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Eight patients with severe aplastic anemia (SAA) and two patients with congenital hypoplastic anemia (CHA) were treated with absorbed samples of rabbit anti-thymocyte globulin (ATG) at a total dose of 50-75 mg/kg body weight intravenously over 5 days. Duration of the disease was 1-14 years. Median age of the ten patients was 11 (4-15) years. Four of the eight patients with SAA were responders and transfusion independent for a follow up of 11 to 50 months. In two cases thrombocytopenia of about 100 000/mm3 is present. Four patients were non-responders three of them died in the meantime by the underlining disease. Two patients with CHA did not respond to ATG treatment. The CFU-C level of all eight patients with SAA before treatment was severely decreased. In co-culture studies with normal bone marrow no growth inhibition of normal bone marrow could be demonstrated. Also 20% patients serum did not inhibit normal bone marrow growth. The results are compared with data from literature and discussed in respect to alternative therapies.

Graft-versus-host disease (GvHD) is still a frequent complication in clinical marrow transplantation. It causes severe lesions in several tissues, and it also affects hemopoiesis and the immune response of the recipient. When the major histocompatibility complex (MHC) differ, GvHD is uniformly fatal. A major advance in circumventing GvHD was achieved by selection of histocompatible donors. In 1968, Epstein et al. (4) demonstrated, in dogs, that bone marrow could be grafted successfully from DLAmatched littermates. In man, despite the selection of HLA-matched sibling donors and prophylactic immunosuppressive post-transplant therapy, 50% of the recipients get GvHD and about 25% die from it (27). This mortality was attributed to minor histocompatibility antigens undetected by currently employed in vitro test systems. The limited number of compatible siblings and the occurrence of GvHD in HLA-matched combinations led to experimental studies in which GvHD was suppressed by eliminating the immune-reactive cell population of the graft.

Bone marrow removed from leukemic patients during remission, for retransplantation during relapse, may contain residual leukemic cells. Antisera against surface antigens of these cells should not react with hemopoietic stem cells. In order to produce an appropriate antiserum, rabbits were given injections of cells from the common form of acute lymphoblastic leukemia (cALL) of childhood. The resultant antiserum was absorbed with liver-kidney homogenate, chronic lymphoid leukemia cells (CLL), normal peripheral lymphocytes, and lymphoblastoid cells from B cell lines. The purified globulin fraction showed high complement-dependent cytotoxicity against the cells of 39 of 56 patients with ALL. It did not react with the cells of 11 T-ALL, 1 B-ALL, and 5 undifferentiated (without known markers) ALL. Furthermore, the antiserum did not react with the cells from acyte myeloid leukemias, chronic lymphoid leukemias, B-type lymphoblastoid cell lines, normal bone marrow cells, and peripheral blood lymphocytes. Unabsorbed anti-cALL globulin was found to be highly cytotoxic against hemopoietic colony-forming cells (CFU-c) and completely inhibited the growth of marrow cells and CFU-c in diffusion chambers. Absorption of anti-cALL with liver-kidney homogenate, CLL, and peripheral blood lymphocytes removed only part of the cytotoxic antibodies cross-reacting with antigens present on CFU-c. An additional absorption with lymphoblastoid cell lines removed the cytotoxic effect of anti-cALL against CFU-c completely and did not inhibit proliferation of marrow cells and CFU-c in diffusion chambers, while high cytotoxicity against cALL blasts was preserved. It follows that cALL antiserum lacks an inhibitory effect on normal hemopoietic stem cells when measured in CFU-c and diffusion chamber assays.