Mechanisms of cellular cytotoxic innate resistance in tilapia (Oreochromis nilotica)

Mechanisms of innate cytotoxic immunity in tilapia (O. nilotica) were measured by characterization of the activity, distribution and functions of nonspecific cytotoxic cells (NCC). Active cytotoxic cells were obtained from anterior kidney. spleen and peripheral blood whereas nonlytic but anti-NCC monoclonal antibody 5C6 positive cells were obtained from tilapia liver. Thymocytes were not cytotoxic and were mab 5C6+. Unfractionated anterior kidney cells were 6% mab 5C6+ and had very low cytotoxicity of HL-60 target cells. Percoll (45.5%) purified NCC were 44% mab 5C6+ and had 35% HL-60 cytotoxicity (160:1 E:T ratio). Transformed mouse and human target cells were tested for sensitivity to NCC lysis. HL-60, U937, K562, IM-9 and NC-37 human targets were lysed by NCC. YAC-1 targets were insensitive to lysis. The killing of HL-60 targets by tilapia NCC was inhibited by mab 5C6. Experiments to determine optimal conditions for the cytotoxicity assay revealed that tilapia required 15-20h for optimum lysis of targets. Incubation at 37 C produced the highest cytotoxicity. The proliferative competence of Percoll purified anterior kidney cells was determined. A significant increase in in vitro uptake of tritiated thymidine by anterior kidney cells occurred following stimulation by mab 5C6, Con-A, PMA and calcium ionophore A23187. Purified spleen cells also produced significant increased uptake of tritiated thymidine following in vitro activation with PMA and mab 5C6, but not Con-A. These studies indicated that NCC may provide innate cytotoxic immunity similar to that provided by the NCC of catfish.