Killing of Cryptococcus neoformans by rat alveolar macrophages.

The addition of [51Cr]-labeled yeast cells of Cryptococcus neoformans to monolayers of Lewis rat alveolar macrophages (AM phi) provided a sensitive and reproducible in vitro assay of phagocytosis. AM phi and yeast cells were incubated in 10% (v:v) normal rat serum for 1 h, non-AM phi associated yeast cells were removed and the AM phi-associated radioactivity (phagocytosis) determined. Replicate wells were replenished with fresh medium and reincubated. At different times, yeast-AM phi monolayers were treated with a non-cryptococcocidal mixture of DNAse and sodium deoxycholate to release the yeast cells from the AM phi. The fate of the yeast cells was critically evaluated by [51Cr]-release and viable plate counts. Killing was detected by plate counts within an hour following phagocytosis and did not increase significantly during the next 5 h. Strains of C. neoformans with small, medium, or large capsules varied in their susceptibility to killing from 10% to 95% but susceptibility to killing was not directly related to capsule size and the extent of phagocytosis. Release of 51Cr did not correlate with viability as determined by culture. The 51Cr was associated with two pools in the yeast cells; one, representing 15-20% of the radiolabel, was easily released and was probably bound to low molecular weight compounds in the cytoplasm. The majority of label was tightly bound to the particulate alkali-soluble cell wall fraction.