In vivo depletion of murine CD8 positive T cells impairs survival during infection with a highly virulent strain of Cryptococcus neoformans.

Cell-mediated immunity plays an important but incompletely understood role in host defense against Cryptococcus neoformans. Because of their multiple capacities as cytokine-secreting cells, cytotoxic cells, and antigen-specific suppressor cells, CD8 positive T lymphocytes could potentially either enhance or impair host defense against C. neoformans. To determine whether CD8 T cells enhance or inhibit host defence during an infection with a highly virulent strain of C. neoformans, we examined the effect of in vivo CD8 cell depletion on survival and on the number of organisms in mice infected by either the intratracheal or intravenous routes. Adequacy of depletion was confirmed both phenotypically and functionally. Regardless of the route of infection, we found that survival of mice depleted of CD8 T cells was significantly reduced compared to undepleted mice. Surprisingly, however, CD8 depletion did not alter organism burden measured by quantitative CFU assay in mice infected by either route. These data demonstrate that CD8 positive T cells participate in the immune response to a highly virulent strain of C. neoformans. By contrast to minimally virulent isolates that do not cause a life threatening infection, the immune response to a highly virulent isolate does not alter the burden of organisms, but does enhance host defense as it is necessary for the optimal survival of infected mice.