组织特异抗原异位表达与免疫耐受

诱导和维持T细胞耐受是免疫系统区分自我和非我的关键。自身免疫调节因子(autoimmune regulator,AIRE)作为转录因子,在胸腺髓质上皮细胞中可驱动一系列组织特异抗原基因的表达,从而在诱导中枢免疫耐受的过程中发挥重要作用。外周免疫耐受的机制复杂一些,清除耐受是其重要机制之一。外周淋巴结的基质细胞可表达部分组织特异抗原,递呈给T淋巴细胞,激活并最终清除它。中枢免疫耐受和外周免疫耐受机制可清除潜在的自身反应性T淋巴细胞,维持对自身组织耐受。一旦免疫耐受被打破,将发生自身免疫反应和自身免疫疾病。     

免疫耐受的形成机制

免疫系统如何识别“自己”和“非己”,这是免疫学理论的核心问题。由于在胚胎期自身反应性细胞克隆被排斥,即形成对自身抗原的耐受;但对外源性“非己”成分能产生免疫应答予以清除,此即免疫系统识别“自己”和“非己”的机制。近年来研究证实外周成熟淋巴细胞中存在着自身反应性T细胞,但处于功能失活状态;谓之外周耐受。目前已明确免疫耐受的形成涉及多种机制的参与,包括免疫细胞的相互作用、免疫细胞的分子识别、信号传递、基因表达等不同层次的调节。对免疫耐受机理的研究可为阐明免疫应答及免疫调节的机制提供依据,必将推动免疫学基础理论研究的发展。     

Activation-threshold tuning in an affinity model for the T-cell repertoire

Naive T cells respond to peptides from foreign proteins and remain tolerant to self peptides from endogenous proteins. It has been suggested that self tolerance comes about by a 'tuning' mechanism, i.e. by increasing the T-cell activation threshold upon interaction with self peptides. Here, we explore how such an adaptive mechanism of T-cell tolerance would influence the reactivity of the T-cell repertoire to foreign peptides. We develop a computer simulation model in which T cells are tolerized by increasing their activation-threshold dependent on the affinity with which they see self peptides presented in the thymus. Thus, different T cells acquire different activation thresholds (i.e. different cross-reactivities). In previous mathematical models, T-cell tolerance was deletional and based on a fixed cross-reactivity parameter, which was assumed to have evolved to an optimal value. Comparing these two different tolerance-induction mechanisms, we found that the tuning model performs somewhat better than an optimized deletion model in terms of the reactivity to foreign antigens. Thus, evolutionary optimization of clonal cross-reactivity is not required. A straightforward extension of the tuning model is to delete T-cell clones that obtain a too high activation threshold, and to replace these by new clones. The reactivity of the immune repertoires of such a replacement model is enchanced compared with the basic tuning model. These results demonstrate that activation-threshold tuning is a functional mechanism for self tolerance induction.     

Preferential recognition of self antigens despite normal thymic deletion of CD4(+)CD25(+) regulatory T cells

T cell tolerance to self Ags is in part established in the thymus by induction of apoptosis or anergy of potentially autoreactive thymocytes. Some autospecific T cells nevertheless migrate to peripheral lymphoid organs but are kept under control by the recently identified CD4(+)CD25(+) regulatory T cell subset. Because these cells inhibit autoimmunity more efficiently than useful non-self Ag-specific immune responses, they are probably autospecific, posing important questions as to how they develop in the thymus. In this study we show that significantly more peripheral CD4(+)CD25(+) regulatory T cells recognize self than non-self Ags. However, we also show for a large panel of endogenous superantigens as well as for self peptide/MHC complexes that autospecific CD4(+)CD25(+) thymocyte precursors are normally deleted during ontogeny. Combined, our data firmly establish that the repertoire of regulatory T cells is specifically enriched in autospecific cells despite the fact that their precursors are normally susceptible to thymic deletion.     

Recognition of self and altered self by T cells in autoimmunity and allergy

T cell recognition of foreign peptide antigen and tolerance to self peptides is key to the proper function of the immune system. Usually, in the thymus T cells that recognize self MHC+ self peptides are deleted and those with the potential to recognize self MHC+ foreign peptides are selected to mature. However there are exceptions to these rules. Autoimmunity and allergy are two of the most common immune diseases that can be related to recognition of self. Many genes work together to lead to autoimmunity. Of those, particular MHC alleles are the most strongly associated, reflecting the key importance of MHC presentation of self peptides in autoimmunity. T cells specific for combinations of self MHC and self peptides may escape thymus deletion, and thus be able to drive autoimmunity, for several reasons: the relevant self peptide may be presented at low abundance in the thymus but at high level in particular peripheral tissues; the relevant self peptide may bind to MHC in an unusual register, not present in the thymus but apparent elsewhere; finally the relevant self peptide may be post translationally modified in a tissue specific fashion. In some types of allergy, the peptide+ MHC combination may also be fully derived from self. However the combination in question may be modified by the presence of other ligands, such as small drug molecules or metal ions. Thus these types of allergies may act like the post translationally modified peptides involved some types of autoimmunity.     

Activation of naive CD4+ T cells in vivo by a self-peptide mimic: mechanism of tolerance maintenance and preservation of immunity

Intrathymic selection generates a peripheral repertoire of CD4(+) T cells with receptors that retain low affinity for self-peptide MHC complexes. Despite self-recognition, T cells remain tolerant even in the setting of microbial challenge and resultant costimulatory signals. We demonstrate here a novel mechanism for tolerance maintenance under conditions of self-recognition and strong costimulation. TCR engagement in vivo with a low-avidity peptide, as a mimic of self, provided with poly(I:C) (dsRNA) led to division of naive T cells that was dependent upon costimulatory signals; however, the dividing cells rapidly underwent deletion. By contrast, the surviving cells that were activated as evidenced by up-regulation of CD69 did not become effectors upon restimulation with the same ligand and maintained an effective response against agonist peptide. We suggest TCR engagement with self-peptide MHC complexes promotes tolerance maintenance during pathogen challenge, while preserving efficient reactivity for subsequent encounter with foreign Ags.     

Regulating the discriminatory response to antigen by T-cell receptor

The cell-mediated immune response constitutes a robust host defense mechanism to eliminate pathogens and oncogenic cells. T cells play a central role in such a defense mechanism and creating memories to prevent any potential infection. T cell recognizes foreign antigen by its surface receptors when presented through antigen-presenting cells (APCs) and calibrates its cellular response by a network of intracellular signaling events. Activation of T-cell receptor (TCR) leads to changes in gene expression and metabolic networks regulating cell development, proliferation, and migration. TCR does not possess any catalytic activity, and the signaling initiates with the colocalization of several enzymes and scaffold proteins. Deregulation of T cell signaling is often linked to autoimmune disorders like severe combined immunodeficiency (SCID), rheumatoid arthritis, and multiple sclerosis. The TCR remarkably distinguishes the minor difference between self and non-self antigen through a kinetic proofreading mechanism. The output of TCR signaling is determined by the half-life of the receptor antigen complex and the time taken to recruit and activate the downstream enzymes. A longer half-life of a non-self antigen receptor complex could initiate downstream signaling by activating associated enzymes. Whereas, the short-lived, self-peptide receptor complex disassembles before the downstream enzymes are activated. Activation of TCR rewires the cellular metabolic response to aerobic glycolysis from oxidative phosphorylation. How does the early event in the TCR signaling cross-talk with the cellular metabolism is an open question. In this review, we have discussed the recent developments in understanding the regulation of TCR signaling, and then we reviewed the emerging role of metabolism in regulating T cell function.     

Strong homeostatic TCR signals induce formation of self‐tolerant virtual memory CD8 T cells

Virtual memory T cells are foreign antigen‐inexperienced T cells that have acquired memory‐like phenotype and constitute 10–20% of all peripheral CD8⁺ T cells in mice. Their origin, biological roles, and relationship to naïve and foreign antigen‐experienced memory T cells are incompletely understood. By analyzing T‐cell receptor repertoires and using retrogenic monoclonal T‐cell populations, we demonstrate that the virtual memory T‐cell formation is a so far unappreciated cell fate decision checkpoint. We describe two molecular mechanisms driving the formation of virtual memory T cells. First, virtual memory T cells originate exclusively from strongly self‐reactive T cells. Second, the stoichiometry of the CD8 interaction with Lck regulates the size of the virtual memory T‐cell compartment via modulating the self‐reactivity of individual T cells. Although virtual memory T cells descend from the highly self‐reactive clones and acquire a partial memory program, they are not more potent in inducing experimental autoimmune diabetes than naïve T cells. These data underline the importance of the variable level of self‐reactivity in polyclonal T cells for the generation of functional T‐cell diversity.     

Breaking T cell tolerance with foreign and self co-immunogens. A study of autoimmune B and T cell epitopes of cytochrome c.

The initiation of autoimmune B cell and T cell responses by self Ag or by foreign pathogens (molecular mimics) is not well understood. In the present study, cytochrome c (cyt c) was used as a model autoantigen to investigate how self-proteins are involved in the priming of autoimmune T cell responses. Immunization with foreign cyt c has been extensively analyzed in previous studies as a model for both humoral and cellular immune responses. Mice do not, however, make antibody or T cell responses to immunization with self (mouse) cyt c. In addition, T cell tolerance can be broken by autoreactive B cells that are readily elicited by immunization with cross-reactive foreign cyt c. These immune B cells presumably bind self cyt c and process and present the self Ag to stimulate an autoreactive T cell response. Autoreactive T cell clones derived by this mechanism are all specific for determinants within amino acids 1-80 of the cyt c protein presented by I-Ek. No T cell responses were observed to the carboxyl terminal 81-104 fragment that dominates the response to foreign cyt c. All clones derived in this study are stimulated by a polypeptide encompassing amino acids 54-68 and utilized the V beta 8.2 TCR gene. In contrast, T cells stimulated by foreign cyt c did indeed respond to fragment 81-104 and appear to utilize alternate TCR genes. Our data demonstrate that B cells specific for linear determinants distributed along the entire length of the foreign cyt c molecule can provide the stimulus required for breaking T cell tolerance to self cyt c. The applications of this work to understanding the mechanisms of autoimmune disease are discussed.     

Immunological discrimination between self and non-self by precursor depletion and memory accumulation

We study processes by which T-lymphocytes "learn" to discriminate "self" from "non-self". We show that intrinsic features of the T cell activation and proliferation process are sufficient to tolerize (self) reactive T-lymphocyte clones. Self vs non-self discrimination therefore develops without any down-regulatory (e.g. suppressive) interactions. T-lymphocyte clones will expand by proliferation only if the IL2 concentration is high enough to induce a proliferation rate larger than the rate of cell decay. This concentration is the proliferation threshold. Because effector T cells are short-lived the proliferation threshold must be quite high. Such high numbers of cells producing IL2 are achieved only when sufficient (memory) precursors are activated. Self and non-self antigens differ with respect the number of (memory) precursor cells they accumulate, as a result of two processes, i.e. precursor depletion and memory accumulation, and can thus be discriminated. Precursor depletion: the dynamics of long-lived precursors can cause tolerization. In neonatal circumstances precursor influx is still low, newborn cells reacting with self antigens are immediately activated, generating (few), i.e. fewer than the proliferation threshold, effectors that decay rapidly. Thus total lymphocyte numbers remain low, yielding self tolerance. Conversely, large doses of similar antigens introduced in mature systems push "their" lymphocyte clone over the proliferation threshold because a large (accumulated) precursor population is rapidly activated. Small doses are however low zone tolerized. Memory accumulation: peripheral T-lymphocyte populations in fact consist of a mixture of virgin precursors and memory cells. If the formation process of (long-lived) memory cells is taken into account and virgin precursors are made short-lived, the proliferation threshold again accounts for self non-self discrimination. Memory cells accumulate when antigenic restimulation is low; it is low when the antigen concentration and/or the antigen affinity is low. Therefore self antigens, which are present in relatively high concentrations, fail to accumulate high affinity memory cells, and are hence tolerated. Memory cells crossreacting to self antigens with low affinity, however accumulate neonatally, pushing those clones over the proliferation threshold whenever "their" high affinity antigen enters the immune system. Thus the model generates differences in the antigenicity (i.e. memory precursor frequency) of self and non-self.(ABSTRACT TRUNCATED AT 400 WORDS)     

Critical analysis of the immunological self/non-self model and of its implicit metaphysical foundations

An examination of the concepts used in immunology prompts us to wonder about the origins and the legitimacy of the notions of self and non-self, which constitute the core of the dominant theoretical model in this science. All theoretical reflection concerning immunology must aim at determining a criterion of immunogenicity, that is, an operational definition of the conditions in which an immune reaction occurs or does not occur. By criticizing both conceptually and experimentally the self/non-self vocabulary, we can demonstrate the inaccuracy and even the inadequacy of the dichotomy of self/non-self. Accordingly, the self/non-self model must be reexamined, or even rejected. On the basis of this critique, we can suggest an alternative theoretical hypothesis for immunology, based on the notion of continuity. The 'continuity hypothesis' developed here attempts to give a criterion of immunogenicity that avoids the reproaches leveled at the self model.     

Linked foreign T-cell help activates self-reactive CTL and inhibits tumor growth

Transgenic mice expressing membrane-bound OVA under the rat insulin promoter, RIP-mOVA, has previously been suggested to display deletional tolerance toward the dominant CTL epitope, SIINFEKL, and provide an elegant model system to test the hypothesis that the lack of T cell help contributes to the tolerance. To understand how the CD8 tolerance is maintained in these mice, a set of neo-self-Ags, OVA, modified to contain a foreign Th peptide, were constructed and tested for their ability to induce CTL responses in RIP-mOVA mice. Immunization with these Th peptide-modified OVA molecules and not with the wild-type OVA induced self-reactive CTLs recognizing dominant CTL peptide, SIINFEKL. Importantly, immunization with the modified OVA constructs also prevented the growth of OVA-expressing tumors in transgenic mice. Since endogenous OVA Th peptides did not contribute toward breaking self CTL tolerance, these results also highlighted a very robust CD4 T cell tolerance toward OVA in RIP-mOVA mice that has not been previously described. These results therefore provide direct evidence that it is the tolerance in the CD4 Th cell compartment that helps maintain the CTL tolerance against self-Ag in these mice. Since the CTL tolerance can be broken or bypassed by foreign Th peptides inserted into a self Ag, potential of using this approach in generating effective therapeutic cancer vaccines is discussed.     

Molecular and cellular aspects of immunologic tolerance.

This review seeks to explain the most exciting recent data concerning the nature of self/non-self discrimination by the immune system in a manner accessible to a biochemical readership. The nature of recognition in the two great lymphocyte families, B cells and T cells, is described with special emphasis on the nature of the ligands recognized by each. The history of the field of immunologic tolerance is surveyed, as are the key experiments on conventional mice which provided a conceptual framework. This suggested that tolerance was essentially due to 'holes' in the recognition repertoires of both the T and B cell populations so that lymphocytes competent to react to self antigens were not part of the immunologic dictionary. There were essentially two ways to achieve this situation. On the one hand, self antigens might 'catch' developing lymphocytes early in their ontogeny and delete the cell, a process of clonal abortion. On the other hand, self antigens might signal lymphocytes (particularly immature cells) in a negative manner, reducing or abolishing their capacity for later responses, without causing death. This process is referred to as clonal anergy. Evidence for both processes exists. Special emphasis is placed on a wave of experimentation beginning in 1988 which imaginatively uses transgenic mouse technology to study tolerance. Transgenic manipulations can produce mice which synthesize foreign antigens in a constitutive and/or inducible manner, sometimes only in specific locations; mice which possess T or B lymphocytes almost all expressing a given receptor of known specificity; and mice which are an immunologic time bomb in that the antigen is present and so too are lymphocytes all endowed with receptors for that antigen. These experiments have vindicated the possibility of both clonal abortion and clonal anergy in both T and B cell populations, the choice of which phenomenon occurs depending on a number of operational circumstances. For T cell tolerance, clonal abortion occurs if the self antigenic determinant concerned is present within the thymus; if not, clonal anergy is more likely. For B cell tolerance, the strength of the negative signal and therefore the choice between abortion and anergy depends on the molar concentration of the self antigen, the capacity for multivalent presentation to a B cell, and the affinity of the B cell's receptor for the antigen in question. Some B cells with low affinity for self antigens certainly escape censorship and remain capable of secreting low affinity anti-self antibodies, which however do no harm.(ABSTRACT TRUNCATED AT 400 WORDS)     

The specific T-cell response to antigenic peptides is influenced by bystander peptides

T lymphocytes recognize antigens in the form of peptides presented by major histocompatibility complex (MHC) molecules on the cell surface. Only a small proportion of MHC class I and class II molecules are loaded with foreign antigenic peptides; the vast majority are loaded with thousands of different self peptides. It was suggested that MHC molecules presenting self peptides may serve either to decrease (antagonistic effect) or increase (synergistic effect) the T cell response to a specific antigen. Here, we present our finding that transfected mouse fibroblasts presenting a single antigenic peptide covalently bound to a class II MHC molecule stimulated specific mouse T cell hybridoma cells to an interleukin-2 response less efficiently than fibroblasts presenting a similar amount of antigenic peptide in the presence of class II molecules loaded with heterogenous bystander peptides.     

The stromal and haematopoietic antigen-presenting cells that reside in secondary lymphoid organs

T cells encounter their cognate antigens in specialized compartments of secondary lymphoid organs (SLOs). There, dendritic cells (DCs) present self and non-self antigens to T cells, and promote immunity or tolerance depending on the availability of danger signals. Resident stromal cells orchestrate the interaction between T cells and DCs by recruiting them to T cell zones and guiding their migration within SLOs. Recent studies have shown that SLO-resident stromal cells also have a crucial role in tolerance induction in the periphery. In this Review, we discuss the roles of SLO-resident DCs and stromal cells in shaping T cell responses.     

Ability of fixed B-lymphoma cells to present foreign protein antigen fragments and allogenic MHC molecules to a cloned helper-T-cell line

Cloned, L3T4+ T cells have been shown to respond to foreign protein antigens in the context of self-Ia glycoproteins and to non-self Ia glycoproteins. In the case of responses to foreign proteins, fixed antigen-presenting cells can present antigen fragments, but cannot present native proteins. Whether fixed allogenic cells can stimulate has been controversial. We have examined this question using a dual-reactive cloned helper-T-cell line. We find that conditions of fixation that block the presentation of native antigen to this cloned line, but which allow the presentation of antigen fragments, also allow presentation of allogeneic Ia molecules, leading to stimulation of the cloned line. This study also revealed an occult alloreactivity in the cloned T-cell line, which was expressed by fixed, but not by normal, antigen-presenting B lymphoma cells. All of these stimuli proceeded via the same clonotypic receptor, as determined by blocking with anti-T-cell receptor monoclonal antibody. These data suggest that responses to non-self Ia glycoproteins involve direct recognition of the allogeneic Ia molecules and do not require processing and presentation of these antigens by self Ia molecules.     

Vigorous allograft rejection in the absence of danger

Tolerance to self is a necessary attribute of the immune system. It is thought that most autoreactive T cells are deleted in the thymus during the process of negative selection. However, peripheral tolerance mechanisms also exist to prevent development of autoimmune diseases against peripheral self-Ags. It has been proposed that T cells develop tolerance to peripheral self-Ags encountered in the absence of inflammation or "danger" signals. We have used immunodeficient Rag 1-/- mice to study the response of T cells to neo-self peripheral Ags in the form of well-healed skin and vascularized cardiac allografts. In this paper we report that skin and cardiac allografts without evidence of inflammation are vigorously rejected by transferred T cells or when recipients are reconstituted with T cells at a physiologic rate by nude bone graft transplantation. These results provide new insights into the role of inflammation or "danger" in the initiation of T cell-dependent immune responses. These findings also have profound implications in organ transplantation and suggest that in the absence of central deletional tolerance, peripheral tolerance mechanisms are not sufficient to inhibit alloimmune responses even in the absence of inflammation or danger.     

Extrathymic tolerance of mature T cells: clonal elimination as a consequence of immunity

The mechanism by which T lymphocytes are tolerized to self or foreign antigens is still controversial. Clonal deletion is the major mechanism of tolerance for immature thymocytes; for mature T cells, tolerance is considered to reflect anergy rather than deletion, and to be a consequence of defective presentation of antigen. This paper documents a novel form of tolerance resulting when mature T cells encounter antigen in immunogenic form. Evidence is presented that exposure of mature T cells to Mlsa antigens in vivo leads to specific tolerance and disappearance of Mlsa-reactive V beta 6+ T cells. Surprisingly, the clonal elimination of V beta 6+ cells is preceded by marked expansion of these cells. Thus, tolerance induction can be the end result of a powerful immune response. These data raise important questions concerning the relationship of tolerance and memory.     

CD4+ CD25+ suppressor T cells: more questions than answers

Several mechanisms control discrimination between self and non-self, including the thymic deletion of autoreactive T cells and the induction of anergy in the periphery. In addition to these passive mechanisms, evidence has accumulated for the active suppression of autoreactivity by a population of regulatory or suppressor T cells that co-express CD4 and CD25 (the interleukin-2 receptor alpha-chain). CD4+ CD25+ T cells are powerful inhibitors of T-cell activation both in vivo and in vitro. The enhancement of suppressor-cell function might prove useful for the treatment of immune-mediated diseases, whereas the downregulation of these cells might be beneficial for the enhancement of the immunogenicity of vaccines that are specific for tumour antigens.