Immunity to a pulmonary Cryptococcus neoformans infection requires both CD4+ and CD8+ T cells

The role of CD4+ and CD8+ T cells in mediating pulmonary clearance of a cryptococcal infection was investigated. Intratracheal inoculation of BALB/c and C.B-17 mice with a moderately virulent strain of Cryptococcus neoformans (52D) resulted in a pulmonary infection, which was cleared by a T cell-dependent mechanism. During this clearance, there was a significant influx of both CD4+ and CD8+ T cells into the lungs. Depletion of CD4+ T cells by injections of CD4-specific monoclonal antibody (mAb) prevented pulmonary clearance and also resulted in significant colonization of the brain and spleen of infected mice. CD4 depletion did not prevent the influx of CD8+ T cells into the lungs. Surprisingly, depletion of CD8+ T cells by mAb also ablated pulmonary clearance. CD8-depleted mice also had a small but significant increase in brain and spleen colony-forming unit compared to control mice by the end of the study. CD4+ T cell pulmonary influx was independent of the presence of CD8+ T cells. The lungs of T cell- depleted mice were examined histologically. CD4+ and CD8+ T cells each mediated a degree of inflammatory influx seen in the lungs of infected mice and raised the possibility that CD4+ and CD8+ T cells may synergize to generate the inflammatory response in the lungs. Numerous phagocytized but intact cryptococci were seen in the inflammatory foci of CD8-depleted mice but not in control or CD4-depleted mice. We propose that CD4+ T cells may recruit and activate effector phagocytes while CD8+ T cells predominantly function to lyse cryptococcus-laden unactivated phagocytes similar to the function of CD8+ T cells during listeria and mycobacteria infections.