In vitro tests correlating with presence or absence of graft-vs-host disease in DLA nonidentical canine radiation chimeras: evidence that clonal abortion maintains stable graft-host tolerance.

Twenty random bred canine littermate pairs were selected for marrow transplantation on the basis of nonidentity at the major canine histocompatibility complex, DLA, including reactivity in mixed leukocyte culture (MLC). Recipient (host) lymphocytes were cryopreserved before transplantation for subsequent use as stimulator cells in MLC. Recipients were conditioned with 1200 R total body irradiation and given a hemopoietic graft from their respective littermates. All recipients received methotrexate for 102 days post-transplant in an attempt to ameliorate graft-vs-host disease (GVHD). Ten recipients developed fatal acute GVHD and peripheral blood mononuclear cells (of donor origin) from all 10 animals retained their reactivity in MLC to preserved host lymphocytes. Six dogs developed late onset or delayed fatal GVHD, although five had a prolonged period of clinical good health before its onset. During this period of good health, chimera cell reactivity in MLC to host lymphocytes was markedly diminished, but strong reactivity was detectable with the onset of delayed GVHD. Chimera cells reactive to host antigens were thus detectable in all chimeras during acute and delayed GVHD. Four dogs did not develop GVHD and survived 142 days, > 485, > 577, and > 1005 days post-transplant. Cells from all four had virtually absent reactivity to host lymphocytes while retaining good reactivity to third party lymphocytes. The nonreactivity was specific for the DLA-D antigens of the host, but extended to unrelated DLA-D homozygous dogs if they expressed the same DLA-D antigens as the host. Chimera lymphocytes cytotoxic to host cells could not be detected in cell-mediated lympholysis. The relative MLC unresponsiveness of cells from these healthy long-term chimeras was not mediated by circulating suppressor cells nor by serum-blocking factors. The mechanism of complete operational tolerance in these chimeras is compatible with the concept of clonal abortion.