A soluble inhibitor of granulocyte macrophage colony growth, to which we shall refer to as T-de-rived colony-inhibiting activity (Td/CIA), was obtained from the supernatant of T cells from 5 healthy donors and 5 patients with severe aplastic anemia (SAA) in remission, following immunosuppressive therapy. The su-pernatants were purified by an AC A 44 column and the suppressor activity found in fractions of 70,000–80,000 daltons. Experiments were then performed to test for (a) endogenous productions of Td/CIA in normal marrow cells (NBM), (b) reversibility of suppression, and (c) competitive inhibition with human placenta-conditioned medium (HPCM). The results of this study can be summarized as follows: (1) the endogenous production of Td/CIA can be elicited by addition of mitogens to NBM, and is prevented if the marrow is T-depleted or treated with cyclosporin A; (2) suppression is completely reversible if Td/CIA is removed from NBM by washing at 1, 48, 72 and 96 h; (3) CFU-c which have been exposed to Td/CIA once, and freed from Td/CIA by washing, are still sensitive to a second exposure of Td/CIA; (4) there is no clear competitive inhibition between Td/CIA and HPCM. These experiments represent an in vitro model of a lymphokine-mediated regulation of CFU-c growth not associated with death of progenitor cells

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