Local versus systemic interleukin-2: Tumor formation by wild-type and B7-1-positive murine melanoma cells

Modification of murine K1735 melanoma cells to express the immune costimula tor B7-1 had no effect on tumor formation in syngeneic mice. In contrast, < 40% of mice inoculated with K1735 cells modified to secrete murine interleu kin-2 (IL-2) formed tumors, and no tumors formed when the K1735 cells coexp ressed both murine IL-2 and B7-1. However, administration of systemic recom binant human IL-2 had no detectable effect on the formation of rumors by th e B7-1-expressing K1735 cells. By contrast, admixtures of IL-2-secreting an d B7-1-expressing K1735 cells formed fewer tumors than either cell type alo ne. Murine IL-2 was effective only when secreted locally, because the IL-2- secreting cells inoculated into the right flank did not affect the growth o f the B7-1-expressing cells inoculated into the opposite flank.