ABSTRACTPulmonary surfactant protein D (SP-D) is a collagenous C-type lectin (collectin) that is secreted into the alveoli and distal airways of the lung. We have studied the interactions of SP-D and alveolar macrophages withKlebsiella pneumoniae, a common cause of nosocomial pneumonia. SP-D does not agglutinate encapsulatedK. pneumoniaebut selectively agglutinates spontaneous, unencapsulated phase variants, such asKlebsiellastrain K50-3OF, through interactions with their lipopolysaccharides (LPS). These effects are calcium dependent and inhibited with maltose but not lactose, consistent with involvement of the SP-D carbohydrate recognition domain. Precoating of K50-3OF with SP-D enhances the phagocytosis and killing of these organisms by rat alveolar macrophages in cell culture and stimulates the production of nitric oxide by the NR-8383 rat alveolar macrophage cell line. SP-D similarly enhances the NO response to K50-3OF LPS adsorbed to Latex beads under conditions where soluble LPS or SP-D, or soluble complexes of SP-D and LPS, do not stimulate NO production. Our studies demonstrate that interactions of SP-D with exposed arrays ofKlebsiellaLPS on a particulate surface can enhance the host defense activities of alveolar macrophages and suggest that activation of macrophages by SP-D requires binding to microorganisms or other particulate ligands. Because unencapsulated phase variants are likely to be responsible for the initial stages of tissue invasion and infection, we speculate that SP-D-mediated agglutination and/or opsonization ofK. pneumoniaeis an important defense mechanism for this respiratory pathogen in otherwise healthy individuals.

Ofek, I., Mesika, A., Kalina, M., Keisari, Y., Podschun, R., Sahly, H., Chang, D., McGregor, D., & Crouch, E. (2001). Surfactant Protein D Enhances Phagocytosis and Killing of Unencapsulated Phase Variants ofKlebsiella pneumoniae. Infection and Immunity, 69(1), 24–33.