IntroductionCD4 ϩ T cells function as both regulators and effectors in the immune response. 1 CD4 ϩ T cells produce cytokines, providing help in the immune response to pathogens, and have often been referred to as Th cells. 2 However, over the past 2 decades, the ability of CD4 ϩ T cells to become cytotoxic has been reported in both mice and humans against alloantigens, microbe-infected cells, and cancer. 1,[3][4][5][6][7][8] Previous studies in mice demonstrate that CD4 ϩ cytotoxic T lymphocytes (CTLs) cause target cell lysis primarily via Fas/FasL interactions. [9][10][11][12] Later, studies on human CD4 ϩ T cells demonstrated that CD4 ϩ CTLs also use perforin and granzymebased granule exocytosis. [13][14][15] This pathway involves disruption of the target cell membrane through extracellular Ca 2ϩ -dependent polymerization of perforin, accompanied by uptake of granzymes, which are responsible for downstream activation of caspases and subsequent DNA fragmentation, resulting in target cell apoptosis.Although the importance of CTL-mediated killing of tumor or microbe-infected cells is well established, direct CTL-mediated microbial killing has been underinvestigated. Evidence has suggested that CTLs have very potent direct antimicrobial activity. This activity requires cell contact and involves regulated exocytosis of granules from the cytoplasm of the effector cells. 16 Perforin and granzymes are well-known cytolytic molecules 17 ; however, granulysin is less well recognized. 16,18 The human granulysin cDNA was originally isolated by subtractive hybridization in a search for T-cell-specific molecules expressed "late" (3-5 days) after activation. 19 Granulysin, a member of the saposin-like protein family of lipid binding proteins, colocalizes with perforin and granzymes in the cytolytic granules of human CTLs and natural killer (NK) cells. [20][21][22] Two major protein products of granulysin, 15 and 9 kDa, are detected in CTL and NK cells. 22 It is apparent that the 9-kDa form is processed from the larger, precursor 15-kDa form, 22 and recombinant 9-kDa granulysin disrupts artificial liposomes and cell membranes, damages mitochondria, and activates caspase 9 to induce apoptosis. 23,24 Additionally, granulysin exhibits potent cytotoxic activity against a broad panel of microbial pathogens and tumor cells. 18,20 Our previous observation, using Cryptococcus neoformans, one of the most important microbial pathogens in patients with compromised CD4 ϩ T-cell-dependent immunity, such as the acquired immunodeficiency syndrome (AIDS), [25][26][27] indicated that granulysin is required for the CD8 ϩ T-cell-mediated direct antifungal activity. 28 By contrast, NK cells use perforin, rather than granulysin, as the effector molecule to kill C neoformans. 29 Although CD4 ϩ and CD8 ϩ T cells and NK cells are important in cryptococcal host defense, 30-32 defective CD4 ϩ T cells are clearly the major risk factor for disease in human immunodeficiency virus (HIV)-infected patients. 33 Previous studies have demonstrated that IL-2-ac...