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Interleukin 2 (IL 2) has several potential uses in cancer therapy including: the augmentation of specific T cell mediated anti-tumor immunity and the activation of non-specific cytolytic effector cells, termed lymphokine-activated killer (LAK) cells. The current review will present data from our laboratory demonstrating in animal models the feasibility of both potential approaches. Studies to be reviewed show that: IL 2 can induce the proliferation and expansion in number of tumor-reactive T cells in vitro; T cells grown in culture in IL 2 can be effective reagents in vivo for specific tumor therapy; the administration of exogenous IL 2 can induce the growth and augment the function of cultured T cells in vivo; however, as a corollary, T cells cultured long-term in vivo with IL 2 are functionally limited in vivo without the administration of exogenous IL 2 in vivo; by contrast, T cells grown in vitro with specific antigen, as opposed to IL 2, as the major stimulus for proliferation are able to proliferate rapidly in vivo, distribute widely in host lymphoid organs, and mediate therapy of disseminated murine leukemia; importantly, such antigen-driven long-term cultured T cells can survive long-term in vivo and provide specific immunologic memory, and, the administration of low-dose IL 2 in vivo can induce the growth of antigen-driven long-term cultured T cells in vivo and thereby increase the number of functional memory T cells; the culture of lymphoid cells in high concentrations of IL 2 can induce LAK cells in vitro capable of lysing leukemia in vitro; LAK cells generated in vitro can mediate a small but detectable anti-tumor effect in vivo against disseminated leukemia as an adjunct to chemotherapy; and, high-dose IL 2 administered in vivo can activate LAK cells in vivo and cure disseminated murine leukemia. Therefore, it is highly likely that IL 2 can become an effective reagent for the therapy of human cancer.

In the current study we used the therapy of established murine leukemia to identify the lymphocyte subsets responsible for toxicity and for therapeutic efficacy of high-dose IL-2. Initial results confirmed that high-dose IL-2 induces marked proliferation of a variety of host cells, including NK cells, Lyt-2+ T cells, L3T4+ T cells, and B cells. Infusion of antibody to NK-1.1 depleted NK-1.1+ cells in vivo and greatly reduced the toxicity of IL-2, but did not decrease therapeutic efficacy. By marked contrast, depletion of host T cells, either Lyt-2+ or L3T4+, had no effect on toxicity but greatly reduced therapeutic efficacy. The requirement for host T cells for the curative effect of IL-2 gives credence to the possibility that substantial efficacy of high-dose IL-2 against established malignancy may require existent host antitumor immunity. Since the human tumors that have been shown to have the most substantial responses to IL-2 (i.e., malignant melanoma and renal cell carcinoma) are those long considered to be immunogenic in the autochthonous host, the current study predicts that for these, as well as other immunogenic human tumors, it should be possible to decrease the toxicity and thus increase the therapeutic index of IL-2 by selectively depleting NK cells in vivo.

The aim of the current study was to determine the ability of antigen-driven cloned helper cell independent cytotoxic T lymphocytes (HITc) to proliferate and to survive in vivo and to mediate tumor therapy. The HITc clone utilized (denoted 1.B6) was specifically cytolytic to FBL-3, a syngeneic Friend virus-induced murine leukemia. Activation in vitro (48 hr) with FBL-3 induced secretion of interleukin 2 (IL 2), expression of IL 2 receptors (IL 2R), and in vitro proliferation. These cells could be "rested" for several weeks without stimulation, which resulted in reduced expression of IL 2R; however, restimulation with antigen resulted in reinduction of IL 2R and proliferation. The ability of cloned HITc to proliferate and to survive in vivo was examined in cyclophosphamide (CY) pretreated donor mice congenic for the Thy-1 gene. Adoptively transferred cloned HITc could be found in large numbers, and were widely distributed in vivo 1 wk after transfer. In tumor therapy, 1.B6 cells when injected into a site of tumor (i.p.) and used as an adjunct to CY were effective against disseminated FBL-3. In this circumstance, cloned 1.B6 cells could be recovered from cured mice 125 days after transfer and were shown to specifically lyse tumor and proliferate in vitro in response to FBL-3. Thus as an adjunct to CY, tumor-specific cloned HITc are capable of eradicating disseminated leukemia, persisting long-term in vivo, and providing specific immunologic memory.

Interleukin 2 (IL 2) in high concentration induces lymphocytes to become nonspecifically cytolytic to a wide variety of tumor targets. We evaluated the therapeutic potential of such lymphokine-activated killer (LAK) cells in vivo and high-dose II 2 in vivo against disseminated murine leukemia. To quantitate the potential anti-leukemia effect of LAK cells in vivo, B6 mice were injected i.p. with graded doses of FBL-3 leukemia cells followed by LAK cells. In this Winn-type assay, 1 X 10(7) LAK cells were able to prevent the outgrowth of 1 X 10(2) FBL-3 cells in only 50% of mice and did not prevent the outgrowth of 1 X 10(6) tumor cells. Thus LAK cells, highly cytolytic to FBL-3 in vitro, mediated only a limited anti-tumor effect when applied directly to leukemia cells in vivo. LAK cells used as an adjunct to chemotherapy induced a small but non-curative effect against FBL-3, however. In this circumstance, LAK cells were markedly less effective than were immune spleen cells from mice previously sensitized to FBL-3. To test the anti-leukemia effect of high-dose IL 2 in vivo, B6 mice were inoculated with 5 X 10(6) FBL-3 cells followed by repeated doses of IL 2 at dose levels shown to induce LAK in vivo. "LAK-inducing" IL 2 doses on days 5 to 9 after FBL-3 inoculation, when tumor was disseminated, cured 50% of the mice. Treatment on days 5 to 9 was far more effective than on days 0 to 4, implying that the evolution of a host-tumor interaction was essential for the therapeutic effect of IL 2. Mice cured of FBL-3 by high-dose IL 2 were found to be immune to FBL-3, suggesting that tumor eradication resulted from a collaboration between LAK activity and tumor-specific immunity.

One goal of our research has been to define the principles necessary to utilize cultured T cells as reagents in vivo in order to augment specific T cell immunity and to utilize the augmented immunity as a form of cancer therapy. A potential barrier for the use of cultured T cells in vivo has been the previously demonstrated inability of cultured T cells to survive in vivo. As an example, studies to be reviewed below showed that a small precursor population of tumor-specific T cells could be grown to large numbers in vitro by repeated supplementation of media with exogenous Interleukin 2 (IL 2) and that the resultant long-term cultured T cells could mediate specific tumor therapy in vivo. However, T cells grown with IL 2 lost the ability to proliferate in response to immune stimulation by tumor antigen, became dependent upon exogenous IL 2 for survival, and thus died rapidly in vivo without repeated administration of exogenous IL 2. By contrast, T cells grown long-term in vitro in response to antigen-stimulation, as opposed to exogenous IL 2, were able to proliferate in vivo in response to stimulation by tumor antigen, mediate tumor therapy and persist long-term in vivo as functional memory T cells. Thus, the previously demonstrated inability of cultured T cells to survive and persist in vivo apparently resulted from the culture conditions utilized and did not reflect an intrinsic defect of all cultured T cells.