Objective To explore the susceptibility of different strains of mice to P.carinii infection and the host immune response to the organism. . Methods. C57BL/6 and BALB/c strains of mice (15 each) were exposed to SCID mice infected with P.carinii by co-housing. Observations were made on the number of parasites in the lungs, level of CD4+ T cell and CD8+ T cell in BALF, and serum IgG at 4, 5 and 6 wk after contagion.. Results . The number of P.carinii grew in the lungs of BALB/c mice was found much greater than those in C57BL/6 mice. A few number of P.carinii cysts were detected in the lungs of both strains of mice by 4 wk after co-housing, the number of cysts increased at 5 wk in the lungs of BALB/c mice but not in that of C57BL/6 mice. P.carinii-specific IgG in the sera and high level of CD62.{low.} CD4-and CD8-positve T cells in the lungs were found at 5 wk. The parasites were cleared from the lungs at 6 wk in all infected mice, shortly after acquired immune response was initiated. . Conclusion . BALB/c mice are more susceptible to P.carinii than the C57BL/6 mice by natural transmission, and all the immunocompetent mice can clear the infection of P.carinii by cellular and humoral immune responses.
