SURFACTANT PROTEIN-D INTERACTS WITH PNEUMOCYSTIS-CARINII AND MEDIATES ORGANISM ADHERENCE TO ALVEOLAR MACROPHAGES

Pneumocystis carinii interacts with glycoproteins present in the lower respiratory tract through its mannose-rich surface antigen complex termed gpA, Surfactant protein D (SP-D) is a recently described component of the airspace lining material that possesses a calcium-dependent lectin domain capable of interacting with glycoconjugates present on microorganisms and leukocytes, Accordingly, we evaluated the extent and localization of SP-D in the lower respiratory tract during Pneumocystis pneumonia in an immunosuppressed rat model and examined its role in modulating interaction of P. carinii with macrophages. We report that SP-D is a major component of the alveolar exudates that typify P. carinii pneumonia and is present bound to the surface of P. carinii organisms in vivo, We further demonstrate that SP-D binds to P. carinii through saccharide-mediated interactions with gpA present on the surface of the organism, Lastly, we show that SP-D augments binding of P. carinii to alveolar macrophages, but does not significantly enhance macrophage phagocytosis of the organism, The interaction of SPD with gpA represents an additional important component of the host-parasite relationship during P. carinii pneumonia.