Following a course of cyclosporine, syngeneic rat radiation chimeras consistently develop a GVHD-like syndrome. Correlation of the thymic immunopathology with conditions leading to syngeneic graft-versus-host disease (sGVHD) suggested the hypothesis that reconstitution of the normal thymic microenvironment after CsA is necessary for self-tolerance. When thymic regeneration is impaired, as in rats receiving previous mediastinal irradiation, then self-reactive effector cells are not regulated and proceed to damage the target tissues. Alternately, it could be argued that the observed thymic abnormalities are irrelevant to sGVHD. To test the primary hypothesis, post-CsA thymic reconstitution was prevented by total thymectomy in unirradiated rats. These rats consistently developed acute type sGVHD seen at 7 and 21 days post-CsA while rats from the CsA-treated sham thymectomy control group failed to develop sGVHD. Because thymectomy prior to CsA blocks sGVHD, most likely the peripheral effector cells in the post-CsA thymectomy group were derived from the CsA-altered thymus. The absence of sGVHD in the sham group indicates that the thymus led to active regulation of these cells after stopping CsA. If regeneration of the thymus restored only negative selection, then the sham thymectomy group should have also developed sGVHD. Flow cytometry and morphology of the spleen and lymph nodes demonstrated that the thymectomized rats, like CsA-treated radiation chimeras, experienced a significant delay in maturation of T cells following CsA. In contrast to the usual model in radiation chimeras, however, the post-CsA thymectomized rats did not convert to chronic type sGVHD. The importance of an abnormal thymus for this transition was confirmed in syngeneic radiation chimeras. Thymectomy after CsA in these rats also blocked the rapid transition to chronic sGVHD.
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