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The susceptibility of various strains of Cryptococcus neoformans to killing by human polymorphonuclear leukocytes (PMNs) and monocytes was investigated. Five previously characterized strains of C. neoformans serotype A, a capsule-free mutant, and six recent clinical isolates were compared. PMNs and monocytes were isolated from normal peripheral blood and allowed to adhere to the flat-bottom wells of microtiter plates. Yeast cells of C. neoformans were added in the presence of normal human serum, and the plates were incubated at 37 degrees C. After 4 h, killing was determined by comparing the quantitative plate counts of viable yeast cells in experimental wells with counts in control wells containing yeast cells in the absence of leukocytes. No appreciable growth of yeast cells occurred in the wells during the incubation period. Both PMNs and monocytes effectively killed yeast cells at effector-to-target ratios as low as 1:1, although monocytes failed to kill the capsule-free strain 602 at a 1:1 ratio. With 9 of 12 strains, PMNs killed C. neoformans more effectively than did monocytes. Significant interstrain variation in killing occurred for both monocytes and PMNs, and the recent, clinical isolates were more resistant to killing by monocytes and PMNs than were the previously characterized strains. The extent to which different strains were killed by monocytes and PMNs was not consistently related to the size of the capsule or the entire cell. Normal PMNs and monocytes are remarkably effective in killing strains of C. neoformans in the absence of specific antibody and appear to constitute a significant defense mechanism in the peripheral circulation.

Killing of Cryptococcus neoformans strains by human neutrophils and monocytes