A new method based on the von Mises-Fisher distribution shows that a minority of liver-localized CD8 T cells display hard-to-detect attraction toPlasmodium-infected hepatocytes

Abstract

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.

Liver environment-imposed constraints diversify movement strategies of liver-localized CD8 T cells

“…However, in reality multiple T cells may be searching for the same parasite. Indeed, our recent analysis suggests that for 10 5 liver-localized CD8 T cells at least 1 T cell should be within 100 µm of the parasite with 99% certainty 51 . In our experiments we rarely see parasites surrounded by more than 5-8 T cells suggesting that in many situations fewer than 10 T cells are close enough to the parasite to find it within several hours 52,53 .…”

“…S5); however, this remains to be more fully investigated in the future. In particular, our recent work suggests that while initially, Plasmodium-specific CD8 T cells search for the infection site randomly, following formation of a T cell cluster near the parasite, other T cells become moderately attracted to the cluster 51,53 . Whether movement patterns of liver-localized T cells change following formation of T cell cluster near the parasite remains to be investigated.…”

“…4. Novel data on movement of Plasmodium-specific (OT-1) and lymphocytic choriomeningitis virus-specific (P14) CD8 T cells moving in livers of Plasmodium berghei-infected mice [52].…”

“…into naive mice and these mice were 1.5-2 hours later infected with 10 5 Plasmodium berghei-CS 5M sporozoites expressing SIINFEKL epitope from chicken ovalbumin [31,62]. Ten to twenty minutes after infection, livers of infected mice were exposed and movements of T cells were recorded using intravital microscopy [52]. We performed the same analyses as in the main text (Fig.…”

“…into naive mice and these mice were 1.5-2 hours later infected with 10 5 Plasmodium berghei-CS 5M sporozoites expressing SIINFEKL epitope from chicken ovalbumin[31,62]. Ten to twenty minutes after infection, livers of infected mice were exposed and movements of T cells were recorded using intravital microscopy[52]. We performed the same analyses as in the main text (Fig.2) including (A) regression analysis of the MSD with time, (B) distribution of movement lengths, (C)-(D) fitting alternative distributions to the movement data for OT1 (C) or P14 (D) T cells.…”