Marrow grafts after 1200 R of whole-body irradiation were carried out between pairs of unrelated dogs of different breeds that were incompatible for dog histocompatibility antigens. Three groups of recipients were studied. Recipients in all groups were treated with the immunosuppressive agent methotrexate (MTX), 0.4 mg/kg, on days 1, 3, 6, and 11, and then once weekly until day 102 after grafting. Dogs in group 1 were given MTX only. Thirty of 35 recipients died between 11 and 51 days and five survived more than 102 days. Median survival time was 20 days. Dogs in group 2 were treated with rabbit antidog antithymocyte serum (ATS) when graft-versus-host (GVH) disease became established. Eight of ten recipients died between 19 and 76 days and two survived more than 102 days. Median survival time was 45 days. Dogs in group 3 were treated with daily injections of prednisone when GVH disease became apparent. Seven of nine recipients died between 11 and 34 days and two survived more than 102 days. Median survival time was 24 days.
It was concluded that established GVH disease in dogs with histoincompatible marrow grafts can be favorably influenced by ATS with significant (p < 0.01) prolongation of survival. However, there was no evidence that treatment with ATS increased the fraction of long-term survivors in this histoincompatible setting. Prednisone was ineffective and did not prolong survival (p > 0.1).