A new method shows that a minority of liver-localized CD8 T cells display hard-to-detect attraction to Plasmodium-infected hepatocytes

Abstract

AbstractMalaria is a disease caused by Plasmodium parasites, 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. Malaria vaccine candidates tested in mice which induce large numbers of malaria-specific CD8 T cells are able to eliminate all liver stages, preventing fulminant malaria. However, how CD8 T cells find all parasites in 48 hours of the liver stage lifespan is not well understood. Using intravital microscopy of murine livers, we generated unique data on T cell search for malaria liver stages within a few hours after infection. To detect attraction of T cells to an infection site, we used the Von Mises-Fisher distribution in 3D, similar to the 2D von Mises distribution previously used in ecology, to detect attraction of searching T cells towards the infection site. Our results suggested that the vast majority (70-95%) of malaria-specific CD8 T cells and non-specific T cells did not display attraction towards the infection site, suggesting that the search for malaria liver stages occurs randomly. However, a small fraction (15-20%) displayed weak but detectable attraction towards parasites which already had been surrounded by several T cells. We found that speeds and turning angles correlated with attraction, suggesting that understanding mechanisms determining the speed of T cell movement in the liver may improve T cell vaccine efficacy. Stochastic simulations suggested that a small movement bias towards the parasite dramatically reduces the number of CD8 T cells needed to eliminate all malaria liver stages, but to detect such attraction by individual cells requires extremely long imaging experiments which are not currently feasible. Importantly, this is the first demonstration that we know about of how activated/memory CD8 T cells search for the pathogen in nonlymphoid tissues few hours after infection. We have also established a framework for how attraction of individual T cells towards a location in 3D can be rigorously evaluated.