Killing of Klebsiella pneumoniae by human alveolar macrophages

We investigated putative mechanisms by which human surfactant protein A (SP-A) effects killing of Klebsiella pneumoniae by human alveolar macrophages (AMs) isolated from bronchoalveolar lavagates of patients with transplanted lungs. Coincubation of AMs with human SP-A (25 mug/ml) and Klebsiella resulted in a 68% decrease in total colony forming units by 120 min compared with AMs infected with Klebsiella in the absence of SP-A, and this SP-A-mediated effect was abolished by preincubation with N-G-monomethyl-L-arginine. Incubation of transplant AMs with SP-A increased intracellular Ca2+ concentration ([Ca2+](i)) by 70% and nitrite and nitrate (NOx) production by 45% (from 0.24 +/- 0.02 to 1.3 +/- 0.21 nmol.10(6) AMs(-1) . h(-1)). Preincubation with 1,2- bis( 2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester inhibited the increase in [Ca2+](i) and abrogated the SP-A-mediated Klebsiella phagocytosis and killing. In contrast, incubation of AMs from normal volunteers with SP-A decreased both [Ca2+](i) and NOx production and did not result in killing of Klebsiella. Significant killing of Klebsiella was also seen in a cell-free system by sustained production of peroxynitrite (>1 muM/min) at pH 5 but not at pH 7.4. These findings indicate that SP-A mediates pathogen killing by AMs from transplant lungs by stimulating phagocytosis and production of reactive oxygen-nitrogen intermediates.