Nonspecific CD8+ T Cells and Dendritic Cells/Macrophages Participate in Formation of CD8+ T Cell-Mediated Clusters against Malaria Liver-Stage Infection

CD8(+) T cells are the major effector cells that protect against malaria liver-stage infection, forming clusters around Plasmodium-infected hepatocytes and eliminating parasites after a prolonged interaction with these hepatocytes. We aimed to investigate the roles of specific and nonspecific CD8(+) T cells in cluster formation and protective immunity. To this end, we used Plasmodium berghei ANKA expressing ovalbumin as well as CD8(+) T cells from transgenic mice expressing a T cell receptor specific for ovalbumin (OT-I) and CD8(+) T cells specific for an unrelated antigen, respectively. While antigen-specific CD8(+) T cells were essential for cluster formation, both antigen-specific and nonspecific CD8(+) T cells joined the clusters. However, nonspecific CD8(+) T cells did not significantly contribute to protective immunity. In the livers of infected mice, specific CD8(+) T cells expressed high levels of CD25, compatible with a local, activated effector phenotype. In vivo imaging of the liver revealed that specific CD8(+) T cells interact with CD11c(+) cells around infected hepatocytes. The depletion of CD11c(+) cells virtually eliminated the clusters in the liver, leading to a significant decrease in protection. These experiments reveal an essential role of hepatic CD11c(+) dendritic cells and presumably macrophages in the formation of CD8(+) T cell clusters around Plasmodium-infected hepatocytes. Once cluster formation is triggered by parasite-specific CD8(+) T cells, specific and unrelated activated CD8(+) T cells join the clusters in a chemokine-and dendritic cell-dependent manner. Nonspecific CD8(+) T cells seem to play a limited role in protective immunity against Plasmodium parasites.