Mechanisms of evasion of the African trypanosome : trypanosoma brucei from the host innate and adaptive immune system

Abstract

The extracellular bloodstream form Trypanosoma brucei is the causative agent of Human African Trypanosomiasis (HAT). It is highly adapted to escape both the innate and adaptive immune systems of the mammalian host, which is mediated through a dense Variant Surface Glycoprotein (VSG) coat. Two main mechanisms of evasion are known to provide protection for the parasite in the bloodstream: antigenic variation and high rates of host opsonin removal. VSG is essential for trypanosome survival and blocking its synthesis triggers growth inhibition and a precytokinesis cell cycle arrest in vitro, but are rapidly cleared in mice. In this study, I showed that blocking the synthesis of VSG resulted in an increase in trypanosome phagocytosis by mouse macrophages but only if the parasite had been opsonised with anti-VSG antibodies. Further analysis revealed that the induction of VSG RNAi caused a change in swimming behaviour, as stalled cells swam with increased velocity and directionality. Blocking VSG synthesis also resulted in a significant reduction of anti-VSG antibody removal from the parasite surface. However, this was not due to defects in endocytosis as the rates of internalisation of bulk surface VSG, or endocytic markers like tomato lectin, transferrin or dextran were not significantly affected by the VSG synthesis block. Antibody- VSG complexes could enter the trypanosome and reach the lysosome but the kinetics at which antibodies were internalised was reduced, which indicates that selective perturbation of anti- VSG antibody complexes may be operating at the flagellar pocket. These results highlight the importance of efficient elimination of anti-VSG-antibody complexes from the trypanosome cell surface for trypanosome evasion of macrophages. Trypanosome evasion from the host adaptive immune system is known to mainly involve dampening the pro-inflammatory response elicited in the early phases of infection. Here, I demonstrated that in vitro, T. brucei hinders the process of maturation and antigen uptake in dendritic cells (DC). As a consequence, led to in a shift in cytokine production to a type II profile, accompanied by an unusual surge in IL-8, a neutrophil recruiting cytokine. The capacity at which T. brucei stimulated DCs could activate naïve T-cells also decreased over time. While naïve T-cells were able to proliferate in response, there were significant changes in the cytokine profiles. The Th1 cytokine IFN-γ was upregulated while expression of the Th2 cytokine IL-4 was suppressed, indicating a general pro-inflammatory response was induced by T. brucei. Interestingly, I showed for the first time, that Th17 cells may be involved in infection control as Th17-associated cytokines were universally upregulated. Finally, a preliminary drug study in this thesis, showed the potential of the pegylated human recombinant arginase, BCT-100 as an alternative HAT treatment. Depletion of arginine using BCT-100 resulted in growth inhibition and a cell cycle arrest. This study highlights several key aspects regarding evasion from host immunity by bloodstream form T. brucei. Further investigation will allow us to gain insight into the mechanisms involved in mounting an effective response against T. brucei infections and develop novel therapies for the treatment of Human African Trypanosomiasis.