Acute morphine treatment alters cellular immune function in the lungs of healthy rats

Previous work has shown that morphine suppresses the pulmonary immune response to infection and reduces pulmonary inflammation. No published studies have addressed the impact of morphine on lymphocyte function in the lungs without infection. This study addressed this question by assessing the impact of acute morphine treatment on proliferation, cytokine production, and natural killer (NK) cell activity in resident pulmonary lymphocytes from healthy rats. Male Lewis rats received either a single 15 mg/kg morphine sulfate or vehicle injection 1 h prior to sacrifice. Lungs were minced and passed through wire mesh following collagenase digestion. The resulting cell preparations were pooled (2 rats/pool) to yield sufficient cell numbers for the functional assays, and a portion of these suspensions were separated using a density gradient. Crude and purified cell suspensions were used in assays of NK cell activity and mitogen-induced proliferation and cytokine production. Morphine significantly suppressed lymphocyte proliferation and cytokine production in whole cell suspensions, but not in purified cultures. NK activity was enhanced by morphine treatment in purified treated cultures. Studies of nitrate/nitrite levels in crude and purified cultures suggest that macrophage-derived nitric oxide may be a mechanism of the suppression observed in whole cell suspensions following morphine treatment. These data are consistent with previous work showing that morphine suppresses mitogenic responsiveness and NK activity in the spleen and peripheral blood, and may do so through a macrophage-derived nitric oxide mechanism.