Improved rat model for studying Pneumocystis carinii pneumonia.

Sprague-Dawley rats treated for 8 weeks with cortisone acetate (25 mg per rat twice weekly) were immunosuppressed to variable degrees. A total of 55% lost over 12% of their initial body weight, had cortisol concentrations in serum more than five times greater than those of the controls, and had markedly depressed ratios of helper to non-helper T cells, in both the spleen and peripheral blood. Animals that gained weight during immunosuppression had cortisol concentrations in serum only three times higher than those of the controls, had normal ratios of helper to non-helper T cells in the spleen, and had only modestly reduced T-cell ratios in peripheral blood. The degree of Pneumocystis pneumonia was evaluated in impression smears and sections of lungs taken from immunosuppressed rats. Pneumocystis infections were more severe in the rats that showed the greatest weight loss. Weight change during immunosuppression may therefore be used as a reliable means for predicting the degree of Pneumocystis infection in living rats. This protocol allows the selection of uniformly infected rats for studies assessing drug therapy of Pneumocystis pneumonia.