Role of CD8+ lymphocytes in host defense against Pneumocystis carinii in mice.

An improved understanding of host defense against Pneumocystis carinii could provide novel therapeutic modalities directed against this opportunistic pathogen. Immunodeficient mouse models confirm the role of CD4+ lymphocytes in defense against P. carinii, but the role of CD8+ lymphocytes is controversial. BALB/c mice specifically depleted of CD4+ lymphocytes are susceptible to P. carinii, recruiting large numbers of CD8+ lymphocytes to their lungs during infection. Because of this recruitment, we hypothesized that CD8+ lymphocytes could participate in host defense against P. carinii. BALB/c mice were depleted of CD4+ lymphocytes, CD8+ lymphocytes, or both CD4+ and CD8+ lymphocytes. All mice were then inoculated intratracheally with P. carinii. Mice depleted of CD4+ lymphocytes became moderately infected with P. carinii. Mice depleted of CD8+ lymphocytes cleared the inoculum, indicating that CD8+ lymphocytes are unnecessary for defense when CD4+ lymphocytes are available. However, mice depleted of both CD4+ and CD8+ lymphocytes became significantly more intensely infected than mice depleted of CD4+ lymphocytes alone. Therefore, CD8+ lymphocytes participate in defense against P. carinii in vivo during depletion of CD4+ lymphocytes. To determine the mechanisms of this protection, CD8+ lymphocytes were purified from the lungs of CD4-depleted mice during infection. Lung CD8+ lymphocytes proliferated in response to P. carinii antigen and elaborated interferon-gamma in vitro. Thus CD8+ lymphocytes provide defense against P. carinii in vivo, and the elaboration of interferon-gamma likely represents one important mechanism of defense. During states of CD4+ lymphocyte depletion, the modulation of CD8+ lymphocyte function may provide alternative approaches to the host defense against opportunistic pathogens.

Pneumocystis carinii pneumonia in scid mice induced by viable organisms propagated in vitro.

Pneumocystis carinii pneumonia remains a major cause of morbidity and mortality in human immunodeficiency virus-infected individuals, despite the widespread use of prophylaxis and the development of new chemotherapeutic agents. The study of P. carinii and of pulmonary host defenses directed against it has been limited by lack of reliable, reproducible methods to obtain pure populations of organisms in useful quantities. While recent success has been achieved with cultures of rat P. carinii organisms, cultures of mouse P. carinii organisms have not been successful. Experiments were performed to determine whether P. carinii organisms derived from mice could be propagated in vitro. Mouse P. carinii organisms, obtained from the lungs of chronically infected athymic mice, were inoculated into spinner flasks containing HEL299 feeder cells seated on microcarrier beads. The numbers of mouse P. carinii organisms increased significantly over 7 days in culture. To test the viability and pathogenicity of these cultured organisms, P. carinii organisms were harvested after 7 days of culture and were inoculated intratracheally into susceptible scid mice. Four weeks after inoculation, scid mice developed uniformly severe P. carinii pneumonia. These studies demonstrate for the first time that mouse P. carinii organisms can be propagated in vitro. Furthermore, cultured mouse P. carinii organisms maintain their pathogenicity in an in vivo model system. This culture system will have important applications in the study of P. carinii biology and in the study of host defenses directed against this important opportunistic pathogen.