Identification of cellular mechanisms operational in vivo during the regression of established pulmonary metastases by the systemic administration of high-dose recombinant interleukin 2

The systemic administration of high-dose recombinant IL 2 mediated significant reductions of established 3-day pulmonary micrometastases from both weakly immunogenic and nonimmunogenic sarcomas. However, when treatment with IL 2 was delayed for 10 days after the injection of tumor cells in an attempt to treat grossly visible pulmonary macrometastases, only those established from weakly immunogenic sarcomas remained susceptible. Established 10-day pulmonary nodules from the nonimmunogenic sarcomas became refractory to IL 2 therapy. We utilized selective depletion of lymphocyte subsets in vivo by the systemic administration of specific monoclonal antibodies to cells bearing either the L3T4 or Lyt-2 marker or a heteroantiserum to cells bearing the ASGM-1 glycosphingolipid to identify lymphocytes involved in IL 2-induced tumor regression. Cells with potent lymphokine-activated killer (LAK) activity against fresh tumor targets in vitro were identified in the lungs of IL 2-treated mice. By flow cytometry analysis, the majority of these effector cells were Thy-1⁺, L3T4^-, Lyt-2^-, ASGM-1⁺. Depletion in vivo of ASGM-1⁺ cells before the onset of IL 2 administration eliminated the successful therapy of 3-day pulmonary metastases from nonimmunogenic sarcomas, with concurrent elimination of LAK cell activity in the lungs. In mice with 3-day pulmonary metastases from weakly immunogenic sarcomas, both Lyt-2⁺ cells and ASGM-1⁺ cells were involved in IL 2-mediated tumor regression, but Lyt-2⁺ cells appeared to be the more potent mediator in the response. Lyt-2⁺ cells were also involved in the elimination of grossly visible 10-day macrometastases from these weakly immunogenic tumors. Depletion of L3T4⁺ cells had no effect on tumor regression. Thus, although LAK effectors derived from ASGM-1⁺ precursors can eliminate pulmonary micrometastases regardless of tumor immunogenicity, Lyt-2⁺ cells are predominant effectors in the elimination of both pulmonary micro- and macrometastases from weakly immunogenic sarcomas.