Cytotoxic CD4⁺ T cells use granulysin to kill Cryptococcus neoformans, and activation of this pathway is defective in HIV patients

An important mechanism of host defense to Cryptococcus neoformans involves the direct microbicidal activity of lymphocytes. The importance of CD4 ⁺ T cells is illustrated by the incidence of this infection in the acquired immunodeficiency syndrome (AIDS) patients; however, the relative activity of microbicidal CD4⁺ T cells compared with CD8⁺ T cells and natural killer (NK) cells has not been established. Further, although NK cells and CD8⁺ T cells use perforin or granulysin, respectively, to kill C neoformans, the effector molecule used by CD4⁺ T cells is not known. Experiments demonstrated that IL-2-activated peripheral blood lymphocytes from healthy adults acquire anticryptococcal activity, and surprisingly, that CD4⁺ T cells had the most profound effect on this activity. Using SrCl₂-induced degranulation and siRNA knockdown, granulysin was shown to be the effector molecule. Although activation by anti-CD3 + IL-2 resulted in the additional expression of perforin, this did not improve the anticryptococcal activity. Cryptococcal killing by CD4⁺ T cells was defective in human immunodeficiency virus (HIV)-infected patients due to dysregulated granulysin and perforin production in response to IL-2 or anti-CD3 + IL-2. In conclusion, CD4⁺ T cells are the major subset of cells responsible for killing C neoformans in peripheral blood. These cells use granulysin as the effector molecule, and priming is dysregulated in HIV-infected patients, which results in defective microbicidal activity.