Regulation of CD8+ T cell development by thymus-specific proteasomes

Proteasomes are responsible for generating peptides presented by the class I major histocompatibility complex (MHC) molecules of the immune system. Here, we report the identification of a previously unrecognized catalytic subunit called beta5t. beta5t is expressed exclusively in cortical thymic epithelial cells, which are responsible for the positive selection of developing thymocytes. Although the chymotrypsin-like activity of proteasomes is considered to be important for the production of peptides with high affinities for MHC class I clefts, incorporation of beta5t into proteasomes in place of beta5 or beta5i selectively reduces this activity. We also found that beta5t-deficient mice displayed defective development of CD8(+) T cells in the thymus. Our results suggest a key role for beta5t in generating the MHC class I-restricted CD8(+) T cell repertoire during thymic selection.     

Requirement for positive selection of gamma delta receptor-bearing T cells.

The alpha beta and gamma delta T cell receptors for antigen (TCR) delineate distinct T cell populations. TCR alpha beta-bearing thymocytes must be positively selected by binding of the TCR to major histocompatibility complex (MHC) molecules on thymic epithelium. To examine the requirement for positive selection of TCR gamma delta T cells, mice bearing a class I MHC-specific gamma delta transgene (Tg) were crossed to mice with disrupted beta 2 microglobulin (beta 2M) genes. The Tg+beta 2M- (class I MHC-) offspring had Tg+ thymocytes that did not proliferate to antigen or Tg-specific monoclonal antibody and few peripheral Tg+ cells. This is evidence for positive selection within the gamma delta T cell subset.     

Influence of the major histocompatibility complex on positive thymic selection of V beta 17a+ T cells

A monoclonal antibody was used to show directly positive thymic selection of the T cell repertoire in mouse strains expressing the 17a beta-chain variable domain (V beta 17a) of the T cell receptor. In the absence of the potent tolerizing class II major histocompatibility complex (MHC) molecule, I-E, peripheral expression of V beta 17a+ T cell receptors varied with the MHC haplotype of the mouse strain. In the most extreme case, H-2q mice expressed high peripheral levels of CD4+ V beta 17a+ T cells (14 to 19 percent), whereas H-2b mice expressed low levels (3 to 4 percent). Analysis of (b x q)F1 mice and chimeric mice showed that these differences were determined by positive thymic selection and implicated the thymic epithelium as the controlling cell type.     

Requirement for RORgamma in thymocyte survival and lymphoid organ development

Most developing thymocytes undergo apoptosis because they cannot interact productively with molecules encoded by the major histocompatibility complex. Here, we show that mice lacking the orphan nuclear hormone receptor RORgamma lose thymic expression of the anti-apoptotic factor Bcl-xL. RORgamma thus regulates the survival of CD4+8+ thymocytes and may control the temporal window during which thymocytes can undergo positive selection. RORgamma was also required for development of lymph nodes and Peyer's patches, but not splenic follicles. In its absence, there was loss of a population of CD3-CD4+CD45+ cells that normally express RORgamma and that are likely early progenitors of lymphoid organs. Hence, RORgamma has critical functions in T cell repertoire selection and lymphoid organogenesis.     

Thymocyte expression of RAG-1 and RAG-2: termination by T cell receptor cross-linking.

The expression of the V(D)J [variable (diversity) joining elements] recombination activating genes, RAG-1 and RAG-2, has been examined during T cell development in the thymus. In situ hybridization to intact thymus and RNA blot analysis of isolated thymic subpopulations separated on the basis of T cell receptor (TCR) expression demonstrated that both TCR- and TCR+ cortical thymocytes express RAG-1 and RAG-2 messenger RNA's. Within the TCR+ population, RAG expression was observed in immature CD4+CD8+ (double positive) cells, but not in the more mature CD4+CD8- or CD4-CD8+ (single positive) subpopulations. Thus, although cortical thymocytes that bear TCR on their surface continue to express RAG-1 and RAG-2, it appears that the expression of both genes is normally terminated during subsequent thymic maturation. Since thymocyte maturation in vivo is thought to be regulated through the interaction of the TCR complex with self major histocompatibility complex (MHC) antigens, these data suggest that signals transduced by the TCR complex might result in the termination of RAG expression. Consistent with this hypothesis, thymocyte TCR cross-linking in vitro led to rapid termination of RAG-1 and RAG-2 expression, whereas cross-linking of other T cell surface antigens such as CD4, CD8, or HLA class I had no effect.     

Self-nonself discrimination by T cells

The alpha beta T cell receptor (TCR) recognizes antigens that are presented by major histocompatibility complex (MHC)-encoded cell surface molecules by binding to both the antigen and the MHC molecules. Discrimination of self from nonself antigens and MHC molecules is achieved by negative and positive selection of T cells in the thymus: potentially harmful T cells with receptors that bind to self antigens plus self MHC molecules are deleted before they can mount immune responses. In contrast, the maturation of useful T cells with receptors that bind foreign antigens plus self MHC molecules requires the binding of their receptor to MHC molecules on thymic epithelium in the absence of foreign antigen. The binding of the TCR to either class I or class II MHC molecules directs differentiation of the selected cells into either CD4-8+ (killer) or CD4+8- (helper) T cells, respectively.     

A nondeletional mechanism of thymic self tolerance

T cells become tolerant of self antigens during their development in the thymus. Clonal deletion of thymocytes bearing self-reactive T cell receptors is a major mechanism for generating tolerance and occurs readily for antigens expressed by bone marrow-derived cells. Tolerance to antigens expressed on the radioresistant thymic stromal elements is demonstrated here to occur via a nondeletional mechanism. For minor lymphocyte stimulatory (Mls-1a) and major histocompatibility complex (MHC) antigens, this alternate form of tolerance induction results in clonal anergy.     

Peripheral selection of the T cell repertoire

T lymphocytes undergo selection events not only in the thymus, but also after they leave the thymus and reside in the periphery. Peripheral selection was found to be dependent on T cell receptor (TCR)-ligand interactions but to differ from thymic selection with regard to specificity and mechanism. Unlike thymic selection, peripheral selection required binding of antigen to the TCR, and it induced expansion of T cell clones. Tolerance to self antigens that are restricted to the periphery occurred through the elimination of self-reactive T cells and by the clonal anergy, which was associated with down-regulation of the alpha beta TCR and CD8.     

Requirement for the SLP-76 adaptor GADS in T cell development

GADS is an adaptor protein implicated in CD3 signaling because of its ability to link SLP-76 to LAT. A GADS-deficient mouse was generated by gene targeting, and the function of GADS in T cell development and activation was examined. GADS- CD4-CD8- thymocytes exhibited a severe block in proliferation but still differentiated into mature T cells. GADS- thymocytes failed to respond to CD3 cross-linking in vivo and were impaired in positive and negative selection. Immunoprecipitation experiments revealed that the association between SLP-76 and LAT was uncoupled in GADS- thymocytes. These observations indicate that GADS is a critical adaptor for CD3 signaling.     

固始鸡免疫器官内细胞凋亡基因Fas和FasL的动态表达

 【目的】探讨固始鸡免疫器官内细胞凋亡基因Fas和FasL的动态表达。【方法】应用免疫组织化学技术和Leica 显微图像处理系统,对细胞凋亡基因Fas和FasL在固始鸡免疫器官内的动态表达进行研究。【结果】Fas在不同发育阶段固始鸡免疫器官内均有表达,但表达量均不相同,呈波浪样动态变化;Fas表达于固始鸡免疫器官内淋巴细胞细胞膜和细胞质,不表达于细胞核;Fas表达阳性细胞在固始鸡不同免疫器官内分布位置不同,呈散在或簇团状分布：法氏囊内阳性细胞主要分布于黏膜靠近上皮细胞的固有膜层内、淋巴小结与淋巴小结之间区域、淋巴小结边缘,淋巴小结内少量淋巴细胞也有Fas表达;胸腺内主要分布于胸腺小叶髓质,极少出现在皮质内;脾脏内主要分布于红髓和边缘区、淋巴小结周围和动脉周围淋巴鞘周围的区域,动脉周围淋巴鞘极少有Fas表达,淋巴小结无Fas表达。FasL在固始鸡免疫器官内的表达与Fas相似,但表达量与Fas相比较少。【结论】细胞凋亡基因Fas和FasL参与了固始鸡免疫器官内淋巴细胞发育分化过程中的凋亡调控,并对免疫器官的稳定发挥重要作用。     

Interleukin-22 drives endogenous thymic regeneration in mice

Endogenous thymic regeneration is a crucial function that allows for renewal of immune competence after stress, infection, or immunodepletion. However, the mechanisms governing this regeneration remain poorly understood. We detail such a mechanism, centered on interleukin-22 (IL-22) and triggered by the depletion of CD4(+)CD8(+) double-positive thymocytes. Intrathymic levels of IL-22 were increased after thymic insult, and thymic recovery was impaired in IL-22-deficient mice. IL-22, which signaled through thymic epithelial cells and promoted their proliferation and survival, was up-regulated by radio-resistant RORγ(t)(+)CCR6(+)NKp46(-) lymphoid tissue inducer cells after thymic injury in an IL-23-dependent manner. Administration of IL-22 enhanced thymic recovery after total body irradiation. These studies reveal mechanisms of endogenous thymic repair and offer innovative regenerative strategies for improving immune competence.     

MHC class I deficiency: susceptibility to natural killer (NK) cells and impaired NK activity.

The role of major histocompatibility complex (MHC) class I expression in natural killer (NK) cell target recognition is controversial. Normal T cell blasts from MHC class I-deficient mutant mice were found to serve as target cells for NK cells in vitro, which suggests that MHC class I molecules are directly involved in NK cell recognition. Spleen cells from the mutant mice were deficient in their ability to lyse MHC class I-deficient target cells or NK-susceptible tumor targets, and mutant mice could not reject allogeneic bone marrow. Thus, class I molecules may participate in the positive selection or tolerance induction of NK cells.     

Clonal deletion versus clonal anergy: the role of the thymus in inducing self tolerance

During development in the thymus, T cells are rendered tolerant to self antigens. It is now apparent that thymocytes bearing self-reactive T cell receptors can be tolerized by processes that result in physical elimination (clonal deletion) or functional inactivation (clonal anergy). As these mechanisms have important clinical implications for transplantation and autoimmunity, current investigations are focused on understanding the cellular and molecular interactions that generate these forms of tolerance.     

Allograft survival: effect of antiserums to thymus glands and lymphocytes

Although antiserums to lymphocytes and thymus cells have similar effects on lymphocytes and thymocytes in vitro, the antiserum to thymus has more persistent lymphopenic effect in vivo and prolongs allograft survival time more markedly. Since only thymus glands of animals treated with antiserum to thymus showed depletion of small lymphocytes, antibody to thymic humoral factor may be operational.     

A thymic precursor to the NK T cell lineage

CD1d-restricted autoreactive natural killer (NK1.1+) T cells function as regulatory cells in various disease conditions. Using improved tetramer tracking methodology, we identified a NK1.1- thymic precursor and followed its differentiation and emigration to tissues by direct cell transfer and in situ cell labeling studies. A major lineage expansion occurred within the thymus after positive selection and before NK receptor expression. Surprisingly, cytokine analysis of the developmental intermediates between NK and NK+ stages showed a T helper cell TH2 to TH1 conversion, suggesting that the regulatory functions of NK T cells may be developmentally controlled. These findings characterize novel thymic and postthymic developmental pathways that expand autoreactive cells and differentiate them into regulatory cells.     

Formation of regulatory patterns during signal propagation in a Mammalian cellular network

We developed a model of 545 components (nodes) and 1259 interactions representing signaling pathways and cellular machines in the hippocampal CA1 neuron. Using graph theory methods, we analyzed ligand-induced signal flow through the system. Specification of input and output nodes allowed us to identify functional modules. Networking resulted in the emergence of regulatory motifs, such as positive and negative feedback and feedforward loops, that process information. Key regulators of plasticity were highly connected nodes required for the formation of regulatory motifs, indicating the potential importance of such motifs in determining cellular choices between homeostasis and plasticity.     

Crystal structure of an early protein-RNA assembly complex of the signal recognition particle

The signal recognition particle (SRP) is a universally conserved ribonucleoprotein complex that mediates the cotranslational targeting of secretory and membrane proteins to cellular membranes. A crucial early step in SRP assembly in archaea and eukarya is the binding of protein SRP19 to specific sites on SRP RNA. Here we report the 1.8 angstrom resolution crystal structure of human SRP19 in complex with its primary binding site on helix 6 of SRP RNA, which consists of a stem-loop structure closed by an unusual GGAG tetraloop. Protein-RNA interactions are mediated by the specific recognition of a widened major groove and the tetraloop without any direct protein-base contacts and include a complex network of highly ordered water molecules. A model of the assembly of the SRP core comprising SRP19, SRP54, and SRP RNA based on crystallographic and biochemical data is proposed.     

Murine leukemia: a virus-induced autoimmune disease?

Thymocytes from mice carrying Moloney murine leukemia virus since birth are cytotoxic for normal syngeneic fibroblasts; they are much less cytotoxic for the same cells infected with this virus. The cytotoxic thymocytes appear to increase in number with age of the carrier mice and are present both during preleukemic and leukemic periods. These results suggest that lymphomas in carrier mice result from a sequence of events initiated by intrathymic destruction of normal cells by virus-infected cells, and culminating in the unrestricted proliferation of autoaggressive clones in the thymic cortex.     

Failure of T cell receptor V beta negative selection in an athymic environment

The mature T cell receptor (TCR) repertoire is the result of selection events during T cell development. Previous assessment of TCR beta-chain selection with serologic and molecular probes demonstrated both positive and negative selection. Although this work suggested a critical role for the thymus, no direct assessment has been made of the requirement for a thymus in TCR V beta selection. A comparison of TCR V beta expression in four different congenic pairs of normal and nu/nu (athymic) mice indicated that the normal V beta deletions associated with tolerance to self minor lymphocyte stimulating (Mlsc) antigens or to self major histocompatibility complex (MHC)-encoded E alpha E beta products did not occur in most athymic mice. Thus, the thymus has a critical role in mediating self tolerance by negative selection.