Experimental Pneumocystis carinii pneumonia in C3H/HeJ and C3HeB/FeJ mice.

C3H mice develop heavier degrees of Pneumocystis carinii pneumonia than other mouse strains tested. We have compared P. carinii pneumonia in two strains of C3H mice: C3H/HeJ mice, which are unresponsive to the effects of bacterial lipopolysaccharide (LPS), have defects in macrophage function, and have increased antibody responses to orally administered T-dependent antigens; and C3HeB/FeJ mice, which are immunologically normal. P. carinii pneumonia was induced by corticosteroids, and the intensity of the infection was judged by a semiquantitative histopathologic scoring system. Heavier degrees of infection were found in C3H/HeJ mice than in C3HeB/FeJ mice. Serum antibodies to P. carinii, measured by an indirect fluorescent antibody technique, were mainly of the IgG class in both strains of mice and varied inversely with the intensity of P. carinii infection in the lungs. Antibody levels were significantly higher in C3H/HeJ mice than in C3HeB/FeJ mice. These data suggest that C3H/HeJ have increased susceptibility to the effects of steroids of host defenses against P. carinii, and heightened serum antibody responses to the organism.