Pulmonary surfactant protein A interactions with human macrophages: Binding and trafficking of SP-A, and alteration of the respiratory burst

Alveolar macrophages, important host defense cells that continuously phagocytose respired environmental and infectious particulates, are prototypical alternatively activated macrophages, with up-regulated innate immune receptor expression, down-regulated co-stimulatory molecule expression, and limited production of reactive oxygen intermediates in response to stimuli. There has been much interest in what induces this activation state in alveolar macrophages. Surfactant protein A (SP-A), an abundant protein in pulmonary surfactant, has been shown to alter several macrophage functions. Here we show that SP-A purified from human lavage significantly inhibits the macrophage oxidative response to PMA or serum-opsonized zymosan through both opsonic and non-opsonic activities. SP-A did not scavenge superoxide, but significantly decreased macrophage NADPH oxidase oxygen consumption. Additionally, SP-A reduced association of NADPH oxidase component p47phox with zymosan phagosomes, suggesting that SP-A inhibits NADPH oxidase activity by altering oxidase assembly.; SP-A effects on macrophage function are believed to be mediated through binding to specific cell surface receptors. We have demonstrated high-affinity, specific binding of SP-A to primary human macrophages that is calcium-dependent. Mannan did not inhibit SP-A binding, suggesting that binding does not involve carbohydrate recognition. Our laboratory has previously shown that SP-A is endocytosed by macrophages into discrete vesicles. Although previous work indicates SP-A is ultimately degraded by murine macrophages over time, its trafficking pathway through macrophages following uptake has not been reported, and is potentially biologically important. We have shown that SP-A is endocytosed by human macrophages through clathrin-coated pits and colocalizes sequentially over time with early endosome marker EEA1, late endosome marker lamp-1, and lysosome marker cathepsin D. We conclude that SP-A binds to receptor(s) on human macrophages, is endocytosed by a receptor-mediated, clathrin-dependent process, and traffics through the endolysosomal pathway. These findings support a role for SP-A in the induction and/or maintenance of the alveolar macrophage alternative activation phenotype, and provide further insight into interactions of SP-A with macrophages which may play important roles in SP-A modulation of lung macrophage biology.