Cellular and humoral immune responses of mice subclinically infected with Pneumocystis carinii.

Cellular and humoral immune responses to Pneumocystis carinii were investigated. ICR and DDD mice were intranasally infected with 10(4) mouse lung-derived P. carinii, and the delayed-type hypersensitivity (DTH) reaction and antibody titers to P. carinii were measured along with the number of P. carinii cysts in the lungs after infection. The number of P. carinii cysts in the lungs peaked at 2 weeks after infection and then decreased to barely detectable levels by 4 weeks. Serum antibody (immunoglobulin G) titers measured by indirect immunofluorescence increased up to 4 weeks. The DTH footpad reaction was most prominent at 2 weeks postinfection and declined thereafter. Thus, the decline in the number of P. carinii cysts in the lungs corresponded well with the time of the peak of the DTH reaction but not with the serum antibody response. Spleen T cells from infected mice mediated the DTH reaction when transferred intravenously into normal recipients and reduced the number of P. carinii cysts in the lungs when transferred intravenously into P. carinii-infected mice. The results indicated that cellular immunity is important for protection from subclinical P. carinii infections.