Human bone marrow lymphoid cells, particularly when enriched with plasma cells, as in multiple myeloma, respond to pokeweed mitogen and to antiserum to immunoglobulin but not to phytohemagglutinin. Cells of patients with X-linked agammaglobulinemia of the bursal deficient type failed to respond to either pokeweed or to the antiserum to immunoglobulin. Leukocytes of the agammaglobulinemia patients however responded in a normal fashion to phytohemagglutinin. Just as the in vitro response to phytohemagglutinin is taken as an index of the thymus-dependent system, the in vitro response to both antiserum to immunoglobulin and pokeweed may be considered an index for the bursaldependent system. Human bone marrow, therefore, contains bursal cells and probably very few or no thymus cells.

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In five patients with idiopathic aplastic anemia, the colony-forming unit (CFU-c) assay was used to quantitate stem cells and test the possible presence of suppressor cells inhibiting bone marrow differentiation. All five marrows failed to form CFU-c. In one out of the five cases tested, coculture of the patient's marrow with normal marrow suppressed the latter to form CFU-c. Removal of T-cells from the patient's bone marrow freed the aplastic marrow to produce more colonies and, in the coculture study, abrogated its inhibitory activity on the normal bone marrow. Treatment of the patient with horse antihuman thymocyte globulin (ATG), improved the peripheral blood count and myelopoiesis. Furthermore, the aplastic marrow increased its colony forming capacity from less than 1% pre-ATG to 46% post-ATG of the normal control, and its inhibitory activity on the normal marrow was ablated. In selected cases of aplastic anemia, with evidence of T-suppressor cell hyperactivity, treatment with ATG may offer an alternative approach to bone-marrow transplantation.

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