T-cell repertoire and thymus

Cytotoxic T lymphocytes to an unmutated tumor rejection antigen P1A: normal development but restrained effector function in vivo

Unmutated tumor antigens are chosen as primary candidates for tumor vaccine because of their expression on multiple lineages of tumors. A critical issue is whether unmutated tumor antigens are expressed in normal cells, and if so, whether such expression imposes special restrictions on cytotoxic T lymphocyte (CTL) responses. In this study, we use a transgenic approach to study the development and effector function of T cells specific for P1A, a prototypical unmutated tumor antigen. We report here that although P1A is expressed at low levels in normal tissues, including lymphoid tissues, the P1A-specific transgenic T cells develop normally and remain highly responsive to the P1A antigen. The fact that transgenic expression of P1A antigen in the thymus induces T cell clonal deletion demonstrates that normal hematopoietic cells can process and present the P1A antigen and that P1A-specific T cells are susceptible to clonal deletion. By inference, P1A-specific T cells must have escaped clonal deletion due to low expression of P1A in the thymus. Interestingly, despite the fact that an overwhelming majority of T cells in the T cell receptor for antigen (TCR)-transgenic mice are specific for P1A, these mice are no more resistant to a P1A-expressing plasmocytoma than nontransgenic littermates. Moreover, when the same TCR-transgenic mice were challenged simultaneously with B7-1(+) and B7-1(-) tumors, only B7-1(+) tumors were rejected. Therefore, even though P1A can be a tumor rejection antigen, the effector function of P1A-specific CTL is restrained in vivo. These results have important implications for the strategy of tumor immunotherapy.

The immune system victorious: selective preservation of self

How the body successfully distinguishes its own tissue cells from those that are foreign and genetically nonidentical to it has been a focus of much research. Clonal deletion maintains that immune system cells with the potential to injure self constituents are eliminated during development, thereby neutralizing their capacity to induce self injury. Selected self-reactive maturing T cell clones undergo deletion in the thymus. A two-step selection process affects immature T cells that enter the thymus. Positive selection makes certain that all surviving cells are able to identify major histocompatibility complex (MHC) proteins present on all body cells. These MHC proteins interact with antigens and present them to T lymphocytes. Negative selection is essential for self-tolerance. It eliminates potentially injurious self-reactive T cells by placing them in contact with a mixture of self antigens in the thymus. Clonal anergy might act together with clonal deletion to maintain self tolerance. Self-reactive T cells in the blood of healthy subjects could represent cells whose affinities for antigen are too weak to initiate an immunologic disease. The fate of T cells reacting to a specific antigen has been traced in transgenic mice. Class I MHC molecules present peptides manufactured within the cell, whereas class II MHC molecules present peptides from extracellular proteins. Interaction of a T cell receptor with its homologous antigen associated with MHC molecules leads to proliferation of that T cell in the presence of costimulatory signals. Investigations elucidating the role of T cell receptors, MHC molecules and antigen peptides in self-nonself discrimination are discussed. The article concludes with an introductory summary of the remaining articles in the issue that address selected topics in self-nonself discrimination.

Quantitation of T-cell receptor V beta chain expression on lymphocytes from blood, brain, and spinal fluid in patients with multiple sclerosis and other neurological diseases

Multiple sclerosis (MS) is an inflammatory, demyelinating disease of the central nervous system in which large numbers of T cells enter the brain and cerebrospinal fluid (CSF). To determine whether these cells represent restricted populations, we studied expression of T-cell receptor V beta chains on paired samples from the central nervous system and blood of patients with MS or other neurological diseases (OND) using a quantitative polymerase chain reaction. The distribution of V beta chain expression in blood was skewed, with a significant preponderance of message from V beta genes 1 through 8 (p = 0.0001). Such skewing was not present in samples from the CSF and brain. Patterns of V beta gene expression were different among paired samples from spinal fluid and blood and were relatively heterogeneous. Blood and CSF samples from a patient with acute meningitis were studied on two separate occasions. The patterns of V beta expression changed over 72 hours in both the blood and the CSF. With one exception, no oligoclonal populations of T cells were observed nor were there disease-specific patterns of V beta gene expression in the blood or CSF. Samples from 2 MS brains and 1 OND brain expressed patterns of V beta genes that were different and less heterogeneous than those in paired blood. In addition, expression of V beta 12 was remarkably increased in the 2 MS brains, suggesting a selective recruitment or expansion of T cells expressing this gene. These data demonstrate that populations of T cells from blood, spinal fluid, and brain differ from one another and can fluctuate during periods of acute inflammation.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental regulation of thymocyte susceptibility to deletion by "self"-peptide

The specificity of the T cell receptor (TCR) repertoire for foreign peptide bound to self-major histocompatibility complex (MHC) molecules is determined in large part by positive and negative selection processes in the thymus, yet the mechanisms of these selection events remain unknown. Using in vitro organ culture of thymi isolated from mice transgenic for a TCR-alpha/beta specific for cytochrome c peptide bound to I-Ek, we analyzed the developmental timing of negative selection (deletion). On the basis of the experiments described below, we conclude that all CD4+8+ thymocytes, and only CD4+8+ thymocytes, are susceptible to negative selection mediated by the cytochrome c peptide antigen. First, we found that deletion of thymocytes resulting from addition of the cytochrome c peptide to the thymic organ cultures can occur at the earliest stage of TCR, CD4, and CD8 coexpression. Second, we found that CD4+8+ thymocytes isolated from positively selecting or nonselecting MHC haplotypes were equally efficiently deleted in vitro, suggesting that positive selection is not a prerequisite for deletion. Third, we examined the effects of TCR/ligand avidity on the developmental timing of deletion by varying the concentration of cytochrome c peptide added to the organ cultures. We detected deletion only at the CD4+8+ stage: intermediate concentrations of peptide that resulted in partial deletion of CD4+8+ cells did not eliminate the appearance of mature CD4+8- cells. Finally, we found that CD4+8- thymocytes were resistant to deletion as well as activation by peptide antigen added to the intact organ cultures. Nevertheless, the CD4+8- thymocytes isolated from the peptide-treated organ cultures responded vigorously to peptide presented by spleen cells in vitro. Thus, the T cells were tolerant of (but not anergized by) self-antigen encountered in thymic organ culture. Together, these results indicate that thymocytes susceptible to negative selection are not developmentally distinct from those susceptible to positive selection, and further, that the thymic microenvironment plays a role in regulating the outcome of TCR/ligand interactions.

Analysis of the immune system with transgenic mice: T cell development

Transgenic mice carrying functionally rearranged T cell receptor genes have contributed significantly to our knowledge of T cell development and thymic positive and negative selection processes. In addition, TCR-transgenic mice have been used to investigate mutations affecting thymocyte development, like scid and lpr. Gene targeting by homologous recombination will allow to analyze more specifically the molecular mechanisms underlying thymic selection and peripheral tolerance.

The role of the T cell receptor in positive and negative selection of developing T cells

Although many combinations of alpha beta T cell receptors are available to the T cells in any given organism, far fewer are actually used by mature T cells. The combinations used are limited by two selective processes, positive selection of T cells bearing receptors that will be useful to the host, and clonal elimination or inactivation of T cells bearing receptors that will be damaging to the host. The ways in which these two apparently contradictory processes occur, and the hypotheses that have been suggested to reconcile them, are discussed.