CD8 T Cells Specific for a Malaria Cytoplasmic Antigen Form Clusters around Infected Hepatocytes and Are Protective at the Liver

Following Anophelesmosquito-mediated introduction into a human host, Plasmodium parasites infect hepatocytes and undergo intensive replication. Accumulating evidence indicates that CD8 T cells induced by immunization with attenuated Plasmodium sporozoites can confer sterile immunity at the liver stage of infection; however, the mechanisms underlying this protection are not clearly understood. To address this, we generated recombinant Plasmodium berghei ANKA expressing a fusion protein of an ovalbumin epitope and green fluorescent protein in the cytoplasm of the parasite. We have shown that the ovalbumin epitope is presented by infected liver cells in a manner dependent on a transporter associated with antigen processing and becomes a target of specific CD8 T cells from the T cell receptor transgenic mouse line OT-I, leading to protection at the liver stage of Plasmo-dium infection. We visualized the interaction between OT-I cells and infected hepatocytes by intravital imaging using two-pho-tonmicroscopy. OT-I cells formed clusters around infected hepatocytes, leading to the elimination of the intrahepatic parasites and subsequent formation of large clusters of OT-I cells in the liver. Gamma interferon expressed in CD8 T cells was dispens-able for this protective response. Additionally, we found that polyclonal ovalbumin-specific memory CD8 T cells induced by de novo immunization were able to confer sterile protection, although the threshold frequency of the protection was relatively high. These studies revealed a novel mechanism of specific CD8 T cell-mediated protective immunity and demonstrated that pro-teins expressed in the cytoplasm of Plasmodium parasites can become targets of specific CD8 T cells during liver-stage infection