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Melphalan-induced Enhancement of Antitumor Immune Reactivity in Thymocytes of Adult BALB/c Mice Bearing a Large MOPC-315 Tumor¹

Department of Microbiology and Immunology, University of Illinois at Chicago Health Sciences Center, Chicago, Illinois 60612

At no stage of tumor growth are thymocytes from MOPC-315 tumor bearers capable of bringing about the generation of enhanced antitumor cytotoxicity when added to immunization cultures of syngeneic normal spleen cells and "autochthonous" tumor cells. However, by Day 7 after low-dose melphalan [L-PAM (L-phenylalanine mustard)] therapy of mice bearing a large (20 mm) s.c. MOPC-315 tumor, their thymocytes exhibit such activity and it persists for at least 17 additional days. The ability of thymocytes from L-PAM-treated MOPC-315 tumor bearers to bring about the generation of enhanced antitumor cytotoxicity when added to immunization cultures of normal spleen cells and MOPC-315 tumor cells is evident over a 10-fold range of responder/stimulator cell ratios, and requires the presence of the thymocytes within the first day after initiation of the 5-day immunization cultures. In addition, immunization cultures containing normal spleen cells and thymocytes from L-PAM-treated MOPC-315 tumor bearers exhibit enhanced antitumor cytotoxicity by Day 4 after culture initiation that persists for at least 3 additional days. Thymocytes from L-PAM-treated MOPC-315 tumor bearers are able to bring about the generation of enhanced antitumor cytotoxicity only in response to stimulation with autochthonous tumor cells but not in response to stimulation with unrelated allogeneic EL4 tumor cells. The apparent specificity of the enhanced antitumor immune reactivity of thymocytes from L-PAM-treated MOPC-315 tumor bearers is not the result of extensive metastasis of tumor cells to the thymus. In fact, no tumor cells were found in the thymuses of MOPC-315 tumor bearers with methods that can detect 1 x 10³ tumor cells, indicating that if MOPC-315 tumor cells metastasize at all into the thymus, the thymuses of mice bearing a large MOPC-315 tumor contain fewer than 1 x 10³ tumor cells. Thus, thymocytes from mice which are engaged in the eradication of a large MOPC-315 tumor display enhanced antitumor immunity in response to stimulation with the autochthonous tumor cells. Such thymocytes may prove important to the outcome of low-dose L-PAM therapy for mice bearing a large MOPC-315 tumor, since the low-dose chemotherapy requires the contribution of T-cell-dependent antitumor immunity for its therapeutic effectiveness.

¹ Supported by Research Grant CA-35761 from the National Cancer Institute and Research Grant IM-435 from the American Cancer Society.

² In partial fulfillment of the requirements for the Doctor of Philosophy degree.

³ To whom requests for reprints should be addressed.

Received 2/ 3/87. Revised 5/14/87. Accepted 6/23/87.