Sublethally (600 R) irradiated (CBA x C57BL)F1 mice were grafted intravenously with parental lymph node cells in doses ranging from 0.2 x 106 to 12 x 106. The transplantation of these lymphoid cells leads to inactivation of the recipient's endogenous CFU (as measured by the diminution of the number of colonies registered on the 10th day after irradiation). A 50% inactivation was observed when the graft size of the CBA cells was 0.52 x 106. This figure for C57BL cells was 10 times more. This experimental system evaluates two simultaneously developing processes: the multiplication of endogenous CFU and the homograft reaction of transplanted lymphocytes against them. Both processes can be quantitatively estimated simultaneously in the same experiment by the determination of the number of colonies in corresponding experimental groups. Thus it was possible in a single experiment to compare quantitatively the effect of immunosuppressants on two points: (a) mitostatic action (suppression of CFU) and (b) lymphotoxic action. The latter, a true immunosuppressive effect, represents suppression of GVH activity of lymphoid cells and is demonstrated by abolition of the inhibition of endogenous colony formation. In the present system we have tested 6-MP, ALS, cyclophosphamide, hydrocortisone, and other drugs. The definite mitostatic and lymphotoxic doses of drugs are ascertained. Cyclophosphamide and ALS proved to be drugs with high dose ranges of selective lymphotoxic action. Hydrocortisone acetate had a more narrow range of selective lymphotoxic effect. 6-MP and Imuran (azathioprine) failed to exert any selective action on lymphoid elements. They possessed pronounced mitostatic efficiency, however.

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