Morphine stimulates phagocytosis of Mycobacterium tuberculosis by human microglial cells: involvement of a G protein-coupled opiate receptor

Opiate-induced immunosuppression has been implicated in the pathogenesis of infections caused by a variety of microorganisms, including human immunodeficiency virus (HIV). Although effects of opiates on lymphocyte function have been studied more extensively, morphine also has been shown to inhibit several functional activities of mononuclear phagocytes (e.g. chemotaxis, respiratory burst activity and phagocytosis). Opiate addiction has been identified as a risk factor for clinical tuberculosis prior to the HIV epidemic, and macrophages are a key cell in the pathogenesis of Mycobacterium tuberculosis. Thus, the hypothesis was tested in the present study that morphine would suppress phagocytosis of M. tuberculosis by human microglial cells, the resident macrophages of the brain. Contrary to this hypothesis, treatment of human fetal microglial cell cultures with morphine (10⁻⁸m) was found to stimulate phagocytosis of nonopsonized M. tuberculosis H37Rv. The stimulatory effect of morphine was blocked by naloxone and the μ opiate receptor selective antagonist β-funaltrexamine. Also, morphine-induced increase in phagocytic activity was markedly inhibited by pertussis toxin and was unaffected by cholera toxin, suggesting the mechanism of morphine's stimulatory effect on microglial cell phagocytosis involves a G_i protein-coupled μ opiate receptor. The results of this in vitro study support the concept that exogenous and endogenous opioids play an immunomodulatory role within the central nervous system through their interaction with G protein-coupled receptors on microglial cells.