Interleukin‐2 transfected prostate cancer cells generate a local antitumor effect in vivo

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Abstract
In an effort to stimulate host‐mediated antitumor response against prostate cancer in an animal model, highly malignant Dunning MAT‐LyLu rat prostate carcinoma cells were transfected with the interleukin‐2 (IL‐2) cDNA, resulting in their ability to secrete large amounts of biologically active IL‐2. Although parental cells form lethal tumors when injected subcutaneously into syngeneic hosts at doses of ≥5,000, injections of IL‐2 secreting cells initially formed tumors and regressed completely in each of over 200 animals at all doses tested (104‐8 × 107 cells). Mixtures of parental and IL‐2 transfected cells were similarly rejected, demonstrating the non‐cell autonomous nature of the response. Histological analysis of regressing tumors revealed a vigorous, predominantly lymphocytic and macrophage infiltrate at day 2 and marked tumor necrosis by day 6. Immunohistochemical staining of infiltrating lymphocytes at this latter time point demonstrated numerous T cells bearing either CD4 or CD8 surface markers, suggesting these cells as possibly mediating the tumor rejection. The ability of athymic mice to reject the IL‐2 secreting tumor cells, however, suggests a non‐T‐cell‐mediated mechanism. Although splenic natural killer (NK) activity is increased following injection of IL2 secreting tumor cells, this activity appears to be unnecessary for tumor elimination since syngeneic animals injected with asialo‐GM1 antiserum to decrease NK activity also rejected IL‐2 transfected cells, albeit slightly less effectively than untreated animals. Immunization of animals with subcutaneous injections of IL‐2 transfected cells protected animals against a subsequent challenge of 104 wild‐type cells 1 to 2 weeks later in 19 of 51 cases; however, immunization did not confer protection against larger doses of parental tumor. These studies indicate that high local concentrations of IL‐2 stimulate the elimination of large local burdens of prostate cancer in this model system, and this elimination results in a weak, but detectable systemic immune response against wild‐type prostate cancer cells.