Graft versus host disease in rats made tolerant for organ allografts.

Tolerance for organ allografts would eliminate acute and chronic rejection as well as the need for nonspecific immunosuppression. A potential hazard of tolerance is the susceptibility to graft vs host disease (GVHD) due to unresponsiveness to alloantigen. This study sought to determine if our model of tolerance induction results in susceptibility to GVHD. Chimeras were created by transplantation of T-cell depleted ACI and Lewis bone marrow into lethally irradiated Lewis rats. Chimerism was determined post-BMTx by flow cytometric analysis of recipient spleens for the presence of ACI cells. ACI/Lew chimeras (ALC), animals that reconstituted only with syngeneic (Lewis) marrow (so-called failed chimeras), and ACI/Lew F1 (LACF1) hybrid rats were all given 200 x 10(6) ACI splenocytes i.v. Animals were examined for evidence of GVHD. GVHD was quantified using the popliteal lymph node enlargement assay. All LACF1 (n = 6) rats developed severe lethal GVHD following ACI splenocyte injection. Similarly, ALC (n = 6) developed fatal GVHD. Animals that reconstituted only with syngeneic Lewis marrow (failed chimeras) showed no signs of illness. GVHD was confirmed histologically and immunohistochemically. Failed chimeras receiving ACI splenocyte challenge showed no evidence of GVHD histologically. Popliteal lymph node enlargement indices reflected the presence of GVHD in the chimeras and hybrids but not in the failed chimeras. We conclude that tolerance induction by mixed chimerism results in susceptibility to GVHD if enough donor lymphoid tissue is given to the host at the time of organ transplant. Animals that are not mixed chimeras (failed bone marrow transplant) rejected the allogeneic splenocytes as evidenced by their lack of disease. Tolerance may therefore make the host defenseless against fatal GVHD.