The anti-tumor efficacy of lymphokine-activated killer cells and recombinant interleukin 2 in vivo: Direct correlation between reduction of established metastases and cytolytic activity of lymphokine-activated killer cells

Our previous studies demonstrated that the incubation of human peripheral blood lymphocytes or murine splenocytes in recombinant interleukin 2 (RIL 2) resulted in the generation of lymphokine-activated killer (LAK) cells capable of lysing a broad spectrum of fresh tumors in short-term chromium-release assays. Moreover, injections of LAK cells plus RIL 2 were highly effective in eliminating established 3 day metastases in the lung and liver (1-3). We have examined several parameters to define whether or not the cytolytic activity of LAK cells as measured in vitro correlated directly with the in vivo anti-tumor efficacy of adoptively transferred LAK cells. LAK cells plus RIL 2 could mediate marked reductions of established pulmonary metastases in mice rendered T cell deficient by adult thymectomy and lethal, total body irradiation followed by reconstitution with T cell-depleted bone marrow and spleen cells. Thus there was no requirement for additional T lymphocytes of host origin for successful therapy with adoptively transferred LAK cells plus RIL 2. Fresh splenocytes depleted of T cells by anti-Thy-1.2 monoclonal antibody plus complement generated LAK cells that were as highly lytic to fresh tumor in vitro and were as effective in reducing established pulmonary metastases as those generated from untreated or complement-treated splenocytes. Thus the precursor to LAK cells with anti-tumor activity in vitro and in vivo did not express the Thy-1 antigenic marker. In contrast, treatment of LAK effector cells (those generated from a 3-day incubation of fresh, normal splenocytes in RIL 2) with anti-Thy-1.2 antibody plus complement reduced or abolished their in vitro cytolytic activity. However, when combined with the systemic administration of RIL 2, these T cell-depleted, non-lytic LAK cells remained as effective in reducing the number of established pulmonary metastases upon adoptive transfer as untreated or complement-treated lytic LAK cells. Analysis by fluorescence-activated cell sorter revealed the reappearance of the Thy-1 marker in the T cell-depleted, non-lytic LAK cell population after in vitro, incubation of the cells with RIL 2 for an additional 3 to 5 days; in vitro cytolytic activity also returned to normal levels by this time. In vivo exogenous RIL 2 administration for 5 days was responsible for the complete regeneration of lytic activity of adoptively transferred T cell-depleted, non-lytic LAK cells. This was evidenced by the isolation of LAK cells with in vitro cytolytic activity directly from the lungs during regression of pulmonary metastatic foci after LAK cells plus RIL 2 administration. In contrast, mice with established 3 day pulmonary metastases that received T cell-depleted, non-lytic LAK cells plus Hanks' balanced salt solution administration showed no reduction in the numbers of tumor nodules; cells recovered directly from the lungs of these mice failed to have any LAK cell cytolytic activity when tested as effectors against fresh tumor targets in a chromium-release assay. Taken together, our in vitro and in vivo results indicate that Thy-1-positive, cytolytic LAK cells mediate regression of established pulmonary metastases in vivo when combined with the systemic administration of RIL 2.