Need for CD8 T cell clusters to eliminate a malaria liver stage arises in part due to heterogeneity in killing efficacy of individual effector T cells

Abstract

AbstractVaccination strategies in mice inducing high numbers of memory CD8 T cells specific to a single epitope are able to provide sterilizing protection against infection with Plasmodium sporozoites. We have recently found that Plasmodium-specific CD8 T cells cluster around sporozoite-infected hepatocytes but whether such clusters are important in elimination of the parasite remains incompletely understood. Here we used our previously generated data in which we employed intravital microscopy to longitudinally image 32 GFP-expressing Plasmodium yoelii parasites in livers of mice that had received activated Plasmodium-specific CD8 T cells after sporozoite infection. We found great heterogeneity in the dynamics of the normalized GFP signal from the parasites (termed “vitality index” or VI) that was weakly correlated with the number of T cells near the parasite. We also found that a simple model assuming mass-action, additive killing by T cells well describes the VI dynamics for most parasites predicting highly variable killing efficacy of individual T cells. Given the estimated median per capita kill rate ofk= 0.031/h we predict that a single T cell is typically incapable to kill the the parasite within 48 hour lifespan of the liver stage. Stochastic simulations of T cell clustering and killing of the liver stage also suggested that 1) three or more T cells per liver stage are required to ensure sterilizing protection; 2) both variability in killing efficacy of individual T cells and resistance to killing by individual parasites may contribute to the observed variability in VI decline, and 3) stable VI of some clustered parasites cannot be explained by measurement noise. Taken together, our analysis for the first time provides estimates of efficiency at which individual CD8 T cells eliminate intracellular infection in vivo.