A minority of liver-localized CD8 T cells demonstrate difficult-to-detect attraction to Plasmodium-infected hepatocytes

Malaria is a disease caused by parasites from genus Plasmodium resulting in over 200 million infections and 400,000 deaths every year. A critical step of malaria infection is when mosquito-injected sporozoites travel to the liver and form liver stages. Several malaria vaccine candidates tested in mice induce high levels of malaria-specific CD8 T cells which are able to eliminate all liver stages, thus providing sterilizing immunity against the disease. However, how CD8 T cells locate the site of infection is not well understood. We generated and analyzed data from intravital microscopy experiments in mice in which the movement of T cells relative to the liver stage was recorded in several different settings. To detect attraction of T cells towards the infection site, we developed a novel metric based on the Von Mises-Fisher (VMF) distribution, which is more powerful than previously used metrics. Our results suggested that the majority ([~] 70 - 95%) of malaria-specific CD8 T cells and T cells of irrelevant specificity did not display any attraction towards the parasite when the parasite was not found by T cells, but some T cells displayed strong attraction when there was a large cluster of Plasmodium-specific CD8 T cells near the parasite. We found that the speed of T cell movement (and small turning angles) correlated with the bias of T cell movement towards the infection site (but several other parameters did not) suggesting that a deeper understanding of what determines the speed of T cell movement in tissues may help with improving T cell vaccine efficacy. Stochastic simulations suggested that a small movement bias towards the parasite dramatically reduces the number of CD8 T cells needed for a complete elimination of the malaria liver stages from the liver, and yet, to detect such attraction exhibited by individual cells requires extremely long imaging experiments. We thus have established a framework for how attraction of moving cells towards a particular location can be rigorously evaluated.