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The selection of T cell receptor specificities must logically not only involve the alpha beta-TCR but, also the CD4 and CD8 molecules, as antigen recognition by the alpha beta-TCR on mature T cells is facilitated by the CD4 and CD8 co-receptors. In this review, the studies that provided key advances in our understanding of the possible role of CD4 and CD8 in T cell development will be discussed.
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To summarize, I have attempted to present evidence that the unselected thymocyte TCR repertoire has such a propensity to recognize MHC/peptide at high affinity that it is likely that, were tolerance induction to be invariably dominant over positive selection, all T cells would be deleted or anergized. If this is true it follows that positive selection must in fact be capable of being the dominant process. I have further suggested that the existence of peptides capable of antagonizing T-cell activation in the thymus could inhibit deletion, and that this might correspond to positive selection.
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In the process of T cell differentiation thymocytes undergo positive and negative selections. The positive and negative selections result from interactions between the T cell antigen receptor (TCR) and self peptides presented by the major histocompatibility complex (MHC) molecules. Positive selection generates functional T cells restricted to the self-MHC. Negative selection eliminates or anergies T cells bearing self reactive TCR with high affinity to self peptides plus MHC. In this study, differentiation and selection of thymocytes were analyzed using bone marrow chimeras. It was demonstrated that differentiation of CD4+CD8+ thymocytes to CD4+ or CD8+ mature thymocytes occurred between 12 and 16 days post bone marrow transplantation. When pigeon cytochrome c 43-58 related peptide antigens were injected intrathymically 13 days after bone marrow transplantation, various changes were observed in resultant T cell repertoire. Intrathymic injection of peptides with high affinity to the MHC class II resulted in specific inhibition of T cell responses to the peptides, whereas injection of peptides with low affinity to the MHC induced no alteration. Furthermore, no modification of T cell responses was observed, when these peptides were administered after 14 days post BMT. The present findings demonstrate that when peptides with high affinity to the MHC class II molecules are administered intrathymically at a critical stage of thymocyte differentiation, negative selection is induced in a manner of one peptide vs one T cell clone.
Thymic nurse cells (TNCs) represent a unique microenvironment in the thymus for MHC restriction and T cell repertoire selection composed of a cortical epithelial cell surrounding 20-200 immature thymocytes. TNCs have been isolated from many classes of animals from fish to humans. Studies performed using TNC lines showed that TNCs bind viable alphabetaTCRlow CD4(+)CD8(+)CD69(-) thymocytes. A subset of the bound cells is internalized, proliferates within the TNC, and matures to the alphabetaTCRhigh CD4(+)CD8(+)CD69(+) stage, indicative of positive selection. A subset of the internalized population is released while cells that remain internalized undergo apoptosis and are degraded by lysosomes within the TNC. A TNC-specific monoclonal antibody added to fetal thymic organ cultures resulted in an 80% reduction in the number of thymocytes recovered, with a block at the double positive stage of development. Together these data suggest a critical role for TNC internalization in thymocyte selection as well as the removal and degradation of negatively selected thymocytes. Recent studies have shown that in addition to thymocytes, peripheral circulating macrophages are also found within the TNC complex and can present antigens to the developing thymocytes. These circulating macrophages could provide a source of self-antigens used to ensure a self-tolerant mature T cell repertoire. A reduction in TNC numbers is associated with a variety of autoimmune diseases including thyroiditis and systemic lupus erythematosis.
Cbl is the product of the protooncogene c-cbl and is involved in T cell antigen receptor (TCR)-mediated signaling. To understand the role of Cbl for immune system development and function, we generated a Cbl-deficient mouse strain. In Cbl-deficient mice, positive selection of the thymocytes expressing major histocompatibility complex class II-restricted transgenic TCR was significantly enhanced. Two factors may have contributed to the altered thymic selection. First, Cbl deficiency markedly up-regulated the activity of ZAP-70 and mitogen-activated protein kinases. The mitogen-activated protein kinase pathway was shown previously to be involved in thymic positive selection. Second, Cbl-deficient thymocytes expressed CD3 and CD4 molecules at higher levels, which consequently may increase the avidity of TCR/major histocompatibility complex/coreceptor interaction. Thus, Cbl plays a novel role in modulating TCR-mediated multiple signaling pathways and fine-tunes the signaling threshold for thymic selection.
Apoptosis plays a critical role in T cell development and thymic selection. Thymocytes which undergo antigen-induced negative selection have been demonstrated to die by apoptosis. Despite this, relatively little is known about the specific apoptotic pathway involved in negative selection. We have examined the role of cyclin-dependent kinase 2 (Cdk2), a key regulator of thymocyte apoptosis, in this process. Stimulation of thymocytes with cognate antigen leads to a large increase in Cdk2 kinase activity. We also show that pharmacological inhibitors of Cdk2 block thymocyte apoptosis in response to antigen. Our data show that Cdk2 activity is essential for the apoptotic pathway used in negative selection.