Pneumocystis carinii Cyst is Associated with Inflammation in the Host

Pneumocystis jirovecii is responsible for much morbidity and mortality in immunosuppressed patients. Pneumocystis carinii is the causative organism in rats and it produces a pneumonia similar to that in humans. Damage to the host is in the form of reduced oxygenation of blood due to a proteinaceous exudate and masses of organisms that fill the alveoli, and to an inflammatory reaction that produces damage to host alveolar epithelial cells. In addition, alveolar macrophages are reduced in number [8], and they exhibit lower phagocytic ability [9] and nitric oxide production [7]. P. carinii has two major life cycle stages, the cyst and the trophozoite. The trophozoite is smaller, and more metabolically active [3]. The cyst appears later during infection [8], has a thick cell wall [2], and is present in infected lungs at 1/100 the number of trophozoites [8]. The response of the host to the cyst form is unknown, despite the possibility that the cyst is an infectious form of the organism. Cyst wall elements can induce pro-inflammatory cytokine release [5,11], but the reaction of an immunosuppressed host to the whole cyst has yet to be determined. The present study sought to determine if the cyst is involved in the inflammatory damage seen in the host during P. carinii pneumonia (PcP).