Gamma Interferon Stimulates Rat Alveolar Macrophages To Kill Pneumocystis carinii by l-Arginine- and Tumor Necrosis Factor-Dependent Mechanisms

Pneumocystis carinii pneumonia remains a serious complication for immunocompromised patients. In the present study, P. carinii organisms interacted with gamma interferon (IFN-γ)-stimulated alveolar macrophages (AMs) to activate the l-arginine-dependent cytocidal pathway involving reactive nitrogen intermediates (RNI) that were assayed as nitrite (NO2−). Unstimulated cultures of AMs produced negligible quantities of RNI. Addition of P. carinii organisms to IFN-γ-primed AMs resulted in greatly enhanced production of RNI. NO2− levels increased from 0.8 ± 0.4 to 11.1 ± 3.8 μM as early as 6 h after P. carinii organisms were incubated with IFN-γ-stimulated AMs and to 35.1 ± 8.9 μM after a 24-h incubation, a near-maximum level. High levels of NO2− were produced by AMs primed with as little as 10 U of IFN-γ per ml in the presence of P. carinii, and a 20-fold increase in IFN-γ concentration resulted in only a further 65% increase in NO2− production. RNI-dependent killing of P. carinii was demonstrated by both a 51Cr release assay and a [35S]methionine pulse immunoprecipitation assay. Addition of either monoclonal tumor necrosis factor alpha (TNF-α) neutralizing antibody or 200 μM NG-monomethyl-l-arginine (l-NGMMA), a competitive inhibitor of the l-arginine-dependent pathway, significantly decreased NO2− production and reduced P. carinii killing. TNF-α alone had no effect on P. carinii viability. These results suggest that (i) the specific interaction of P. carinii organisms with IFN-γ-primed AMs triggers the production of RNI, (ii) RNI are toxic to P. carinii, and (iii) TNF-α likely plays a central role in mediating P. carinii killing by IFN-γ-stimulated AMs.