ T cell development. In mature T cells, the activity of these DGK isoforms aids in the maintenance of self-tolerance by preventing T cell hyper-activation and promoting T cell anergy. In this review, we discuss the roles of DAG-mediated pathways, PA-effectors, and DGKs in T cell development and function. We also highlight recent work that has uncovered previously unappreciated roles for DGK activity, for instance in invariant NKT cell development, anti-tumor and anti-viral CD8 responses, and the directional secretion of soluble effectors.

Elimination of T cell reactivity to pancreatic β cells and partial preservation of β cell activity by peptide blockade of LFA-1:ICAM-1 interaction in the NOD mouse model

In insulin dependent diabetes mellitus (T1D), self-reactive T cells infiltrate pancreatic islets and induce beta cell destruction and dysregulation of blood glucose. A goal is to control only the self-reactive T cells, leaving the remainder of the T cell population free to protect the host. One approach is blockade of the second signal for T cell activation while allowing the first (antigen-specific) signal to occur. This work proposes that small peptides that block interaction of second signals delivered through the counter receptors LFA-1:ICAM-1 will induce attacking T cells (receiving the antigen signal) to become anergic or undergo apoptosis. In NOD mice, the peptides eliminated T cell reactivity against pancreatic antigens and reduced cellular infiltration into islets, which retained stronger density of insulin staining at five weeks after cessation of therapy. In in vitro studies the peptides induced nonresponsiveness during activation of T cells from mice and from human peripheral blood.

T cell Receptor Signal Transduction in T lymphocytes

The T cell receptor (TCR) recognizes self or foreign antigens presented by major histocompatibility complex (MHC) molecules. Engagement of the TCR triggers the formation of multi-molecular signalosomes that lead to the generation of second messengers and subsequent activation of multiple distal signaling cascades, such as the Ca⁺²-calcineurin-NFAT, RasGRP1-Ras-Erk1/2, PKCθ-IKK-NFκB, and TSC1/2-mTOR pathways. These signaling cascades control many aspects of T cell biology. Mechanisms have been evolved to fine-tune TCR signaling to maintain T cell homeostasis and self-tolerance, and to properly mount effective responses to microbial infection. Defects or deregulation of TCR signaling has been implicated in the pathogenesis of multiple human diseases.