Immunohistological localization of immunoglobulins in pneumocystosis of adults.

We examined by direct immunofluorescence open lung biopsies from seven adult patients with Pneumocystis carinii pneumonia. In all of them, immunoglobulin deposits (especially IgA) were found in alveoli that exhibited by light microscopy the classical foamy exudate infested by Pneumocystis. These immunoglobulins were eluted at an acid pH and were able to fix complement fractions in vitro. Moreover, we could reproduce this fixation by incubating previously eluted tissue sections with concentrated immunoglobulins isolated from a bronchopulmonary lavage performed in another case of pneumocystis. This immunohistological pattern suggests the presence of a humoral response in pulmonary parenchyma of patients infected with Pneumocystis carinii, despite the consistent underlying immunocompromised status. Considering these results, we conclude that humoral immune mechanisms do not constitute the main barrier against the development of Pneumocystis carinii in human lungs.