It was shown that it is possible to use cyclophosphamide (Cy) to cause immunologically mediated regression of the immunogenic, Cy-resistant L5178Y lymphoma in syngeneic and semisyngeneic mice. In order to cause tumor regression it was necessary to give Cy shortly before or shortly after tumor implantation. However, regardless of whether Cy was given before or after tumor implantation, tumor regression did not commence until 10 days of progressive tumor growth, by which time the tumor was 1 cm in diameter. Tumor regression was associated with the presence in the spleen of an increased number of Lyt-2+ T-cells capable of passively transferring immunity to tumor-bearing recipients. This augmented level of immunity was sustained throughout the period of tumor regression. In contrast, a lower level of concomitant immunity generated by control tumor bearers decayed after Day 12 of tumor growth. Because the therapeutic effect of Cy could be inhibited by passive transfer of L3T4+ T-cells from normal donor mice it is apparent that the therapeutic effect of Cy is based on its ability to preferentially destroy L3T4+ suppressor T-cells. These putative precursor suppressor T-cells were regenerated 4 days after being destroyed by Cy. Taken together the results represent a striking example of the negative regulatory influence of suppressor T-cells on the immune response to an immunogenic tumor.

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Supported by Grants CA-27794 and CA-16642 from the National Cancer Institute, a grant from RJR Nabisco, and a grant from the Oliver S. and Jennie R. Donaldson Trust.

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