TWENTY-FOURTH ANNUAL EASTERN FISH HEALTH WORKSHOP Royal Pavilion Resort, Atlantic Beach, NC MARCH 9-11, 1999 Protection Of Target Cell Induced Apoptosis In Nonspecific Cytotoxic Cells By Serum Factors G.Reid Bishop, Liliana Jaso-Friedman, and Donald L. Evans Department of Medical Microbiology and Parasitology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602 Innate immunity is critical for survival of teleost fish during challenge by infectious agents. As a primary defense against pathogens, tilapia, (Oreochromus nilotica), utilize circulating nonspecific cytotoxic cells [NCC]. Previously, it was shown that, like mammalian natural killer [NK] cells, NCC from anterior kidney do not normally recycle after target cytolysis. Mechanisms of NCC down-regulation following killing were previously unexplored. Here, flow cytometric cell cycle data are presented which demonstrate that following incubation with HL-60 targets, NCC undergo apoptosis as demonstrated by DNA hypoploidy. In contrast, control experiments reveal that NCC alone do not undergo apoptosis in the absence of targets. These results suggest that NCC are prevented from recycling by undergoing a form of programmed cell death either concomitant with or following target cell interaction. Further flow cytometric data reveal that target-induced apoptosis of NCC can be completely inhibited by incubation of NCC with whole serum isolated from cold-stressed tilapia prior to and during incubation with HL60 targets. These data are in agreement with 51Cr release assays where NCC treated with stressed serum demonstrate enhanced killing of HL-60 target cells with respect to media and heat-inactivated serum controls. Also, NCC treated with the topoisomerase I inhibitor, camptothecin, undergo apoptosis similar to mammalian tumor cell lines. Like target-induced apoptosis, camptothecin-induced apoptosis of NCC is also inhibited by whole stress serum. Combined, these experiments suggest the presence of soluble factors that enhance NCC cytolytic activity by a general anti-apoptotic mechanism which allow NCC to recycle following initial target cell interactions. Experiments designed to biochemically characterize the stress serum factor are currently in progress. Research supported by a BARD grant. Return to 24th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page