The Fantastic Voyage of the Trypanosome: A Protean Micromachine Perfected during 500 Million Years of Engineering

The lymphatic system favours survival of a unique T. brucei population

Trypanosoma brucei colonise and multiply in the blood vasculature, as well as in various organs of the host's body. Lymph nodes have been previously shown to harbour large numbers of parasites, and the lymphatic system has been proposed as a key site that allows T. brucei distribution through, and colonization of the mammalian body. However, visualization of host-pathogen interactions in the lymphatic system has never captured dynamic events with high spatial and temporal resolution throughout infection. In our work, we used a mixture of tools including intravital microscopy and ex vivo imaging to study T. brucei distribution in 20 sets of lymph nodes. We demonstrate that lymph node colonization by T. brucei is different across lymph node sets, with the most heavily colonised being the draining lymph nodes of main tissue reservoirs: the gonadal white adipose tissue and pancreas. Moreover, we show that the lymphatic vasculature is a pivotal site for parasite dispersal, and altering this colonization by blocking LYVE-1 is detrimental for parasite survival. Additionally, parasites within the lymphatic vasculature have unique morphological and behavioural characteristics, different to those found in the blood, demonstrating that across both types of vasculature, these environments are physically separated. Finally, we demonstrate that the lymph nodes and the lymphatic vasculature undergo significant alterations during T. brucei infection, resulting in oedema throughout the host's body.

Organotypic endothelial adhesion molecules are key for Trypanosoma brucei tropism and virulence

Trypanosoma brucei is responsible for lethal diseases in humans and cattle in Sub-Saharan Africa. These extracellular parasites extravasate from the blood circulation into several tissues. The importance of the vasculature in tissue tropism is poorly understood. Using intravital imaging and bioluminescence, we observe that gonadal white adipose tissue and pancreas are the two main parasite reservoirs. We show that reservoir establishment happens before vascular permeability is compromised, suggesting that extravasation is an active mechanism. Blocking endothelial surface adhesion molecules (E-selectin, P-selectins, or ICAM2) significantly reduces extravascular parasite density in all organs and delays host lethality. Remarkably, blocking CD36 has a specific effect on adipose tissue tropism that is sufficient to delay lethality, suggesting that establishment of the adipose tissue reservoir is necessary for parasite virulence. This work demonstrates the importance of the vasculature in a T. brucei infection and identifies organ-specific adhesion molecules as key players for tissue tropism.

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Our study investigates the blood vasculature for T. brucei reservoir establishment

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We show the pancreas is a large extravascular reservoir

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We establish that T. brucei tropism is linked to organotypic adhesion molecules

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Interfering with adhesion molecules impacts parasite virulence and host survival