Phagocytosis and Killing of Cryptococcus neoformans by Rat Alveolar Macrophages in the Absence of Serum

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Abstract
The in vitro interaction between yeast cells of Cryptococcus neoformans and Lewis rat alveolar macrophages (AMÕ) was studied in the absence of serum. AMÕ were harvested by lung lavage, and monolayers of adherent cells were established in wells of microtiter plates. Radiolabeled yeast cells were added to fresh AMÕ monolayers, the plates were incubated at 37°C under 5% CO2, nonadherent yeasts were removed, and phagocytosis (i.e., attachment or ingestion) was determined by measuring adherent radioactivity. AMÕ were able to bind or ingest, and kill, encapsulated strains of C. neoformans in the absence of serum. Serum-free phagocytosis was suboptimal by comparison with phagocytosis in the presence of serum. The mechanism of serum-free phagocytosis Involves a receptor on the AMÕ with affinity for mannose-rich determinants present on the yeast cell walls and unrelated to the capsular polysaccharide. Opsonin-independent phagocytosis was only detected with nonencapsulated, small, and medium encapsulated strains of C. neoformans. Large encapsulated strains were not taken up without serum. Serum-free phagocytosis could be of critical importance in the alveolar spaces, where only marginal concentrations of serum opsonins are initially present.