CD8+ T cells specific for a malaria cytoplasmic antigen form clusters around infected hepatocytes and are protective at the liver stage of infection.

Following Anopheles mosquito-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 Plasmodium infection. We visualized the interaction between OT-I cells and infected hepatocytes by intravital imaging using two-photon microscopy. 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 dispensable 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 proteins expressed in the cytoplasm of Plasmodium parasites can become targets of specific CD8(+) T cells during liver-stage infection.