Immunoregulation of fish nonspecific cytotoxic cell activity by retinolacetate but not poly I:C

Nonspecific cytotoxic cells (NCC) from fish (Ictalurus punctatus) were treated with different concentrations of retinolacetate and poly I:C. Both in vitro and in vivo experiments were conducted. Retinolacetate significantly increased NCC activity against chromium-51 labeled human B-cell lymphoma target cells (NC-37). Preincubation (treatment prior to adding the labeled target cells) of NCC for 4 to 8 h in 10^-3 to 10^-12 M concentration of retinolacetate produced significant increases in NCC activity compared to treatment during the killing assay only. Similar experiments with different concentrations of poly I:C had no NCC augmenting effects when tested by adding poly I:C either during preincubation periods or during the cytotoxicity assay. Retinolacetate probably produces positive modulation of cytotoxicity by increasing the killing effectiveness of individual NCC, rather than recruiting larger numbers of cytolytic cells. In vivo studies were also conducted by injecting catfish (i.p.) with 1 ×, 3 × and 5 × the daily recommended vitamin A dosages and determining NCC activity after 24, 48 and 72 h treatment. The 1 × dose significantly increased NCC activity at 72 h. This increase was not transient because NCC activity after 33-37 days' treatment was significantly higher than controls in the 1 ×, 2 × and 3 × groups. Intraperitoneal injections of fish with poly I:C produced no significant increases in NCC activity at 24 or 72 h post-inoculation.