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CORRELATION OF THE IN VIVO AND IN VITRO ACTIVITIES OF ANTITHYMOCYTE SERA (ATS) WITH THE IMMUNIZING ANTIGEN DOSE

SUMMARY Rabbits were immunized with 1, 3, 10, 30, 100, 300, or 1,000 × 106 murine thymocytes per kg according to the method of Levey and Medawar. Thus, 33 individual and 6 pooled anti-mouse antithymocyte serum (ATS) preparations were obtained and tested for in vivo immunosuppressive (graft-protective) as well as for in vitro thymocytotoxic activity. It was found that: (1) at least 3 × 106 thymocytes/kg were necessary for inducing ATS of appreciable immunosuppressive activity; (2) rabbits immunized with 30 × 106 thymocytes/kg supplied sera of the most potent immunosuppressive activity; (3) the increase of the immunizing antigen dose over 30 × 106 thymocytes/kg resulted in ATS preparations of decreased immunosuppressive activity; (4) the graft-protective activity of an ATS pool corresponded to the average of the activities of the individual ATS preparations from which the pool had been mixed, i.e., the process of pooling itself did not modify the immunosuppressive activity; and (5) there was a good correlation (r = 0.72, P < 0.001) between the in vivo immunosuppressive (graft-protective) activity and the in vitro thymocytotoxic titre of ATS preparations. The theoretical and practical significance of these results is discussed.