Effects of bacterial pneumonitis on development of pneumocystosis in rats.

Disease caused by the protozoan parasite Pneumocystis carinii complicates management of patients with a variety of defects in immune function and is most commonly observed in patients who receive long-term therapy with glucocorticoids. In the rat, disease is readily induced by chronic administration of glucocorticoids. However, rats that have had polymorphonuclear leukocytic pneumonitis induced by Pseudomonas are protected from development of pneumocystosis, whereas rats that have received an intratracheal injection of Staphylococcus, which does not induce a polymorphonuclear leukocytic alveolar exudate, are not protected. It is possibly that accidental contact of polymorphonuclear leukocytes with dormant Pneumocystis is an important element of control of the organism in healthy animals, and suppression of polymorphonuclear leukocytic inflammatory response underlies glucocorticoid-induced and spontaneous activation of the disease.