ADMINISTRATION OF IL-12 INDUCES A CD3(-)CD8(-)B220(+) LYMPHOID POPULATION CAPABLE OF ELICITING CYTOLYSIS AGAINST FAS-POSITIVE TUMOR-CELLS()CD4()

IL-12 has been shown to induce a number of biological effects (I, 2) t hat are consistent with its potential role as an antitumor agent. This cytokine enhances NK (3) and CTL (4) activities, acts as an NK and T cell growth factor (5-7), stimulates secretion of various cytokines, p articularly IFN-gamma (IFN-gamma) by NK and T cells (3, 8), and promot es maturation of the Th1 cell subset (1, 9). In addition, IL-12 induce s lymphoid cells to express a variety of surface molecules, including IL-2R (10) and adhesion molecules (11, 12). Some of these biological p roperties, which may be mutually related, have been considered to cont ribute to controlling tumor growth. The molecular mechanisms involved in the induction of cell-mediated cytotoxicity are still not resolved. Two predominant mechanisms of cytotoxicity have recently been describ ed: namely, cytotoxicity mediated by perforin and,granzymes stored in cytoplasmic granules and that mediated by Fas ligand (FasL)(3) express ed on effector cell surfaces (13). It appears that Fast is induced on various types of lymphoid cells. These include CD8(+) CTL (14, 15), CD 4(+) Th1 clones (16-18), and a unique lymphoid subset (CD3(+)CD4(-)CD8 (-)B220(+)) that spontaneously develops in MRL/lpr mice (19). Previous ly, we showed that administration of rlL-12 into tumor-bearing mice re sults in complete tumor regression in two tumor models (20, 21) and th at the tumor regression is associated with massive infiltration of lym phoid cells in tumor masses (20-22). Tumor regression could be induced by multiple mechanisms mediated by different subsets of lymphoid cell s, some of which have been previously described (22, 23); However, it has not been investigated whether tumor cells express Fas or whether F as-FasL interactions are involved in anti-tumor mechanisms. It is poss ible that lymphoid cells activated following IL-12 administration may exhibit Fast-mediated cytotoxicity if tumor cells express Fas. In the present study we investigated whether administration of IL-12 can gene rate lymphoid cells with the capacity to elicit FasL-mediated cytotoxi city and, if so, which subset(s) of lymphoid cells is responsible for IL-12-induced anti-Fas cytotoxicity. The results show that systemic IL -12 administration induced tumor regression in CSA1M fibrosarcoma (BAL B/c origin) and OV-HM ovarian carcinoma ((B6C3)F-1 origin) models. CSA 1M exhibited Fas Ag, whereas OV-HM did not. Systemic IL-12 administrat ion induced the generation of a lymphoid population with unique phenot ypes (CD3(+)CD4(-)CD8(-)B220(+)) in spleens from both BALB/c and B6C3F (1) mice; A CD3(+)CD4(-)CD8(-)B220(+) cell-enriched splenic population from either strain of mice exhibited considerable degrees of cytotoxi city against CSA1M (Fas(+)), but not against OV-HM (Fas(-)) tumor cell s. The cytotoxicity of CSA1M cells was almost completely inhibited by Fas-Fc, a recombinant fusion protein between the extracellular domain of mouse Fas and Fc of human Ig. Regressing tumor masses following IL- 12 administration exhibited a massive lymphoid cell infiltration and e xpressed significant levels of Fast mRNA. These results indicate that administration of IL-12 to tumor-bearing mice induces a unique populat ion of lymphoid cells (CD3(+)CD4(-)CD8(-)B220(+)) that exhibits Fast-m ediated cytotoxicity on Fas(+) tumor cells and would infiltrate into t umor sites.