Depletion of CD4⁺ (L3T4⁺) lymphocytes in vivo impairs murine host defense to Cryptococcus neoformans

T cell-mediated immunity has been shown to play an important role in the host defense to Cryptococcus neoformans. Infections due to C. neoformans are increased in patients with AIDS who are deficient in the CD4⁺ subset of T lymphocytes. Thus, the effect of CD4⁺ (L3T4⁺) lymphocyte depletion on murine host defenses to C. neoformans was studied. The mAb GK 1.5 was administered to mice, and CD4⁺ T lymphocyte depletion was confirmed by the analysis of T cell subsets in blood, spleen, lymph node, and lung. Evidence of a functional defect was confirmed by demonstrating that the splenocytes of treated mice were unable to proliferate in response to class II incompatible spleen cells. Furthermore, delayed type hypersensitivity to C. neoformans was abrogated by CD4⁺ lymphocyte depletion. Mice depleted of CD4⁺ lymphocytes were inoculated with a virulent strain of C. neoformans by the i.v. or the intratracheal route. After i.v. inoculation of C. neoformans, the survival of mice depleted of CD4⁺ lymphocytes was reduced (27.8 ± 1.8 vs 36.0 ± 3.1 days, p < 0.04). After intratracheal inoculation, C. neoformans disseminated from the lung to extrapulmonary organs. Dissemination occurred earlier in mice depleted of CD4⁺ lymphocytes compared to mice that received control antibody, and the burden of C. neoformans in extrapulmonary organs was greater in mice depleted of CD4⁺ lymphocytes than control mice. Surprisingly, there was no increase in the burden of C. neoformans in the lungs of CD4⁺ lymphocyte-depleted mice. Survival of mice inoculated with C. neoformans and depleted of CD4⁺ lymphocytes was reduced compared to control mice and was related to the increased rate of accumulation of organisms in the brains of treated mice. The mean survival of GK 1.5-treated mice was 34.1 ± 0.9 days compared to control mice with a mean survival of 40.6 ± 9 days (p < 0.001). These data suggest that CD4⁺ lymphocytes play a prominent role in the host defense of infections due to C. neoformans, that CD4⁺ lymphocytes are required in extrapulmonary organs for optimal clearance of C. neoformans and that CD4⁺ lymphocytes are critical for survival of mice infected with C. neoformans.