Resistance of mucoid Pseudomonas aeruginosa to nonopsonic phagocytosis by alveolar macrophages in vitro.

A unique, recently described rat alveolar macrophage cell line (NR8383) was used to study the interaction of the pulmonary immune system with a mucoid cystic fibrosis isolate of Pseudomonas aeruginosa (SRM-3), its nonmucoid revertant (SRM-3R), and a non-cystic fibrosis isolate (PAO-1). Strain SRM-3 was cultivated in a chemostat system to allow maintenance of an entirely mucoid population. The alveolar macrophage response to the mucoid and nonmucoid strains of P. aeruginosa was determined by visually quantitating phagocytosis in acridine orange-stained monolayers and measuring the induction of an oxidative burst as indicated by chemiluminescence and H2O2 production. In all experiments, fewer than 2% of the NR8383 cells engulfed the mucoid SRM-3 isolate, while SRM-3R and PAO-1 were phagocytized by 15 and 41%, respectively. Opsonization by normal serum (complement) provided minimal phagocytic enhancement of these strains, whereas specific anti-P. aeruginosa antibody slightly elevated phagocytic responses to strains with nonmucoid phenotypes while providing a sevenfold increase in uptake of SRM-3. Chemiluminescent and H2O2 responses were comparable with the levels of phagocytosis observed, with very little or no response to the mucoid strain SRM-3. The data indicate that the strains with mucoid phenotypes are refractile to ingestion and that studies which describe ingestion of mucoid strains were likely measuring ingestion of revertants. Alginic acid (2 mg/ml) was found to inhibit stimulation of macrophage response to the opsonized and unopsonized nonmucoid strain PAO-1.