Resistance to Cryptococcus neoformans infection in the absence of CD4+ T cells

Previous studies of Cryptococcus neoformans infection have revealed a role for CD4(+) T cells and CD8(+) T cells in anticryptococcal resistance in the lungs, but such a role has been revealed only for CD4(+) T cells in the brains of experimentally infected mice. In this study, we found that mice genetically engineered to lack CD4+ T cells could be successfully vaccinated to express resistance to a rechallenge with Cryptococcus neoformans, provided the challenge dose was kept to lower than 1000 organisms per mouse. The challenge infection was uniformly lethal for unvaccinated control mice. Depletion of CD8(+) T cells weakened this resistance to re-challenge: both naive and vaccinated mice that were treated with antibody raised against CD8(+) T cells died significantly earlier than did mice that received an irrelevant control antibody. In vitro, purified CD8(+) T cells taken from draining lymph nodes of antigen-experienced mice were less efficient than were identically prepared CD4(+) T cells at stimulating the cells of a transformed microglial cell line to inhibit C neoformans proliferation, possibly mirroring the inferiority of CD8(+) T-cell-mediated protection observed in vivo. RNase protection assays showed similar IFN-gamma mRNA levels in both lymphocyte subsets. Class II major histocompatibility antigen expression was up-regulated strikingly on microglia cultured with IFN-gamma, but class I expression was less dramatically affected. Therefore microglial cell interaction may be more greatly enhanced with CD4(+) cells than with CD8(+) cells.