Trophozoite-to-cyst ratio increases during recovery from Pneumocystis carinii pneumonia in rats.

Pneumocystis carinii has two developmental stages: the trophozoite and the cyst. The differential specificity of monoclonal antibodies SMoAb (cysts only) and CMoAb (cysts and trophozoites) was used to analyse the trophozoite-to-cyst (T: C) ratio during recovery from P. carinii pneumonia (PCP) in a dexamethasone model in Wistar rats. In tissue homogenates the cysts decreased from week 0 to week 4 (4.10 +/- 1.57 10(6)/g vs 0.069 +/- 0.021 10(6)/g, p < 0.001), whereas the trophozoites decreased much less, leading to a change in T: C ratio from week 0 and 1 (72 +/- 33 and 49 +/- 14, respectively) to week 2 and 4 (1752 +/- 652 and 1163 +/- 482, p < 0.001). A similar tendency was found in immunofluorescence-labelled histological samples quantitated with the VIDAS image analyser. These results demonstrate a hitherto unrecognized and significant change in T: C ratio during recovery from PCP. This change could not be monitored using bronchoalveolar fluid, probably due to adherence of the trophozoites to the alveolar wall. Changes in T: C ratio indicate that in particular the cyst form of P. carinii is sensitive to the host response mounted during recovery, and support the theory that trophozoites probably also multiply in an extracystic asexual fission.