ACTIVE IMMUNITY TO PNEUMOCYSTIS-CARINII REINFECTION IN T-CELL-DEPLETED MICE

Immunocompetent C.B-17 mice were immunized against Pneumocystis carini i by several intratracheal inoculations with infective P. carinii. The se mice and another group of C.B-17 mice naive to P. carinii were then depleted of CD4(+) cells by treatment with both anti-CD4 and anti-Thy 1 monoclonal antibodies. Both groups of mice were then challenged with an infective inoculum containing 10(7) P. carinii organisms by intrat racheal instillation. The mean log(10) counts of P. carinii nuclei in the lungs of the nonimmune mice were 4.98, 5.89, and 6.77 when they we re killed at 4, 10, and 19 days, respectively, after challenge. The P. carinii counts in the lungs of the immune mice were significantly low er at each time point and below detectable levels at 10 and 19 days. A nalysis of P. carinii DNA by PCR revealed no detectable P. carinii in the lungs of the immunized mice at either 10 or 19 days, whereas all o f the nonimmunized mice contained P. carinii DNA at all time points. T he sera of immune but not nonimmune mice contained P. carinii-specific immunoglobulin G. These results indicate that immunization of an immu nocompetent host against P. carinii can protect against P. carinii pne umonia even after the host is depleted of CD4(+) cells. In addition, t he results are consistent with the possibility that antibodies were re sponsible for the observed protection against P. carinii.