A NOVEL IN-SITU MODEL TO STUDY PNEUMOCYSTIS-CARINII ADHESION TO LUNG ALVEOLAR EPITHELIAL-CELLS

Pneumocystis carinii, an extracellular parasite thriving in the lungs of immunosuppressed mammals, is a major cause of death in AIDS patient s in the USA. As a prelude to growth, the parasite adheres mostly to t ype I pneumocytes lining the alveolar spaces. The mechanism of adheren ce remains unknown, largely because of difficulties in isolating type I pneumocytes and maintaining them in vitro. As a first step to unders tand P. carinii adherence to its natural substrate, we developed an in situ method to directly study parasite binding to lung alveolar cells . We used formaldehyde-fixed paraffin-embedded sections of normal rat lung as substrate for adhesion. As in its binding to the lungs in vivo , P. carinii adhered preferentially to type I pneumocytes. Adherence w as saturable, time and dose dependent, and selectively blocked by glyc oconjugates, in particular bovine submaxillary mucin, fetuin, and asia lofetuin, suggesting that it may be mediated by a lectin type of inter action. Further, IgG of rats with P. carinii pneumonia inhibited adher ence, suggesting that it may react with parasite ligands involved in t he recognition of type I cell receptors. Our results demonstrate the u sefulness of the in situ model for studying the mechanisms of P. carin ii adherence to alveolar cells. In addition, this method may be valuab le for identifying neutralizing antibodies and drugs potentially usefu l for controlling the infection in vivo.