Analysis of Pneumocystis carinii cysts with a fluorescence-activated cell sorter

Human sera from Pneumocystis carinii-infected patients and specific rabbit antisera have antibodies against the cyst form of the organism. Lung tissue concentrations from cortisone-treated C3H/HeN mice and six open lung biopsy-positive patients were centrifuged and suspended, and immunofluorescent staining was done. We utilized the fluorescence-activated cell sorter to analyze and sort P. carinii cysts from lung homogenates into a morphologically distinct population. A quantitative basis was used for the definition of the cyst population by displaying the frequency of cells as a function of parameter (fluorescence intensity and light scatter) expression. In 14 of 15 histogram analyses, P. carinii-infected homogenates were differentiated from normal- and bacterial-control homogenates. The parameter range of light scatter (size) was 2 to 8 μm, and the fluorescence intensity was greater than a threshold based on the histogram profile.