PULMONARY SURFACTANT PROTEIN-A MEDIATES ENHANCED PHAGOCYTOSIS OF MYCOBACTERIUM-TUBERCULOSIS BY A DIRECT INTERACTION WITH HUMAN MACROPHAGES

During initial infection with Mycobacterium tuberculosis, bacteria tha t reach the distal airspaces of the lung are phagocytosed by alveolar macrophages in the presence of pulmonary surfactant. Here we have exam ined the role of surfactant-associated protein A (SP-A) in phagocytosi s of the virulent Erdman strain of M. tuberculosis by human monocyte-d erived macrophages (MDMs) and human alveolar macrophages (HAMs). Macro phage monolayers incubated with soluble SP-A from alveolar proteinosis patients (APP SP-A(4)) and recombinant rat SP-A (SP-A(hyp)) demonstra ted enhanced adherence of M. tuberculosis, 82 +/- 17% and 49 +/- 18%, respectively. Removal of SP-A from monolayers by washing before adding bacteria did not diminish the enhanced adherence. Fluorescence micros copy demonstrated that washed monolayers contained intracellular rathe r than surface-bound SP-A. These studies indicated a direct interactio n between SP-A and the macrophage in mediating enhanced adherence of M . tuberculosis. Consistent with this interpretation, macrophage monola yers formed on human or rat SP-A (substrate SP-A) demonstrated enhance d adherence of M. tuberculosis to their apical surface (APP SP-A and n ative rat SP-A increased M. tuberculosis adherence by 102 +/- 16% and 102 +/- 25%, respectively). Electron microscopy demonstrated increased numbers of phagocytosed bacteria in APP SP-A-treated MDM cross-sectio ns. SP-A proteins devoid of carbohydrate failed to enhance M. tubercul osis adherence to macrophages. In contrast, heat-denatured APP SP-A en hanced adherence of bacteria equivalent to that of intact glycoprotein . Thus, the carbohydrate moieties of SP-A appear to be critical in the SP-A-macrophage interaction. Finally, mannan and anti-mannose recepto r Ab completely inhibited the enhanced phagocytosis of M. tuberculosis observed with APP SP-A, providing evidence for up-regulation of macro phage mannose receptor activity. These studies implicate SP-A as an im portant modulator of alveolar macrophage function that results in an e nhanced potential for M. tuberculosis to gain access to its intracellu lar niche.