Human surfactant protein A enhances attachment of Pneumocystis carnii to rat alveolar macrophages

Pneumocystis carinii (PC) pneumonia remains one of the most important opportunistic pulmonary infections. The alveolar macrophage (AM) is likely the primary cell for recognition and removal of PC. The histopathology of PC pneumonia is characterized by a surfactant-like alveolar exudate. We hypothesize that surfactant protein A (SP-A), the major apoprotein of surfactant, mediates attachment of PC to rat AMs by acting as a ligand between the organism and the AM. In this study, attachment of PC was determined using Cr-51-labeld PC incubated at 4 degrees C with normal rat AM monolayers in the presence or absence of human SP-A. SP-A significantly enhanced attachment of PC from 14.2+/-1.2% to 42.0+/-3.8% (P < 0.05). This enhanced attachment was visualized and quantified morphologically with confocal microscopy. PC attachment by SP-A was calcium- and mannose-dependent as SP-A-mediated attachment was significantly reduced in the presence of EGTA and mannose to 13.1+/-1.6% and 19.3+/-2.6%, respectively (P < 0.05), Addition of type V collagen and antibodies to SP-A also significantly reduced SP-A-mediated attachment to 4.9+/-1.2% and 10.1+/-1.2%, respectively (P < 0.05). We conclude that SP-A can function as a ligand between PC and the AM and may represent an important detection and clearance mechanism of PC from the alveolar spaces.