Ultrastructural analysis of leucocyte interaction with tumour targets in a teleost, Cyprinus carpio L

The first step in natural killer activity, target cell binding, was investigated ultrastructurally in order to identify presumptive cytotoxic cells. After incubation of unfractioned peripheral blood cells of the carp, Cyprinus carpio L., with HeLa tumour cells as targets, regular conjugate forming was observed. Targets were surrounded by cells bearing features either of lymphocyte or monocyte/macrophage types. Cells with the usual lymphocytic characteristics contained acid phosphatase positive granules of various density and - after two hours of incubation - paracrystalline inclusions were observed. Cells with monocyte/macrophage morphology were characterized by abundant organelles, varying granules and occasional phagolysosomes. In both cell types development of the contact area was similar to those events known from mammalian cytotoxic cells: development of interdigitating microvilli and increase in the reoriented secretory apparatus (both more intense in the monocyte/macrophage cells than in the lymphocytes). The possibility of several cytotoxic cell types instead of one primitive cytotoxic precursor type proposed earlier in literature is discussed.