Abstracts for the 18th Annual Scientific Meeting of the International Society for Biological Therapy of Cancer, Bethesda, Maryland, October 31–November 2, 2003s for the 18th Annual Scientific Meeting of the International Society for Biological Therapy of Cancer, Bethesda, Maryland, October 31–November 2, 2003 Natural Killer Cells & Innate Immunity CD40 Ligation Induces Antitumor Reactivity of Murine Macrophages Via an Interferon Gamma-Dependent Mechanism Ilia N Buhtoiarov, Hillary E Lum, Paul M Sondel, Alexander L Rakhmilevich. Department of Human Oncology and Comprehensive Cancer Center, University of Wisconsin, Madison, WI; I.N.B. Is Supported by the UICCAmerican Cancer Society Fellowship Grant for Beginning Investigators, Geneva, Switzerland. We investigated if macrophages can be activated via CD40 ligation in vivo and in vitro to mediate antitumor effects. Intraperitoneal injection of an agonistic anti-CD40 mAb into C57BL/6 mice resulted in activation of peritoneal macrophages capable of suppressing B16 melanoma cell proliferation in vitro. This antitumor activity of macrophages from anti-CD40 mAb-treated mice was greatly increased in the presence of lipopolysaccharide (LPS). Anti-CD40 mAb also primed macrophages in vitro, similar to exogenous interferon gamma (IFN), to mediate the cytostatic effect against B16 cells in the presence of LPS. This cytostatic activity of macrophages against tumor cells directly correlated with nitric oxide (NO) production. Activation of macrophages by anti-CD40 mAb depended on endogenous IFN, as both in vivo and in vitro induction of the cytostatic effect of macrophages by anti-CD40 mAb was abrogated in IFNknockout mice. Similarly, in vitro activation of macrophages from immunocompetent C57BL/6 mice with anti-CD40 mAb was reduced in the presence of antiIFNmAb. Macrophages obtained either from C57BL/6 mice depleted of T and NK-cells by antibody treatment, or from Scid/beige mice, were still activated by anti-CD40 mAb. These results argued against the role of NK and T cells as the sole source of exogenous INFfor macrophage activation by CD40 ligation, and suggested that antiCD40 mAb-activated macrophages could produce IFN. We confirmed this hypothesis by detecting intra-cytoplasmic IFNin macrophages stimulated with anti-CD40 mAb in vitro. Taken together, our results show that CD40 ligation induces antitumor reactivity of macrophages which depends on endogenous IFN. Supported by RO1 grant CA87025-01 from the NCI, NIH. Cytotoxicity of IL-2 Activated Cord Blood Mononuclear Cells Stanka Derzic, Vicki Slone, Mike Schwartz, Leonard Sender. CHOC Research Institute, Cancer Research Lab, Children’s Hospital of Orange