Colony inhibition of fibroblasts from chimeric dogs mediated by the dogs' own lymphocytes and specifically abrogated by their serum.

Nine canine irradiation chimeras were studied between 173 days and 7.5 years after 1200 to 1500 r of total body irradiation and transplantation of allogeneic bone marrow. Skin fibroblasts from chimeras and normal dogs were tested for colony inhibition by exposure to sera and peripheral blood lymphocytes from both chimeric and normal dogs. Lymphocytes from the chimeric dog were found to inhibit colony formation by its "own" fibroblasts while lymphocytes from other chimeras or from normal dogs did not. Serum from the chimera specifically abrogated this inhibitory effect. These results indicate that the immunological "tolerance" of the chimeric dog is mediated in vivo by blocking serum factors.