Immunosuppressive effects of morphine on immune responses in mice.

Implantation of a 75-mg morphine sulfate pellet subcutaneously into mice of different strains and sexes caused profound immunosuppression of their spleen cell primary in vitro antibody responses to sheep red blood cells. No sex differences were observed. In mice of the C3H lineage, naltrexone blocked the immunosuppression. In mice in the C57BL/6J lineage, naltrexone was ineffective in blocking the effects of morphine and was itself suppressive. In beige C57BL/6J bgJ/bgJ mice, placebo pellets were also suppressive. The mechanism of the morphine-induced immunosuppression was investigated in C3HeB/FeJ mice. Addition of normal splenic macrophages to in vitro cultures restored immune responses, as did IL-1, IL-6 and IFN-gamma, suggesting that morphine-induced immunosuppression is due to a deficit in macrophage function. Morphine pellet implantation induced splenic atrophy. Whether suppression is attributable to decreased macrophage numbers or to decreased functional capacity of individual macrophages is currently under investigation.