Pneumocystis carinii stimulates in vitro production of tumor necrosis factor-α by human macrophages

The ability of Pneumocystis carinii to induce tumor necrosis factor (TNF)-α release by monocytes/macrophages from immunocompetent humans was investigated. Monocytes and monocyte-derived macrophages from healthy individuals produced an increased amount of TNF-α when exposed to P. carinii cysts obtained from rats with steroid-induced pneumocystosis. The cysts induced increased TNF-α production in a dose-dependent manner; baseline TNF-α production was restored after addition of an anti-P. carinii hyperimmune serum. Kinetics experiments showed that the secretion of TNF-α occurs early and reachs a maximal peak after 8 h. Since TNF-α is directly lethal to P. carinii in vitro, it is suggested that the production of this cytokine in response to the cysts may be one of the mechanisms for the control of this parasitic infection.