IGM-MEDIATED OPSONIZATION AND CYTOTOXICITY IN THE SHARK

Two types of cytotoxic reactions have been observed using cells from t he nurse shark: spontaneous cytotoxicity mediated by cells of the macr ophage lineage and antibody-dependent killing carried out by a differe nt effector cell population. Previous data showed that removal of phag ocytic cells using iron particles abolished macrophage-mediated killin g, but not antibody-dependent reactions. The current study used single cell assays and showed that the effector of antibody-driven reactions was the neutrophil. Surprisingly, the mechanism of killing was shown to be phagocytosis mediated by both 7S and 19S immunoglobulin M (IgM). Reactions proceeded with as little as 0.01 mu g of purified 19S or 7S IgM and were complete within 4-6 h. In contrast, purified immunoglobu lin did not adsorb to macrophages and had no effect on target cell bin ding or cytotoxicity. Pretreatment of cells with cytochalasin D abolis hed the phagocytic reaction, but not spontaneous cytotoxicity. These d ata show that antibody-mediated killing results from opsonization and phagocytosis; the mechanism of macrophage killing is currently unknown . In addition, these data show that the shark neutrophil, not the macr ophage lineage, carries a receptor for Fe mu.