Correlation of Organism Burden and Alveolar Macrophage Counts during Infection withPneumocystis cariniiand Recovery

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Abstract
Changes in the number of alveolar macrophages were correlated with organism burden duringPneumocystis cariniiinfection. The lungs of healthy, dexamethasone-treated, and dexamethasone-treated andP. carinii-infected rats were lavaged with phosphate-buffered saline. Counting of alveolar macrophages in the lavage fluids revealed thatP. cariniiinfection caused a 58% decrease in the number of alveolar macrophages and that higherP. cariniiorganism burdens caused a more rapid decrease in alveolar macrophage number. As a control, healthy rats were challenged with the same number of organisms as that normally used to generateP. cariniiinfections in dexamethasone-treated rats. Thirteen days after challenge, these rats had a profound (54%) increase in alveolar macrophage number in response to the challenge, while the number of alveolar macrophages in immunosuppressed andP. carinii-infected rats had decreased significantly by this time point. These experiments created the first animal model to mimic human pneumocystis pneumonia in alveolar macrophage number alterations. Reduction ofP. cariniiorganism numbers by treatment of rats with trimethoprim and sulfamethoxazole brought a slow rebound in alveolar macrophage number, while recovery fromP. cariniiinfection by cessation of immunosuppression brought a rapid rebound in alveolar macrophage number. These results suggest that both the immune state of the host andP. cariniiburden affect alveolar macrophage number.