Contributions of IFN-γ and granulysin to the clearance ofPlasmodium yoeliiblood stage

P.vivax-infected Retics (iRetics) express human leukocyte antigen class I (HLA-I), are recognized by CD8(+)T cells and killed by granulysin (GNLY) and granzymes. However, howPlasmodiuminfection induces MHC-I expression on Retics is unknown. In addition, whether GNLY helps controlPlasmodiuminfectionin vivohas not been studied. Here, we examine these questions using rodent infection with theP.yoelii17XNL strain, which has tropism for Retics. Infection withP.yoeliicaused extramedullary erythropoiesis, reticulocytosis and expansion of CD8(+)CD44(+)CD62L(-)IFN-gamma-producing T cells that form immune synapses with iRetics. We now provide evidence that MHC-I expression by iRetic is dependent on IFN-gamma-induced transcription of IRF-1, MHC-I and beta 2-microglobulin (beta 2-m) in erythroblasts. Consistently, CTLs from infected wild type (WT) mice formed immune synapses with iRetics in an IFN-gamma- and MHC-I-dependent manner. When challenged withP.yoelii17XNL, WT mice cleared parasitemia and survived, while IFN-gamma KO mice remained parasitemic and all died. beta 2-m KO mice that do not express MHC-I and have virtually no CD8(+)T cells had prolonged parasitemia, and 80% survived. Because mice do not express GNLY,GNLY-transgenic mice can be used to assess thein vivoimportance of GNLY. Parasite clearance was accelerated inGNLY-transgenic mice and depletion of CD8(+)T cells ablated the GNLY-mediated resistance toP.yoelii. Altogether, our results indicate that in addition to previously described mechanisms, IFN-gamma promotes host resistance to the Retic-tropicP.yoelii17XNL strain by promoting MHC-I expression on iRetics that become targets for CD8(+)cytotoxic T lymphocytes and GNLY. Author summary CD8(+)cytotoxic T lymphocytes (CTLs) are important for immune defense against intracellular pathogens, such as viruses, bacteria and parasites, and tumor surveillance. CTLs, which recognize peptide epitopes presented by MHC-I molecules expressed in nucleated cells, become activated and kill infected target cells by releasing the contents of cytotoxic granules into the immunological synapse. Since mostPlasmodium spp. infect erythrocytes that are enucleated and do not express MHC-I, the role of CD8(+)T cells in the blood-stage of malaria has been neglected. We recently showed thatP.vivax-infected reticulocytes express MHC-I and are killed in a manner dependent on granulysin (GNLY), a cytotoxic granule effector protein. However, the protective role of CD8(+)T cells is controversial and the role of GNLYin vivoremains to be demonstrated. Here, we show that CTLs and GNLY mediate mouse resistance to blood-stage infection withP.yoelii, a rodent malaria parasite that preferably infects reticulocytes.