Defective de novo thymocyte maturation in cyclosporin A (CsA)-induced autoimmunity: expression of costimulatory and activation molecules

Lethally x-irradiated Lewis rats, reconstituted with syngeneic bone marrow and transiently treated with CsA for 4 weeks, will develop an autoimmune disease about 2–3 weeks after cessation of CsA therapy. CsA-induced autoimmunity is a thymus-dependent and T cell-mediated autoimmune disease. CsA is thought to generate autoreactive T cells by interference with negative selection in the thymus; x-irradiation is required to eliminate the peripheral autoregulatory T cell circuit. In this study we re-evaluate the effect of CsA on thymic atrophy and thymocyte maturation. Subsequently we examine the expression of costimulatory and activation molecules (CD2, CD5, CD11a, CD11b, CD25, CD28, CD43, CD54, OX-40, RT-1A, RT-1B and RT-1D) during distinct maturational stages in order to detect possible clues to the observed effects of CsA on thymocyte maturation and selection. The results revealed that CsA blocks maturation of double-positive TCRint to double-positive TCRhigh thymocytes and preferentially inhibits the development of mature CD4 single-positive thymocytes. Furthermore, CsA administration resulted in a reduced expression of the costimulatory CD2 molecule. Although it is a matter of debate whether this defective CD2 expression is involved in the aberrant maturation and selection of thymocytes, it is speculated that reduced costimulation via CD2 may influence differentiation into distinct T cell subsets

Infection with rat cytomegalovirus (CMV) in the immunocompromised host is associated with the appearance of a T cell population with reduced CD8 and T cell receptor (TCR) expression

Infection with human cytomegalovirus (HCMV) mostly results in a chronic subclinical infection; the immune system is unable to eliminate the virus and is apparently in equilibrium with the persistent virus. In the immunosuppressed host this equilibrium is disturbed, resulting in clinical infection. Rat cytomegalovirus (RCMV) infection in its host can be used as a model for HCMV infection. Using flow cytometry we examined the effect of acute RCMV infection on the composition of leucocyte subsets in the peripheral blood of both immunocompetent and immunosuppressed (5 Gy total body irradiation) Lewis rats. Special attention was paid to the natural killer (NK) cells and the CD8+ T cells known to be involved in the control of viral infections. Furthermore, we determined the presence of leucocyte subsets in the internal organs by immunohistochemistry. In immunocompetent rats, infection caused a small increase in NK cells and a large increase in CD8+ T cells. In contrast, infection of immunosuppressed rats caused a marked increase in NK cells and a small increase in CD8+ T cells, consisting of T cells with reduced expression of both CD8 and TCR. This phenomenon is characteristic of anergic CD8+ T cells, possibly explaining the ability of the virus to escape elimination by the immune system. The increase of NK cells in the peripheral blood of immunosuppressed, RCMV-infected rats could also be detected in kidney, liver, lung and pancreas, but not in salivary gland. This could explain the long persistence of infectious virus in the salivary gland

Cyclosporin-A differentially affects apoptosis during in vivo rat thymocyte maturation

Maturation arrest and interference with selection are two well-documented effects of cyclosporin-A (CsA) on the thymus. We recently hypothesized that these effects are related and owing to the reduced T-cell receptor (TCR)-CD3 complex-mediated signal transduction in thymocytes upon CsA treatment. In this hypothesis, the maturation arrest is the result of the additional depletion of thymocytes that normally survive by positive selection, whereas the impaired self-tolerance induction is caused by an increased survival of thymocytes that normally undergo negative selection. In this view, it is anticipated that CsA differentially affects thymocyte apoptosis during in vivo thymocyte maturation. Indeed, we report in this study a strong increase in apoptotic cells in the thymic cortex on in situ analysis. Simultaneously, the number of apoptotic cells had decreased at the cortico-medullary zone which is held to be the site for negative selection. Rapamycin (Rapa) also interferes with thymocyted maturation by inhibiting cytokine-driven proliferation. Hence, Rapa preferentially affects the early maturational stages of thymoctye development and is considered not to alter thymocyte selection and subsequent apoptotic events. Indeed, the number of apoptotic events appears not to be altered. However, possibly owing to the decrease in cortical macrophages, the apoptotic cells revealed an atypical enumeration around blood vessels. Taken together, our results favour the hypothesis that the dominant effect of CsA on the thymus is the reduction of the TCR-CD3 complex-mediated signal transduction in thymocytes upon interaction with stromal cells. Furthermore, the preferential localization of apoptotic cells next to blood vessels upon Rapa administration may indicate that endothelial cells are a back-up system for the removal of apoptotic cells