Immunological Tolerance 免疫耐受 Immunological Tolerance Introduction: Tolerance refers to the specific immunological non-reactivity to an antigen resulting from a previous exposure to the same antigen. While the most important form of tolerance is non-reactivity to self antigens, it is possible to induce tolerance to non-self antigens. When an antigen induces tolerance, it is termed tolerogen.

概论 免疫耐受是机体对抗原刺激表现为“免疫不应答”的现象 免疫耐受具有免疫特异性 自身耐受可以避免自身免疫病的发生

第一节 免疫耐受的形成及表现 胚胎期 自身或外来的抗原刺激 不成熟的T、B细胞 形成免疫耐受 出生后 对相同抗原不引起应答

一、胚胎期及新生期接触抗原所致免疫耐受 Owen的观察 于1945年首先报道了在胚胎期接触同种异型Ag所致的免疫耐受现象

异卵双生的牛 Need for tissue and organ graft drove the curiosity to understand the mechanisms of tolerance. The observations a zoologist, Owens, that Dizygotic bovine twins could accept grafts from each other but their siblings from other pregnancies could not tolerate such grafts led Medawar to perform a series of experiments to induce tolerance in mice.

免疫耐受实验的诱导 He and his collaborators Brent and Billingham under the hypothesis that early exposure to foreign antigens might facilitate tolerance induction performed this experiment.

嵌合体（chimaeric）小鼠 A white (Balb/c) mouse made tolerant to C57Bl (black) mouse tissues. Tolerance to tissue and cell antigens can be induced by injection of hemopoietic (stem) cells in neonatal or severely immuno-compromised (by lethal irradiation or drug treatment) animals. Also, grafting of thymus in early life results in tolerance to the donor type cells and tissues. Such animals are known as chimeras. These findings are of significant practical application in bone marrow grafting.

二、后天接触抗原导致的免疫耐受 适宜的抗原刺激，可产生特异性免疫应答 不适宜的抗原量，特殊的Ag表位及Ag表位的变异，均会导致免疫耐受 T细胞活化缺乏第二信号；缺乏生长及分化因子，使T细胞克隆不能扩增，可导致免疫耐受

抗原因素与免疫耐受 抗原剂量 抗原剂量 抗原剂量过低，不足以激活T及B细胞 低带耐受 诱导Ts细胞活化 抗原剂量过高 抑制免疫应答 高带耐受

T细胞耐受易于诱导，所需抗原量低， 耐受持续时间长（数月~数年） B细胞耐受的诱导，需要较大剂量的抗原， B细胞耐受持续时间短（数周）

抗原类型 蛋白单体 不能被APC细胞提呈 T细胞不被活化 B细胞不产生抗体 蛋白聚体，情况正好相反 B细胞产生抗体

抗原不同部位的决定基，其作用不同，可诱导或抑制免疫应答 抗原免疫途径 口服抗原易导致全身耐受 抗原决定基特点 抗原不同部位的决定基，其作用不同，可诱导或抑制免疫应答

Factors affecting tolerance role of antigen Factors which affect response Favor immune response Favor tolerance Physical form of antigen Route of injection Dose of antigen Large, aggregated, complex molecules, properly processed Subcutaneous or intramuscular Optimal dose soluble, aggregate-free, simple small molecules, not processed Oral or, sometimes, intravenous Very large or very small dose Tolerance to soluble antigens: A state of tolerance to a variety of T-dependent and T-independent antigens has been achieved in various experimental models. Based on these observations it is clear that a number of factors determine whether an antigen will stimulate an immune response or tolerance (Table 1).

Factors affecting tolerance the role of host Factors that affect response Favor immune response Favor tolerance Age of responding animal Differentiation state of cells Fully differentiated; memory T & B cells Older, immuno-logically mature Newborn (mice), immuno-logically immature Relative undifferentiated B cell with only IgM, T cells in the thymic cortex

Immunologic features of tolerance It is an antigen-induced, active process Like immunologic memory, it is antigen specific Like immunologic memory, it can exist in B cells, T cells or both Like immunologic memory, its easier to induce and lasts longer in T cells than in B cell Induction of tolerance is very similar to induction of an immune response. Immunologic features of tolerance: Tolerance is different from non-specific immunosuppression, and immunodeficiency. It is an active antigen dependent process in response to the antigen. Like immune response tolerance is specific and like immunological memory, it can exist in T-cell, B cells or both and like immunological memory, tolerance at the T cell level is longer lasting than tolerance at the B cell level.

Tolerance in T and B cells Induction of tolerance in T cells is easier and requires relatively smaller amounts of tolerogen than tolerance in B cells. Maintenance of immunological tolerance requires persistence of antigen. Tolerance can be broken naturally (as in autoimmune diseases) or artificially (as shown in experimental animals, by x‑irradiation, certain drug treatments and by exposure to cross reactive antigens). Tolerance may be induced to all epitopes or only some epitopes on an antigen and Tolerance to a single antigen may exist at B cell level or T cells level or at both levels.

High and low dose tolerance

Host age and antigen dose affect tolerance newborn adult

T细胞及B细胞分别在胸腺及骨髓微环境中发育，此间进行阴性选择 第二节 免疫耐受机制 中枢耐受 T细胞及B细胞分别在胸腺及骨髓微环境中发育，此间进行阴性选择 启动细胞凋亡，致克隆消除 减少生后自身免疫病的发生

外周耐受 绝大多数组织特异性抗原浓度太低， 不足以活化相应的T及B细胞 克隆无能及免疫忽视 绝大多数组织特异性抗原浓度太低， 不足以活化相应的T及B细胞 如抗原浓度适宜，自身反应性T细胞与组织细胞MHC-I—自身Ag复合物接触，产生第一信号，但组织细胞不表达协同刺激分子，无第二信号，导致克隆无能状态（clonal anergy)→凋亡→克隆消除

有些克隆无能淋巴细胞长期存活→IL-2→克隆扩增→引起自身免疫 体内某些抗原浓度不足以诱导T细胞耐受，但其浓度足以活化效应T细胞，而有致自身免疫病的危险，这种自身抗原与自身反应性T细胞并存状态称免疫忽视（immunological ignorance） 自身抗原浓度适宜时，虽能活化自身反应性B细胞，但Th不活化，不提供细胞因子，B细胞呈无能状态。如旁路Th细胞被活化，则引起自身免疫病

如耐受血液中除去淋巴细胞，则没有免疫耐受发生 免疫抑制细胞的作用 淋巴细胞 另一 小鼠 免疫耐受 小鼠 正常 小鼠 取皮肤移植 存活 如耐受血液中除去淋巴细胞，则没有免疫耐受发生

免疫隔离部位的抗原在生理条件下不致免疫应答 免疫隔离部位：脑、眼的前房 生理屏障 抑制性细胞因子

Mechanism of tolerance induction Clonal deletion Thymus: negative selection Bone marrow: IgM+, IgD- B cells encountering self antigen Clonal anergy Lack of co-stimulatory(B7) molecules Exposure to large amounts of antigen Improper antigen presentation Lack of antigenic stimulus Receptor editing Anti-idiotype antibodies Suppressor T cells The exact mechanism of induction and maintenance of tolerance is not fully understood. Experimental data, however, point to several possibilities. Clonal deletion: Functionally immature cells of a clone encountering antigen undergo a programmed cell death. For example, auto-reactive T-cell are eliminated in the thymus following interaction with self antigen during their differentiation (negative selection). Clonal deletion has been shown to occur also in the periphery. B cells expressing only IgM (no IgD) on their surface when exposed to antigen are eliminated.

Clonal deletion: negative selection in the thymus Clonal deletion: Functionally immature cells of a clone encountering antigen undergo a programmed cell death. For example, auto-reactive T-cell are eliminated in the thymus following interaction with self antigen during their differentiation (negative selection). Clonal deletion has been shown to occur also in the periphery. B cells expressing only IgM (no IgD) on their surface when exposed to antigen are eliminated.

Negative selection of B cells in bone marrow Clonal deletion: Functionally immature cells of a clone encountering antigen undergo a programmed cell death. For example, auto-reactive T-cell are eliminated in the thymus following interaction with self antigen during their differentiation (negative selection). Clonal deletion has been shown to occur also in the periphery. B cells expressing only IgM (no IgD) on their surface when exposed to antigen are eliminated.

Clonal anergy in T cells Clonal anergy: Auto-reactive T cells when exposed to antigenic peptides that do not possess co-stimulatory molecules (B7-1 or B7-2) become anergic to the antigen.

Clonal anergy in B cells Also, B cells when exposed to large amounts of soluble antigen down regulate their surface IgM and become anergic and short lived. These cells also up regulate Fas molecules on their surface. An interaction of these B cells with Fas-ligand bearing cells result in their death via apoptosis.

Tolerance due to lack of helper T cells Clonal ignorance: T cells reactive to self antigen not represented in the thymus will mature and migrate to the periphery, but they may never encounter the appropriate antigen because it is sequestered in inaccessible tissues. Such cells may die out for lack of stimulus. Auto-reactive B cells that escape deletion may not find the antigen or the specific helper T-cells and hence not be activated and die out.

Receptor editing among B cells Receptor editing: B cells which encounter large amounts of soluble antigen, as they do in the body, and bind to this antigen with very low affinity become activated to re-express their RAG1 and RAG2 genes. These gene cause them to undergo gene recombination and change their specificity.

Antiidiotype antibody in tolerance Anti-idiotype antibody: Anti‑idiotype antibodies produced experimentally have been demonstrated to inhibit immune response to specific antigens. Anti-idiotype antibodies are produced during the process of tolerization and such antibodies have been demonstrated in tolerant animals. These antibodies prevent the receptor from combining with antigen. Suppressor cells: Both low and high doses of antigen may induce suppressor T cells which can specifically suppress immune responses of both B and T cells.

Breakdown of tolerance Immunosuppression Lack of antigen during differentiation of new clones Lack of antigen exposure As above Cross reactive antigens Termination of tolerance: Experimentally induced tolerance can be terminated by prolonged absence of exposure to the tolerogen, by treatments which severely damage the immune system (x‑irradiation) or by immunization with cross reactive antigens. These observations are of significance in the conceptualization of autoimmune diseases.

第三节 免疫耐受与医学 口服免疫原，建立全身耐受 静脉注射抗原，建立全身耐受性 移植骨髓及胸腺，建立或恢复免疫耐受 第三节 免疫耐受与医学 口服免疫原，建立全身耐受 静脉注射抗原，建立全身耐受性 移植骨髓及胸腺，建立或恢复免疫耐受 脱敏治疗，防止IgE抗体产生 防止感染 诱导产生特异拮抗性免疫细胞，抑制效应免疫细胞对靶细胞的攻击 自身抗原肽拮抗剂的使用

多重抗感染措施，防止病原体产生抗原拮抗分子 打破免疫耐受 免疫原及免疫应答分子用于肿瘤患者的治疗 细胞因子及其抗体的合理使用 多重抗感染措施，防止病原体产生抗原拮抗分子