免疫耐受的生理基础

免疫耐受是针对特定抗原(主要为自身抗原)的特异性不应答状态,是免疫系统稳定机体内环境的生理功能基础,其产生和维持的主要机制包括中枢耐受和外周耐受2种类型。其中,中枢耐受主要通过阴性选择的过程将自身高反应性免疫细胞进行克隆清除,其主要发生在胚胎期和新生期的动物体内;外周耐受的机制相对多样而复杂,主要包括克隆清除、克隆无能、免疫忽视状态和免疫隔离部位等多种方式。免疫耐受也会受到调节性T细胞等免疫调节细胞及分子的调控,从而保障免疫系统的正常生理状态。对免疫耐受的生理基础及其相关的进展进行了简要的介绍和总结。     

CD4~+CD25~+ Treg细胞与移植免疫耐受

诱导器官移植受者对供者抗原的免疫耐受是防治同种异型移植排斥反应的最理想途径。目前认为,免疫耐受形成的主要机制包括:胸腺及骨髓阴性选择引起的克隆清除(Clonal deletion)、组织特异性自身抗原低表达引起的克隆忽视(Clonal ignorance)、阻断T细胞共刺激信号引起的克隆无能(Clonal anergy)、嵌合体(Chimerism)的形成、调节性T细胞(Regulatory Tcell,Treg)介导的克隆抑制(Clonal suppression)等。近年来CD4+CD25+ Treg细胞的研究已成为免疫学界的热门课题之一。已知CD4+CD25+ Treg细胞存在于小鼠、大鼠和人体中,是机体自然存在的具有主动调节活性的T细胞,对维持自我耐受和控制自身免疫病发挥着重要作用。本文着重就CD4+CD25+ Treg细胞的免疫调节机制及其在诱导移植免疫耐受方面的研究进展做一综述。     

影响CD4+CD25+T细胞分化发育的细胞分子机制

免疫耐受的精髓即机体对外界病原体抗原产生免疫应答的同时对自身抗原不应答.近两年对CD4 CD25 调节性T细胞(CD4 CD25 regulatory T cell, Treg)所发挥的免疫耐受功能的研究取得了令人瞩目的长足进展,对此群细胞所具有的维持外周免疫耐受的独特地位已无可争议.但调节性T细胞的多种生物学特征特别是Treg细胞分化发育的分子机制与信号需求并不清楚,因此探索有关Treg的发生发育及其影响机制已成为近两年研究Treg细胞的热点.综述最近的相关研究数据,了解胸腺以及外周影响Treg细胞分化发育和功能产生的多种细胞分子机制,有助于进一步研究此群细胞的功能及其在抑制自身免疫性疾病、诱导移植耐受等方面的应用.     

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。     

树突细胞的免疫耐受及其在Ⅰ型糖尿病中的作用

树突细胞(dendritic cells,DCs)通过将抗原提呈给初始T淋巴细胞(native T lymphocyte),从而诱导CD8~+细胞毒性T细胞(cytotoxic T cell,CTL)和CD4~+效应T细胞的分化并启动获得性免疫应答。此外DCs在诱导并维持免疫耐受方面也具有重要作用。现就DCs的免疫耐受机制及其在I型糖尿病(type 1 diabetes mellitus,T1DM)中的作用的研究进展作一综述,为T1DM等自身免疫性疾病及移植免疫疾病的细胞免疫治疗提供新思路。     

粘膜耐受在预防和治疗自身免疫病中的作用

粘膜应用自身抗原诱导外周免疫耐受状态 ,依赖应用抗原剂量 ,诱导产生两种粘膜耐受机制 :高剂量偏向产生T细胞克隆无能 /排除 ,低剂量偏向产生分泌抑制性细胞因子(TGF β ,IL 4,IL 10 )的T细胞克隆扩增。大量研究表明鼻内或口服应用抗原诱导粘膜耐受可有效预防几种实验性自身免疫病 (EAE ,EAMG ,EAN ,EAU ,IDDM和CIA) ,在同等剂量鼻内应用比口服诱导粘膜耐受更有效。基于动物实验结果 ,对人类自身免疫病MS、RA和葡萄膜炎的临床试验正在进行。耐受原与CTB偶联可拓宽粘膜耐受的有效性 ,加强对临床疾病的抑制。然而 ,粘膜免疫与抗原应用途径、种类和疾病发病时间有关 ,可能表现为双重作用 ,尤其在发展中的自身免疫病 ,粘膜应用抗原可能加重病情。     

人FasL基因转染成熟树突状细胞对T淋巴细胞增殖和凋亡的影响

探讨转染人FasL基因的成熟树突状细胞(DC)对异体T淋巴细胞增殖和凋亡的影响,为实现临床器官移植免疫耐受提供初步实验依据.从健康成年人外周静脉血中获得成熟树突状细胞.将人FasL基因成功转染成熟树突状细胞,检测其表面分子的表达和自身凋亡情况,并对其抗原递呈功能进行分析.从异体健康成人外周血中获取T淋巴细胞,将转染成功的树突状细胞与T淋巴细胞混合培养,检测其对T淋巴细胞增殖和凋亡的影响.结果表明:人FasL基因转染没有明显影响成熟树突状细胞表面分子CD40、CD80、CD86和HLA-DR的表达;没有诱导树突状细胞自身发生凋亡;没有影响DC的抗原递呈功能.转染FasL基因后的树突状细胞使异体T淋巴细胞刺激指数明显下降,凋亡增加.因此认为,人FasL基因转染对成熟树突状细胞的表面分子表达、自身凋亡、抗原递呈等生物学性状无影响;转染FasL基因的树突状细胞使异体T淋巴细胞的增殖能力减弱,并能明显诱导T淋巴细胞凋亡.     

树突状细胞诱导外周免疫耐受的机制

树突状细胞既能启动免疫应答,又能诱导免疫耐受。目前对树突状细胞外周耐受方面的研究进展迅速,本文就未成熟树突状细胞、免疫抑制因子处理的树突状细胞及转基因树突状细胞在诱导外周免疫耐受中的作用作一综述,这可能是治疗自身免疫性疾病和移植排斥反应的新途径。     

调节性 T 细胞发育的一个关键转录因子 Foxp3

调节性 T 细胞是目前免疫学领域研究的热点,对于维持机体免疫耐受和免疫应答稳态具有非常重要的作用 . 对其发育和功能机制的深入认识,不但有助于了解错综复杂的免疫系统理论,而且在自身免疫性疾病、肿瘤和艾滋病的治疗以及移植耐受的诱导等方面具有广泛的应用前景 . 最近的研究发现,转录因子 Foxp3 对于调节性 T 细胞的发育具有重要的作用,是调节性 T 细胞发育的一个关键转录因子 .     

1型调节性T细胞的表型和功能

1型调节性T细胞(Tr1)在免疫耐受的诱导和维持过程中发挥重要作用。Tr1细胞通常在免疫耐受的环境中由外来抗原诱导产生,通过产生高水平的IL-10而发挥免疫抑制作用。而CD4 CD25 调节性T细胞(Tregs)可在胸腺中天然产生也可在外周被抗原诱导产生,通过细胞接触发挥抑制作用。现对Tr1细胞的表型、功能及其抑制作用机制等方面的研究进展进行综述。     

An Exhaustion-Like Phenotype Constrains the Activity of CD4+ T Cells Specific for a Self and Melanoma Antigen

While the immune system has the capacity to recognize and destroy melanoma, tolerance mechanisms often hinder the development of effective anti-tumor immune responses. Since many melanoma antigens are self proteins expressed in normal melanocytes, self antigen exposure before tumor development can negatively impact the function of T cells specific for these self/tumor antigens. However, the contribution of self tolerance to anti-melanoma T cell dysfunction remains largely unexplored. We have previously described a TCR transgenic (Tg) mouse model in which T cells specific for the self/melanoma antigen, tyrosinase-related protein 1 (TRP1), develop in the presence of endogenous TRP1 expression (Ag+) and diminished antigen presentation due to the absence of gamma-interferon-inducible lysosomal thiol reductase (GILT-/-). We show that TRP1-specific T cells from these Ag+GILT-/-Tg mice do not protect from melanoma tumor growth, fail to induce autoimmune vitiligo, and undergo diminished proliferation compared to T cells from Ag-GILT+/+Tg mice. Despite an increased frequency of TRP1-specific Treg cells in Ag+GILT-/-Tg mice compared to Ag-GILT+/+Tg animals, Treg cell depletion only partially rescues the proliferative capacity of T cells from TRP1-expressing mice, suggesting the involvement of additional suppressive mechanisms. An increased percentage of melanoma-specific T cells from Ag+GILT-/-Tg animals express PD-1, an inhibitory receptor associated with the maintenance of T cell exhaustion. Antibody blockade of PD-1 partially improves the ability of TRP1-specific T cells from Ag+GILT-/-Tg mice to produce IL-2. These findings demonstrate that melanoma-specific T cells exposed to a self/melanoma antigen in healthy tissue develop an exhaustion-like phenotype characterized by PD-1-mediated immunosuppression prior to encounter with tumor.     

Antigen presentation in acquired immunological tolerance

In acquired tolerance, previous exposure to antigen under certain conditions induces specific unresponsiveness instead of specific immunological memory. It has been studied as an approach to the mechanisms of self-tolerance that operate on immunocompetent T and B lymphocytes once they leave their sites of origin in the thymus and the bone marrow. Possible mechanisms involve induction of specific suppressor cells or inactivation of antigen-specific lymphocytes (clonal anergy) as a consequence of abortive antigen presentation, in which the antigen receptor is effectively engaged but certain poorly defined accessory signals the T lymphocytes require are lacking. We propose that small, resting B lymphocytes, which lack these accessory signals, are the inactivating antigen-presenting cells in acquired tolerance to proteins and to the class II transplantation antigens. B lymphocytes, which can use their antigen receptors to gather and process antigens that are present at very low concentrations, may play a role in self-tolerance. In addition, B lymphocytes and T lymphocytes rendered anergic by encounter with self antigens could persist as self-specific suppressor cells to block an autoimmune response of autoreactive clones that had escaped deletion or anergy.     

T-Cell Tolerance: Central and Peripheral

Somatic recombination of TCR genes in immature thymocytes results in some cells with useful TCR specificities, but also many with useless or potentially self-reactive specificities. Thus thymic selection mechanisms operate to shape the T-cell repertoire. Thymocytes that have a TCR with low affinity for self-peptide–MHC complexes are positively selected to further differentiate and function in adaptive immunity, whereas useless ones die by neglect. Clonal deletion and clonal diversion (Treg differentiation) are the major processes in the thymus that eliminate or control self-reactive T cells. Although these processes are thought to be efficient, they fail to control self-reactivity in all circumstances. Thus, peripheral tolerance processes exist wherein self-reactive T cells become functionally unresponsive (anergy) or are deleted after encountering self-antigens outside of the thymus. Recent advances in mechanistic studies of central and peripheral T-cell tolerance are promoting the development of therapeutic strategies to treat autoimmune disease and cancer and improve transplantation outcome.T cells recognize pathogen fragments in the context of surface MHC molecules on host cells. As such, they have the potential to do enormous damage to healthy tissue when they are not appropriately directed, that is, when they respond to self-antigens as opposed to foreign antigens. T lymphocyte tolerance is particularly important, because it impacts B-cell tolerance as well, through the requirement of T cell help in antibody responses. Thus, failure of T-cell tolerance can lead to many different autoimmune diseases. The tolerance of T cells begins as soon as a T-cell receptor is formed and expressed on the cell surface of a T-cell progenitor in the thymus. Tolerance mechanisms that operate in the thymus before the maturation and circulation of T cells are referred to as “central tolerance.” However, not all antigens that T cells need to be tolerant of are expressed in the thymus, and thus central tolerance mechanisms alone are insufficient. Fortunately, additional tolerance mechanisms exist that restrain the numbers and or function of T cells that are reactive to developmental or food antigens, which are not thymically expressed. These mechanisms act on mature circulating T cells and are referred to as “peripheral tolerance.”     

Medullary thymic epithelial cells, the indispensable player in central tolerance

Crosstalk between thymocytes and thymic epithelial cells is critical for T cell development and the establishment of central tolerance. Medullary thymic epithelial cells (mTECs) play important roles in the late stage of T cell development, especially negative selection and Treg generation. The function of mTECs is highly dependent on their characteristic features such as ectopic expression of peripheral tissue restricted antigens (TRAs) and their master regulator—autoimmune regulator (Aire), expression of various chemokines and cytokines. In this review, we summarize the current understanding of cellular and molecular mechanisms of mTEC development and its functions in T cell development and the establishment of central tolerance. The open questions in this field are also discussed. Understanding the function and underlying mechanisms of mTECs will contribute to the better control of autoimmune diseases and the improvement of immune reconstitution during aging or after infection, chemotherapy or radiotherapy.     

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)     

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.     

Ablation of "tolerance" and induction of diabetes by virus infection in viral antigen transgenic mice

To address the mechanisms of tolerance to extrathymic proteins, we have generated transgenic mice expressing the lymphocytic choriomeningitis viral (LCMV) glycoprotein (GP) in the beta islet cells of the pancreas. The fate of LCMV GP-specific T cells was followed by breeding the GP transgenic mice with T cell receptor transgenic mice, specific for LCMV and H-2Db. These studies suggest that "peripheral tolerance" of self-reactive T cells does not involve clonal deletion, clonal anergy, or a decrease in the density of T cell receptors or accessory molecules. Instead, this model indicates that self-reactive cytotoxic T cells may remain functionally unresponsive, owing to a lack of appropriate T cell activation. Infection of transgenic mice with LCMV readily abolishes peripheral unresponsiveness to the self LCMV GP antigen, resulting in a CD8+ T cell-mediated diabetes. These data suggest that similar mechanisms may operate in several so-called "T cell-mediated autoimmune diseases."     

Autoimmunity in Coxsackievirus B3 induced myocarditis: role of estrogen in suppressing autoimmunity

Picornaviruses are small, non-enveloped, single stranded, positive sense RNA viruses which cause multiple diseases including myocarditis/dilated cardiomyopathy, type 1 diabetes, encephalitis, myositis, orchitis and hepatitis. Although picornaviruses directly kill cells, tissue injury primarily results from autoimmunity to self antigens. Viruses induce autoimmunity by: aborting deletion of self-reactive T cells during T cell ontogeny; reversing anergy of peripheral autoimmune T cells; eliminating T regulatory cells; stimulating self-reactive T cells through antigenic mimicry or cryptic epitopes; and acting as an adjuvant for self molecules released during virus infection. Most autoimmune diseases (SLE, rheumatoid arthritis, Grave's disease) predominate in females, but diseases associated with picornavirus infections predominate in males. T regulatory cells are activated in infected females because of the combined effects of estrogen and innate immunity.     

B cells as antigen presenting cells

Several characteristics confer on B cells the ability to present antigen efficiently: (1) they can find T cells in secondary lymphoid organs shortly after antigen entrance, (2) BCR-mediated endocytosis allows them to concentrate small amounts of specific antigen, and (3) BCR signaling and HLA-DO expression direct their antigen processing machinery to favor presentation of antigens internalized through the BCR. When presenting antigen in a resting state, B cells can induce T cell tolerance. On the other hand, activation by antigen and T cell help converts them into APC capable of promoting immune responses. Presentation of self antigens by B cells is important in the development of autoimmune diseases, while presentation of tumor antigens is being used in vaccine strategies to generate immunity. Thus, detailed understanding of the antigen presenting function of B cells can lead to their use for the generation or inhibition of immune responses.