Characterisation of lipid specific T cell responses in macaque and human Mycobacterium tuberculosis infection

Abstract

Mycobacterium tuberculosis (Mtb) continues to cause a global health pandemic and the emergence of new drug-resistant strains requires new therapeutic interventions. Mtb is characterised by a lipid-rich cell wall, thus T cells that respond to lipid antigens are likely to play a critical role in host immunity. I investigate the hypothesis that lipid-responsive CD1b- and CD1drestricted T cells may play critical roles in host immunity to tuberculosis (TB). Invariant natural killer T cells (iNKTs) are a CD1d restricted T cell population found in humans and macaques and previous in vitro and in vivo studies have identified their key anti-Mtb roles. I focus on the macaque model of Mtb infection, due to their accurate recapitulation of human disease, to identify iNKT correlates of protection. Using validated methods to identify iNKTs, and systematic analyses across four genetically distinct groups, I first demonstrate that peripheral blood iNKT frequency associates with disease susceptibility. I also observe skewed iNKT CD4+ /CD8+ subsets in animals more susceptible to Mtb. Furthermore, I demonstrate that CD8+ iNKTs associate with better disease outcome in both the Chinese cynomolgus and Indian Rhesus macaque. My results warrant further investigation into the mechanisms behind the observed iNKT based correlates in macaque TB infection. CD1b, the second lipid presenting molecule that I have focused on, is expressed on the surface of professional antigen presenting cells (APCs) and I show for the first time its expression in the human lung granuloma. It presents mycolic acids (MA) from the Mtb cell wall to Germ-line-Encoded-Mycolyl lipid specific T cells (GEMs). I use multiple in vitro cellular models to study the structure-function relationships of a large panel (>20) of synthetic Mtb MAs, representative of the major families of MAs in the Mtb cell wall. My results demonstrate that structurally diverse MA lipid tails vary in their ability to stimulate GEM T cells in a mechanism which is independent of antigen processing. The mycolic acid antigenic potential is dictated by the type of meromycolate tail functional group and relative stereochemistry. In silico molecular modelling of MAs onto CD1b support my in vitro observations and suggest that stimulatory MAs form seemingly more stable CD1b lipid complexes. Finally, my observations are further corroborated in ex vivo T cell responses from TB patient clinical samples and increased cytokine responses after stimulation with different mycolic acids. Taken together, our findings delineate immunogenic Mtb lipids which paves the way for lipid based anti-Mtb vaccines and therapeutics targeting GEM T cells.

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Identifiers

ORCID for Paul Elkington: orcid.org/0000-0003-0390-0613

ORCID for Salah Mansour: orcid.org/0000-0002-5982-734X

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