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Killing of Cryptococcus neoformans strains by human neutrophils and monocytes
1Department of Microbiology and Immunology, Duke University Medical Center, Durham, North Carolina 27710.
Abstract:
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.
Insights
Human immune cells, polymorphonuclear leukocytes (PMNs) and monocytes, effectively kill Cryptococcus neoformans strains. Recent clinical isolates showed increased resistance to these immune cells, highlighting strain-specific defenses against fungal infections.
Area of Science:
- Immunology
- Mycology
- Infectious Diseases
Background:
- Cryptococcus neoformans is an opportunistic fungal pathogen causing serious infections.
- Human immune cells, including polymorphonuclear leukocytes (PMNs) and monocytes, play a crucial role in combating fungal infections.
Purpose of the Study:
- To investigate the susceptibility of various Cryptococcus neoformans strains to killing by human PMNs and monocytes.
- To compare the killing efficacy of PMNs and monocytes against different C. neoformans strains, including clinical isolates.
Main Methods:
- Isolation of PMNs and monocytes from normal human peripheral blood.
- Incubation of C. neoformans strains with leukocytes at 37°C in the presence of human serum.
- Quantitative assessment of yeast cell killing by comparing viable cell counts after 4-hour incubation.
Main Results:
- Both PMNs and monocytes effectively killed most C. neoformans strains at low effector-to-target ratios.
- Monocytes showed reduced efficacy against a capsule-free mutant strain.
- PMNs demonstrated superior killing activity compared to monocytes for the majority of strains tested.
- Recent clinical isolates exhibited greater resistance to killing by both PMNs and monocytes than previously characterized strains.
- Killing efficacy was not consistently correlated with capsule size or overall cell dimensions.
Conclusions:
- Normal PMNs and monocytes are potent in killing C. neoformans strains, even without specific antibodies.
- These innate immune cells represent a significant defense mechanism against C. neoformans in the peripheral circulation.
- Interstrain variations in susceptibility and increased resistance in clinical isolates warrant further investigation into fungal defense mechanisms.