Nonspecific cytotoxic cells in fish (Ictalurus punctatus). IV. Target cell binding and recycling capacity.

The morphology of nonspecific cytotoxic cells (NCC) was identified. NCC were purified by target cell conjugate formation and density gradient separation. NCC are monocyte-like. They have reniform nuclei and a low nucleus/cytoplasm ratio. Cytoplasmic granules were not seen after giemsa staining. Scanning electron microscopy demonstrated moderate surface villi and target cell attachment occurred via long membraneous filament-like surface projections extending to the target cell membranes. Transmission electron microscopy of effector:target cell conjugates revealed membrane contact areas without fusion or fragmentation. The nucleus of the NCC had accentuated peripheral chromation and a prominent Golgi apparatus; the cytoplasm contained osmiophilic granules. Michaelis-Menten and Lineweaver-Burk transformation of target cell binding revealed a Vmax of 11-15,000 and a Km of 40,000. The percentage of NCC bound to target cells was 16-18%. Results of these studies were combined with the conjugate experiment to obtain an estimated percentage of active NCC (5-7%). A maximum recycling capacity of .16-.30 indicated that once attachment by NCC to the target cell occurred (and a lethal signal delivered by an effector cell), either the NCC did not recycle or a long lag period was required to restore its cytotoxic capability.