The specificity of T cell regulation that enables self-nonself discrimination in the periphery.

It was recently shown that perceiving the avidity of T cell activation can be translated into peripheral T cell regulation to control autoimmune disease. This regulation is achieved by CD8(+) T cells that recognize a common surrogate target structure, Qa-1/Hsp60sp, preferentially expressed by activated T cells of intermediate but not high avidity. A truncated self-reactive repertoire, devoid of high-avidity T cells, generated by thymic negative selection, allows selective down-regulation of intermediate-avidity T cells to accomplish self-nonself discrimination in the periphery. Identification of the common surrogate target structure expressed on intermediate-avidity T cells opens up a conceptual theme to understand the relationship between the specificity of peripheral immune regulation and self-nonself discrimination. Here, we investigated peptide vaccination induced cross-protection mediated by CD8(+) T cells in two autoimmune disease models, experimental allergic encephalomyelitis (EAE) and type 1 diabetes (T1D). We show that Qa-1 restricted CD8(+) T cells cross-protect animals from either EAE or T1D without abrogating the immune response to foreign antigens. Cross-protection occurs because potentially pathogenic self-reactive T cells included in the pool of intermediate-avidity T cells are capable of preferentially expressing common surrogate target structures on their surface to render themselves subject to the down-regulation, independent of the specificity of the antigens that they are triggered by. Thus, like in the thymus, the immune system discriminates self from nonself, during adaptive immunity in the periphery, not by recognizing the structural differences between self and foreign antigens, but rather by perceiving the avidity of T cell activation.

How the immune system achieves self-nonself discrimination during adaptive immunity.

We propose an "Avidity Model of Self-Nonself Discrimination" in which self-nonself discrimination is achieved by both central thymic selection and peripheral immune regulation. The conceptual framework that links these two events is the understanding that both in the thymus and in the periphery the survival or the fate of T cells is determined by the avidity of the interactions between T cell receptors (TCRs) on T cells, specific to any antigens and MHC/antigen peptides presented by antigen-presenting cells (APCs). We envision that the immune system achieves self-nonself discrimination, during adaptive immunity, not by recognizing the structural differences between self versus foreign antigens, but rather by perceiving the avidity of T cell activation. Intrathymic deletion of high avidity T cell clones responding to the majority of self-antigens generates a truncated peripheral self-reactive repertoire composed of mainly intermediate and low but devoid of high avidity T cells compared with the foreign-reactive repertoire. The existence of intermediate avidity self-reactive T cells in the periphery represents a potential danger of pathogenic autoimmunity inherited in each individual because potentially pathogenic self-reactive T cells are included in the pool of intermediate avidity T cells and can often be functionally activated to elicit autoimmune diseases. The distinct composition of peripheral T cell repertoires to self versus to foreign antigens provides a unique opportunity for the immune system to discriminate self from nonself, in the periphery, by selectively downregulating intermediate avidity T cells to both self and foreign antigens. Selective downregulation of the intermediate avidity T cell populations containing the potentially pathogenic self-reactive T cells enables the immune system to specifically control autoimmune diseases without damaging the effective anti-infection immunity, which is, largely, mediated by high avidity T cells specific to the infectious pathogens. In this regard, it has been recently shown that Qa-1-restricted CD8(+) T cells selectively downregulate intermediate avidity T cells, to both self and foreign antigens, and as a consequence, specifically dampen autoimmunity yet optimize the immune response to foreign antigens. Selective downregulation of intermediate avidity T cells is accomplished via specific recognition, by the Qa-1-restricted CD8(+) T cells, of particular Qa-1/self-peptide complexes, such as Qa-1/Hsp60sp, which function as a common surrogate target structure and preferentially expressed on the activated intermediate avidity T cells. This regulatory pathway thus represents one example of the peripheral mechanisms that the immune system evolved to complete self-nonself discrimination that is achieved, imperfectly, by thymic negative selection, in order to maintain self-tolerance. The conceptual framework of the "Avidity Model" differs from, but contains intellectual wisdom of certain conceptual elements of, the "Tunable Activation Thresholds Hypothesis," the "Danger Model," and the "Ergotypic Regulation Phenomenon." It provides a unified and simple paradigm to explain various seemingly unrelated biomedical problems inherent in immunological disorders that cannot be uniformly interpreted by any currently existing paradigms. The potential impact of the conceptual framework of the "Avidity Model" on our understanding of the development and control of commonly seen autoimmune diseases is also discussed.

Suppression of experimental autoimmune encephalomyelitis by selective blockade of encephalitogenic T-cell infiltration of the central nervous system.

Multiple sclerosis (MS) is a devastating neuroinflammatory disorder of the central nervous system (CNS) in which T cells that are reactive with major components of myelin sheaths have a central role. The receptor for advanced glycation end products (RAGE) is present on T cells, mononuclear phagocytes and endothelium. Its pro-inflammatory ligands, S100-calgranulins, are upregulated in MS and in the related rodent model, experimental autoimmune encephalomyelitis (EAE). Blockade of RAGE suppressed EAE when disease was induced by myelin basic protein (MBP) peptide or encephalitogenic T cells, or when EAE occurred spontaneously in T-cell receptor (TCR)-transgenic mice devoid of endogenous TCR-alpha and TCR-beta chains. Inhibition of RAGE markedly decreased infiltration of the CNS by immune and inflammatory cells. Transgenic mice with targeted overexpression of dominant-negative RAGE in CD4+ T cells were resistant to MBP-induced EAE. These data reinforce the importance of RAGE-ligand interactions in modulating properties of CD4+ T cells that infiltrate the CNS.

Perceiving the avidity of T cell activation can be translated into peripheral T cell regulation.

The structure recognized by regulatory T cells that enables them to discriminate self from nonself in the periphery is one of the central issues of regulatory T cell biology. A link between immunoregulation and self-nonself discrimination has emerged from experiments showing that Qa-1-restricted CD8(+) T cells selectively down-regulate target T cells activated by the intermediate avidity of their own T cell antigen receptor-ligand interactions. Because the peripheral self-reactive T cell repertoire is devoid of high-avidity T cells compared with the foreign-reactive repertoire, as a result of thymic negative selection, the selective down-regulation of intermediate but not high-avidity T cells enables the immune system to suppress autoimmunity without damaging the ongoing immune response to foreign pathogens. However, the molecular mechanism delineating how avidity of T cell activation is perceived by the regulatory T cells has not been elucidated. Here we show that a heat shock peptide (Hsp60sp), coupled with the MHC class Ib molecule Qa-1, is a surrogate target structure that is preferentially expressed at a higher level on the intermediate avidity T cells and specifically recognized by the Qa-1-restricted CD8(+) T cells. The biological significance of this observation was confirmed by the ability of Hsp60sp-loaded relevant dendritic cells to induce a Qa-1-restricted CD8(+) T cell-mediated protection from autoimmune encephalopathy in the experimental allergic encephalomyelitis model. Thus, perceiving the avidity of T cell activation can be translated into peripheral T cell regulation to discriminate self from nonself in the periphery to maintain self-tolerance.

An affinity/avidity model of peripheral T cell regulation.

We show in these studies that Qa-1-dependent CD8+ T cells are involved in the establishment and maintenance of peripheral self tolerance as well as facilitating affinity maturation of CD4+ T cells responding to foreign antigen. We provide experimental evidence that the strategy used by the Qa-1-dependent CD8+ T cells to accomplish both these tasks in vivo is to selectively downregulate T cell clones that respond to both self and foreign antigens with intermediate, not high or low, affinity/avidity. Thus, the immune system evolved to regulate peripheral immunity using a unified mechanism that efficiently and effectively permits the system to safeguard peripheral self tolerance yet promote the capacity to deal with foreign invaders.

Qa-1/HLA-E-restricted regulatory CD8+ T cells and self-nonself discrimination: an essay on peripheral T-cell regulation.

By discriminating self from nonself and controlling the magnitude and class of immune responses, the immune system mounts effective immunity against virtually any foreign antigens but avoids harmful immune responses to self. These are two equally important and related but distinct processes, which function in concert to ensure an optimal function of the immune system. Immunologically relevant clinical problems often occur because of failure of either process, especially the former. Currently, there is no unified conceptual framework to characterize the precise relationship between thymic negative selection and peripheral immune regulation, which is the basis for understanding self-non-self discrimination versus control of magnitude and class of immune responses. In this article, we explore a novel hypothesis of how the immune system discriminates self from nonself in the periphery during adaptive immunity. This hypothesis permits rational analysis of various seemingly unrelated biomedical problems inherent in immunologic disorders that cannot be uniformly interpreted by any currently existing paradigms. The proposed hypothesis is based on a unified conceptual framework of the "avidity model of peripheral T-cell regulation" that we originally proposed and tested, in both basic and clinical immunology, to understand how the immune system achieves self-nonself discrimination in the periphery.

An integrated view of suppressor T cell subsets in immunoregulation.

The immune system evolved to protect organisms from a virtually infinite variety of disease-causing agents but to avoid harmful responses to self. Because immune protective mechanisms include the elaboration of potent inflammatory molecules, antibodies, and killer cell activation--which together can not only destroy invading microorganisms, pathogenic autoreactive cells, and tumors, but also mortally injure normal cells--the immune system is inherently a "double-edged sword" and must be tightly regulated. Immune response regulation includes homeostatic mechanisms intrinsic to the activation and differentiation of antigen-triggered immunocompetent cells and extrinsic mechanisms mediated by suppressor cells. This review series will focus on recent advances indicating that distinct subsets of regulatory CD4+ and CD8+ T cells as well as NK T cells control the outgrowth of potentially pathogenic antigen-reactive T cells and will highlight the evidence that these suppressor T cells may play potentially important clinical roles in preventing and treating immune-mediated disease. Here we provide a historical overview of suppressor cells and the experimental basis for the existence of functionally and phenotypically distinct suppressor subsets. Finally, we will speculate on how the distinct suppressor cell subsets may function in concert to regulate immune responses.

Expression of the alpha1beta1 integrin, VLA-1, marks a distinct subset of human CD4+ memory T cells.

The alpha1beta1 integrin, very late antigen-1 (VLA-1), is a collagen receptor expressed in many CD4+ T cells localizing to inflamed tissues. Here we show that the expression of VLA-1 is a stable marker of a distinct subset of CD4+ memory T cells. Thus, in human peripheral blood lymphocytes (PBLs), approximately 1-4% of the CD4+ T cells express VLA-1, and following T cell receptor activation ex vivo, the percentage of VLA-1+ cells increases within the CD45RO+ population. Importantly, the activated VLA-1+ and VLA-1- cells can be isolated and maintained in culture as phenotypically stable subsets. Functionally, CD4+ memory T cells, operationally defined as the cells that divide rapidly following stimulation with a recall antigen, are highly enriched for VLA-1+ cells. Moreover, depletion of the small fraction of VLA-1+ cells present in CD4+ PBLs prior to stimulation significantly abrogated the proliferative response to recall antigens. Notably, the VLA-1+ cells in fresh CD4+ PBLs are composed of resting CD45RO+/RA-, CCR7-, CD62L+, CD25-, and VLA-4hi cells. Interestingly, this VLA-1+ subset is enriched for Th1-type cells, and Th1-polarizing conditions during T cell activation favor the emergence of VLA-1+ cells. Thus, VLA-1 expression is a stable marker of a unique subset of human memory CD4+ T cells that predominantly differentiates into Th1 cells.

Characterizing the circulating, gliadin-specific CD4+ memory T cells in patients with celiac disease: linkage between memory function, gut homing and Th1 polarization.

Celiac disease (CD) is a chronic, immune-mediated disorder of the gut, driven by T cells reacting locally to a distinct antigen, gliadin. Thus, CD offers the opportunity to study the T cell memory response to gliadin and whether gut tropism and T helper cell type 1 (Th1) polarization, which characterize the effector phase, are preserved in the memory progeny. It is notable that previous studies yielded conflicting results as to the presence of gliadin-specific memory CD4+ T cells in the peripheral blood of CD patients. However, we used a different and highly sensitive approach based on fluorescein-derived label dilution, whereby the memory cells are identified operationally by their greater capacity to proliferate upon re-encounter with antigen. Thus, using flow cytometry, we could resolve multiple successive generations as well as immunophenotype the dividing cells. Here, we show that the peripheral blood lymphocyte of some CD patients on a gliadin-free diet, but not healthy donors, contains a detectable population of CD4+ memory T cells specific for deamidated gliadin. Moreover, these gliadin-specific memory T cells are marked by a distinctive phenotype: They express high levels of the gut-homing beta7 integrins and primarily produce interferon-gamma and tumor necrosis factor alpha. We conclude that memory for gliadin-derived antigens within the circulating CD4+ T cells is linked with gut tropism as well as Th1 polarization.

Suppression of murine cardiac allograft arteriopathy by long-term blockade of CD40-CD154 interactions.

BACKGROUND: The interaction between CD40 on antigen-presenting cells and CD40L on T cells is critical in allograft rejection. CD154 blockade suppresses allograft rejection, but the role of this pathway in allograft vasculopathy remains obscure. METHODS AND RESULTS: A vascularized murine heterotopic cardiac transplant model was used to test whether perioperative CD154 blockade suppresses allograft vasculopathy or whether long-term CD154 blockade is required to suppress allograft vasculopathy. Perioperative CD154 blockade consisted of MR1 given on days -1, 1, and 3; long-term blockade consisted of MR1 given on days -1, 1, and 3 and continued twice weekly for 8 weeks. Allografts treated with perioperative or long-term CD154 blockade survived indefinitely. Perioperative and long-term treatment with control antibody (Ha4/8) resulted in uniform early rejection. Perioperative CD154 blockade transiently reduced early T-cell and macrophage infiltration in parallel with a transient reduction in endothelial adhesion receptor expression. Although perioperative CD154 blockade prevented allograft failure, it did not reduce allograft vasculopathy; mean neointimal cross-sectional area in perioperative MR1-treated and Ha4/8-treated recipients was 43+/-7% and 50+/-12%, respectively (P=NS). In contrast, mean neointimal cross-sectional area in long-term, MR1-treated recipients was 19+/-3% (P<0.001 versus perioperative MR1). Long-term CD154 blockade also suppressed endothelial E-selectin, P-selectin, and intracellular adhesion molecule-1 expression and improved graft function 3.5-fold versus control (P<0.05). CONCLUSIONS: These data show that perioperative CD154 blockade mitigates acute rejection but long-term CD154 blockade may result in decreased allograft endothelial activation and is required to suppress allograft arteriopathy.

HLA-E-restricted regulatory CD8(+) T cells are involved in development and control of human autoimmune type 1 diabetes.

A key feature of the immune system is its ability to discriminate self from nonself. Breakdown in any of the mechanisms that maintain unresponsiveness to self (a state known as self-tolerance) contributes to the development of autoimmune conditions. Recent studies in mice show that CD8(+) T cells specific for the unconventional MHC class I molecule Qa-1 bound to peptides derived from the signal sequence of Hsp60 (Hsp60sp) contribute to self/nonself discrimination. However, it is unclear whether they exist in humans and play a role in human autoimmune diseases. Here we have shown that CD8(+) T cells specific for Hsp60sp bound to HLA-E (the human homolog of Qa-1) exist and play an important role in maintaining peripheral self-tolerance by discriminating self from nonself in humans. Furthermore, in the majority of type 1 diabetes (T1D) patients tested, there was a specific defect in CD8(+) T cell recognition of HLA-E/Hsp60sp, which was associated with failure of self/nonself discrimination. However, the defect in the CD8(+) T cells from most of the T1D patients tested could be corrected in vitro by exposure to autologous immature DCs loaded with the Hsp60sp peptide. These data suggest that HLA-E-restricted CD8(+) T cells may play an important role in keeping self-reactive T cells in check. Thus, correction of this defect could be a potentially effective and safe approach in the therapy of T1D.

Lymphocytes expressing alpha1beta1 integrin (very late antigen-1) in peripheral blood of patients with arthritis are a subset of CD45RO(+) T-cells primed for rapid adhesion to collagen IV.

We report that very late antigen-1 (VLA-1(+)) CD3(+)CD45RO(+) T-cells are selectively segregated from VLA-1(-) peripheral blood (PB) mononuclear cells (MC), in which CD3(+) T-cells are evenly CD45RO(+) and CD45RO(-), when PBMC are stained with a monoclonal antibody (mAb) to VLA-1 and passaged on immunomagnetic columns. In contrast, both VLA-1(+) and VLA-1(-) MC isolated from synovial fluid (SF) are mainly CD45RO(+)CD3(+) T-cells. VLA-1(+) MC formed 13 +/- 5.3% of MC eluting from columns loaded with PBMC of patients with seropositive rheumatoid arthritis (n = 6) and 2.3 +/- 1.6% of patients (n = 4) with other arthritides (P < 0.022). Importantly, only the VLA-1(+) MC from PB and SF adhered to collagen IV upon triggering with phorbol 12-myristate 13-acetate. Moreover, adhesion and migration on collagen IV were preferentially maintained in lines cultured from VLA-1(+) T-cells, and both were inhibited by mAb to the VLA-1 alpha1 I domain. These results suggest that VLA-1(+) CD45RO(+) T-cells in patients with arthritis could play a role in both systemic and local inflammation by rapidly adhering to collagen IV.

Lovastatin and phospholipase Cgamma regulate constitutive and protein kinase C dependent integrin mediated interactions of human T-cells with collagen.

We previously reported that human interleukin (IL)-2 dependent T cell lines derived from very late antigen (VLA)-1(+) CD45RO(+) peripheral blood (PB) T-cells adhere constitutively to collagen type IV, whereas lines from VLA-1(-) PB lymphocytes (L) adhere weakly. Here we report that the latter are induced to adhere by phorbol 12-myristate 13-acetate (PMA). Both PMA dependent and constitutive adhesion, including that of a Herpes Virus Saimiri (HVS) infected CD4(+)VLA-1(+) clone (HVST) were inhibited by anti-VLA-1 monoclonal antibodies (mAb), by inhibitors of phospholipase C (PLC)gamma and by lovastatin but not by a MEK1 inhibitor, whereas only PMA induced adhesion was blocked by inhibition of protein-kinase (PK) C. Furthermore, lovastatin enhanced PLCgamma and anti VLA-1 mAb blockade, and its effect was not reversed by mevalonic acid (MVA). Lovastatin also inhibited interferon (IFN)gamma secretion by T cells triggered with anti-CD3 and in cells detaching from collagen IV. These results suggest new ways for functional modulation of activated T-cells interacting with collagen.