Defective positive selection results in T cell lymphopenia and increased autoimmune diabetes in ADAP-deficient BDC2.5-C57BL/6 mice

Thymosin Beta 15 Alters the Spatial Development of Thymic Epithelial Cells

The thymus is the most sensitive organ under various pathophysiological conditions, such as aging, starvation, and infection. As a key stromal cell for T cell development, it is well-known that thymic epithelial cells (TECs) play an important role in the thymus response to the external environment. Thymosin beta 15 (Tβ15) is a G-actin binding protein secreted by TECs, it plays an important role in maintaining the dynamic balance of actin, angiogenesis, axonal formation, and wound healing, but the relationship between Tβ15 and TECs is not clear yet. Here, we show the impact of Tβ15 on the TEC's spatial development, as well as the T-cell differentiation and thymic output. As a result, TEC is the main effector cell of Tβ15 in the thymus. Tβ15 OX inhibits the chemotaxis of TECs to the medulla and subsequently blocks the positive selection of thymocytes from CD3⁺TCRβ⁺CD4⁺CD8⁺ double positive cells to CD3⁺TCRβ⁺CD4⁺CD8^- single-positive (CD4SP) cells. Tβ15-knockdown accelerates the reticular differentiation of astral TECs and medullary TECs. Importantly, mice implanted with Tβ15-knockdown iTECs show high thymic output but low peripheral T cell maturity and activity. In a word, our results explain the role of Tβ15 on the differentiation and function of TECs and provide a new perspective for understanding the process of thymus development and degeneration.

Lymphopenia, Lymphopenia-Induced Proliferation, and Autoimmunity

Immune homeostasis is a tightly regulated system that is critical for defense against invasion by foreign pathogens and protection from self-reactivity for the survival of an individual. How the defects in this system might result in autoimmunity is discussed in this review. Reduced lymphocyte number, termed lymphopenia, can mediate lymphopenia-induced proliferation (LIP) to maintain peripheral lymphocyte numbers. LIP not only occurs in normal physiological conditions but also correlates with autoimmunity. Of note, lymphopenia is also a typical marker of immune aging, consistent with the fact that not only the autoimmunity increases in the elderly, but also autoimmune diseases (ADs) show characteristics of immune aging. Here, we discuss the types and rates of LIP in normal and autoimmune conditions, as well as the coronavirus disease 2019 in the context of LIP. Importantly, although the causative role of LIP has been demonstrated in the development of type 1 diabetes and rheumatoid arthritis, a two-hit model has suggested that the factors other than lymphopenia are required to mediate the loss of control over homeostasis to result in ADs. Interestingly, these factors may be, if not totally, related to the function/number of regulatory T cells which are key modulators to protect from self-reactivity. In this review, we summarize the important roles of lymphopenia/LIP and the Treg cells in various autoimmune conditions, thereby highlighting them as key therapeutic targets for autoimmunity treatments.

MicroRNA-146a Overexpression Impairs the Positive Selection during T Cell Development

MicroRNAs play crucial roles in modulating immune system. miR-146a, a potent feedback suppressor of NF-κB signaling, was shown to limit the innate immune response and myelopoiesis in a knockout mouse model. Here, we observed high lymphopoiesis demonstrated as mild splenomegaly and severe lymphadenopathy in a miR-146a transgenic mouse model. Overexpression of miR-146a resulted in enhanced proliferation and reduced apoptosis of T cells. More activated CD4⁺ T cells or effector memory T cells were observed in transgenic mice even under physiological conditions. Importantly, as one of the key steps to generate central tolerance, the positive selection of thymocytes is impaired in transgenic mice, resulting in more CD4⁺CD8⁺ double-positive thymocytes but fewer CD4⁺CD8⁻ and CD4⁻CD8⁺ single-positive thymocytes. The maturation of selected CD4⁻CD8⁺ thymocytes was also impaired, leading to more severe loss of CD4⁻CD8⁺ than CD4⁺CD8⁻ thymocytes in thymus of transgenic mice. Gene expression profiling analysis identified nine positive selection-associated genes, which were downregulated in transgenic mice, including genes encoding major histocompatibility complex class I/II molecules, IL-7 receptor α chain, and Gimap4, whose downregulation may contribute to the impairment of positive selection. Gimap4 was verified as a novel target of miR-146a. These findings further extend our understanding of the function of miR-146a in T cell biology and identify a novel regulatory mechanism underlying the positive selection during T cell development.

Mechanisms of diabetic autoimmunity: I--the inductive interface between islets and the immune system at onset of inflammation

The mechanisms of autoimmune reactivity onset in type 1 diabetes (T1D) remain elusive despite extensive experimentation and discussion. We reconsider several key aspects of the early stages of autoimmunity at four levels: islets, pancreatic lymph nodes, thymic function and peripheral immune homeostasis. Antigen presentation is the islets and has the capacity to provoke immune sensitization, either in the process of physiological neonatal β cell apoptosis or as a consequence of cytolytic activity of self-reactive thymocytes that escaped negative regulation. Diabetogenic effectors are efficiently expanded in both the islets and the lymph nodes under conditions of empty lymphoid niches during a period of time coinciding with a synchronized wave of β cell apoptosis surrounding weaning. A major drive of effector cell activation and expansion is inherent peripheral lymphopenia characteristic of neonates, though it remains unclear when is autoimmunity triggered in subjects displaying hyperglycemia in late adolescence. Our analysis suggests that T1D evolves through coordinated activity of multiple physiological mechanisms of stimulation within specific characteristics of the neonate immune system.

Mechanisms of diabetic autoimmunity: II--Is diabetes a central or peripheral disorder of effector and regulatory cells?

Two competing hypotheses aiming to explain the onset of autoimmune reactions are discussed in the context of genetic and environmental predisposition to type 1 diabetes (T1D). The first hypothesis has evolved along characterization of the mechanisms of self-discrimination and attributes diabetic autoimmunity to escape of reactive T cells from central regulation in the thymus. The second considers frequent occurrence of autoimmune reactions within the immune homunculus, which are adequately suppressed by regulatory T cells originating from the thymus, and occasionally, insufficient suppression results in autoimmunity. Besides thymic dysfunction, deregulation of both effector and suppressor cells can in fact result from homeostatic aberrations at the peripheral level during initial stages of evolution of adaptive immunity. Pathogenic cells sensitized in the islets are efficiently expanded in the target tissue and pancreatic lymph nodes of lymphopenic neonates. In parallel, the same mechanisms of peripheral sensitization contribute to tolerization through education of naïve/effector T cells and expansion of regulatory T cells. Experimental evidence presented for each individual mechanism implies that T1D may result from a primary effector or suppressor immune abnormality. Disturbed self-tolerance leading to T1D may well result from peripheral deregulation of innate and adaptive immunity, with variable contribution of central thymic dysfunction.

The Immune Adaptor ADAP Regulates Reciprocal TGF-β1-Integrin Crosstalk to Protect from Influenza Virus Infection

Highly pathogenic avian influenza virus (HPAI, such as H5N1) infection causes severe cytokine storm and fatal respiratory immunopathogenesis in human and animal. Although TGF-β1 and the integrin CD103 in CD8+ T cells play protective roles in H5N1 virus infection, it is not fully understood which key signaling proteins control the TGF-β1-integrin crosstalk in CD8+ T cells to protect from H5N1 virus infection. This study showed that ADAP (Adhesion and Degranulation-promoting Adapter Protein) formed a complex with TRAF6 and TAK1 in CD8+ T cells, and activated SMAD3 to increase autocrine TGF-β1 production. Further, TGF-β1 induced CD103 expression via an ADAP-, TRAF6- and SMAD3-dependent manner. In response to influenza virus infection (i.e. H5N1 or H1N1), lung infiltrating ADAP-/- CD8+ T cells significantly reduced the expression levels of TGF-β1, CD103 and VLA-1. ADAP-/- mice as well as Rag1-/- mice receiving ADAP-/- T cells enhanced mortality with significant higher levels of inflammatory cytokines and chemokines in lungs. Together, we have demonstrated that ADAP regulates the positive feedback loop of TGF-β1 production and TGF-β1-induced CD103 expression in CD8+ T cells via the TβRI-TRAF6-TAK1-SMAD3 pathway and protects from influenza virus infection. It is critical to further explore whether the SNP polymorphisms located in human ADAP gene are associated with disease susceptibility in response to influenza virus infection.

T cell-independent modulation of experimental autoimmune encephalomyelitis in ADAP-deficient mice

The adhesion- and degranulation-promoting adaptor protein (ADAP), expressed in T cells, myeloid cells, and platelets, is known to regulate receptor-mediated inside-out signaling leading to integrin activation and adhesion. In this study, we demonstrate that, upon induction of active experimental autoimmune encephalomyelitis (EAE) by immunization with the myelin oligodendrocyte glycoprotein35-55 peptide, ADAP-deficient mice developed a significantly milder clinical course of EAE and showed markedly less inflammatory infiltrates in the CNS than wild-type mice. Moreover, ADAP-deficient recipients failed to induce EAE after adoptive transfer of myelin oligodendrocyte glycoprotein-specific TCR-transgenic T cells (2D2 T cells). In addition, ex vivo fully activated 2D2 T cells induced significantly less severe EAE in ADAP-deficient recipients. The ameliorated disease in the absence of ADAP was not due to expansion or deletion of a particular T cell subset but rather because of a strong reduction of all inflammatory leukocyte populations invading the CNS. Monitoring the adoptively transferred 2D2 T cells over time demonstrated that they accumulated within the lymph nodes of ADAP-deficient hosts. Importantly, transfer of complete wild-type bone marrow or even bone marrow of 2D2 TCR-transgenic mice was unable to reconstitute EAE in the ADAP-deficient animals, indicating that the milder EAE was dependent on (a) radio-resistant nonhematopoietic cell population(s). Two-photon microscopy of lymph node explants revealed that adoptively transferred lymphocytes accumulated at lymphatic vessels in the lymph nodes of ADAP-deficient mice. Thus, our data identify a T cell-independent mechanism of EAE modulation in ADAP-deficient mice.

The adapter protein ADAP is required for selected dendritic cell functions

Background

The cytosolic adaptor protein ADAP (adhesion and degranulation promoting adapter protein) is expressed by T cells, natural killer cells, myeloid cells and platelets. ADAP is involved in T-cell-receptor-mediated inside-out signaling, which leads to integrin activation, adhesion and reorganization of the actin cytoskeleton. However, little is known about the role of ADAP in myeloid cells. In the present study, we analyzed the function of ADAP in bone-marrow-derived dendritic cells (BMDCs) from ADAP-deficient mice.

Results

ADAP-deficient BMDCs showed almost normal levels of antigen uptake, adhesion, maturation, migration from the periphery to the draining lymph nodes, antigen-specific T-cell activation, and production of the proinflammatory cytokines IL-6 and TNF-∝. Furthermore, we provide evidence that the activation of signaling pathways after lipopolysaccharide (LPS) stimulation are not affected by the loss of ADAP. In contrast, ADAP-deficient BMDCs showed defects in CD11c-mediated cellular responses, with significantly diminished production of IL-6, TNF-∝ and IL-10. Actin polymerization was enhanced after CD11c integrin stimulation.

Conclusions

In summary, we propose that the adapter molecule ADAP is critical for selected CD11c integrin-mediated functions of dendritic cells.

Immunosuppressive therapy exacerbates autoimmunity in NOD mice and diminishes the protective activity of regulatory T cells

Mounting evidence indicates that immunosuppressive therapy and autologous bone marrow transplantation are relatively inefficient approaches to treat autoimmune diabetes. In this study we assessed the impact of immunosuppression on inflammatory insulitis in NOD mice, and the effect of radiation on immunomodulation mediated by adoptive transfer of various cell subsets. Sublethal radiation of NOD females at the age of 14 weeks (onset of hyperglycemia) delayed the onset of hyperglycemia, however two thirds of the mice became diabetic. Adoptive transfer of splenocytes into irradiated NON and NOD mice precipitated disease onset despite increased contents of CD25(+)FoxP3(+) T cells in the pancreas and regional lymphatics. Similar phenotypic changes were observed when CD25(+) T cells were infused after radiation, which also delayed disease onset without affecting its incidence. Importantly, irradiation increased the susceptibility to diabetes in NOD and NON mice (71-84%) as compared to immunomodulation with splenocytes and CD25(+) T cells in naïve recipients (44-50%). Although irradiation had significant and durable influence on pancreatic infiltrates and the fractions of functional CD25(+)FoxP3(+) Treg cells were elevated by adoptive cell transfer, this approach conferred no protection from disease progression. Irradiation was ineffective both in debulking of pathogenic clones and in restoring immune homeostasis, and the consequent homeostatic expansion evolves as an unfavorable factor in attempts to restore self-tolerance and might even provoke uncontrolled proliferation of pathogenic clones. The obstacles imposed by immunosuppression on abrogation of autoimmune insulitis require replacement of non-specific immunosuppressive therapy by selective immunomodulation that does not cause lymphopenia.

Analyses of T cell phenotype and function reveal an altered T cell homeostasis in systemic sclerosis. Correlations with disease severity and phenotypes

We investigated in systemic sclerosis (SSc) patients the T cell homeostasis and its relationship with the clinical course of the disease. Distribution of peripheral T cell subsets, thymic output, lymphocyte proliferation and apoptosis were analyzed by flow cytometry or ELISA. Age inappropriate levels of naive CD4(+) T cells and thymic output were observed. Proliferation of CD4(+) T cells, lymphocyte apoptosis and CD4(+) regulatory T (Treg) cell frequency were significantly higher than those observed in controls and significantly correlated with clinical phenotypes and clinical progression parameters i.e., diffusing capacity of the lung for carbon monoxide (DLCO) and disease activity. These data indicate that the evaluation of the T cell homeostasis can represent a valuable prognostic tool for SSc patients and it is useful to distinguish between limited and diffuse phenotypes. A therapeutic intervention targeted at reversing T cell homeostasis abnormalities would therefore potentially be helpful in counteracting disease progression.

Lymphocyte proliferation in immune-mediated diseases

Defects in T cell homeostatic mechanisms can result in T cell lymphopenia, defined as decreased numbers of lymphocytes. Lymphopenia results in homeostatic proliferation in order to maintain T cell homeostasis. It has been proposed that homeostatic proliferation can expand the pool of autoreactive T cells that promote autoimmunity, and indeed recent studies have further substantiated this observation in both animal models and humans. Conversely, homeostatic proliferation can promote tumor immunity by allowing tumor-specific T cells to accumulate. In this review, we discuss how the outcome of homeostatic proliferation can function both in a deleterious manner in autoimmunity and a beneficial way in tumor immunity. We also discuss the roles of various cytokines and T regulatory cells that control homeostatic proliferation.