title: Cyclosporin A.: a powerful immunosuppressant in the rat
Creators: name: Hutchinson, Ian Fraser
Publisher: name: University of Glasgow
Year: 1984
Subjects: subject: Immunology, pharmacology.

description: Organ transplantation is now an accepted method of treating end stage kidney failure. Other organs are also being transplanted and an increasing number of heart, liver, pancreas and bone marrow transplants are being carried out. Grafts may be unsuccessful for many reasons but the biggest problem is graft rejection e.g. only approximately 40% of all first cadaver kidney grafts are functioning after six years. The first prolonged human cadaver renal allograft survival was reported from Boston in 1962, when a patient was treated with azathioprine, a purine analogue similar to 6-mercaptopurine, to prevent rejection. Since then, major advances in the understanding of the immune system, and the establishment of tissue typing laboratories have taken place. However, since the 1970s graft survival figures have not improved significantly. Indeed, azathioprine in combination with corticosteroids have remained unchallenged as the mainstay of immunosuppressive therapy during the past two decades. Recently, the discovery of Cyclosporin A has raised hopes that a major advance in immunosuppression has been achieved. In 1976 at the Biological and Medical Research Division of Sandoz Limited, Basle, Switzerland, during a screening programme of fungus extracts, a cyclic endecapeptide, Cyclosporin A (CyA), was identified as having marked immunosuppressive properties. This observation led to studies of CyA in different species with a variety of organ transplants, notably at Cambridge, England. Following these, experiments a pilot human study was begun by Professor Caine in Cambridge using CyA in clinical organ grafting, initially as the sole immunosuppressant. The results were encouraging. CyA had been shown to be a potent immunosuppressant yet its mode of action was unclear. Early workers had suggested that it acted primarily on lymphocytes, possibly on T lymphocytes" Further information has revealed that the mode of action may be mediated through a subpopulation of T cells, namely suppressor cells. Using the cardiac allograft model in the rat, the aim of the study described in this thesis was to investigate the mechanisms by which CyA mediated its immunosuppressive activity. (LEW x BN)F1 and (LEW x WF)F1 animals were used as organ donors and LEW animals as recipients. CyA was administered at 15 mg/kg/day for 7 days commencing immediately after transplantation. All grafts functioned normally for more than 100 days in healthy recipients. The humoral arm of the immune response of these grafted and CyA-treated recipients was assessed by examining complement-dependent cytotoxicity, antibody-dependent cell-mediated cytotoxicity and the reverse haemolytic plaque assay. The cell-mediated component was measured by examining lymphocyte-mediated cytotoxicity. Both the cell-mediated and the humoral components were profoundly suppressed throughout the 100 day period after transplantation. The nature of this unresponsiveness was examined in more detail by adoptively transferring thymocytes from CyA-treated animals bearing well-functioning grafts into unmodified animals and performing test heart grafts 24 hours later. Prolongation of these test heart grafts suggested that these transferred cells had suppressor characteristics. The failure of these cells to prolong third party test grafts suggested antigen- specific immunosuppression. Such donor-specific immunosuppression was supported in vitro by the results obtained in the mixed lymphocyte response in which there was significant suppression against donor cells but not third party cells. In both these in vivo and in vitro experiments the donor-specific suppressor activity appeared only after the cessation of CyA treatment. The finding of cells with suppressor characteristics is supported by the results of studies of dynamic responses of certain lymphoid tissues to the cardiac allografts. Previous studies have suggested that T lymphocytes in bone marrow migrate first to the thymic medulla and then to the splenic peri-arterial sheath (PAS) via the marginal zone (MZ). A second line of T lymphocytes may migrate first to thymic cortex and then to the splenic red pulp (RP). In these present experiments medullary thymocytes and T lymphocytes in the splenic PAS and MZ were strikingly reduced in size 1 to 2 weeks after the completion of CyA treatment. It has been postulated that helper and cytotoxic T lymphocytes may develop in the thymic medulla while suppressor T lymphocytes may originate in the thymic cortex. Thus CyA may be predicted to favour preferentially suppressor cell function. In the present experimental studies using the cardiac allograft in the rat it has been demonstrated that CyA is indeed a potent immunosuppressant, suppressing both cell-mediated and humoral immunity. It has also been shown that following the cessation of CyA treatment there is emergence of cells with antigen-specific suppressor characteristics. It is suspected that 'these suppressor cells act by abrogating or overriding the function of all specific helper T cells necessary for initiating differentiation of cytotoxic T lymphocytes and antibody-producing B cells.