Identification and Phenotype of MAIT Cells in Cattle and Their Response to Bacterial Infections.

Matthew D Edmans,Elma Tchilian,Christina Vrettou,Emmanuel Atangana Maze,Tiphany Chrun,Paul Klenerman,Timothy K Connelley,Ligong Liu,Martin Vordermeier,Jeffrey Y W Mak,David P Fairlie,Sidonia B G Eckle,Lindert Benedictus,Siddharth Jayaraman

doi:10.3389/fimmu.2021.627173

Abstract

Mucosal-associated invariant T (MAIT) cells are a population of innate-like T cells that utilize a semi-invariant T cell receptor (TCR) α chain and are restricted by the highly conserved antigen presenting molecule MR1. MR1 presents microbial riboflavin biosynthesis derived metabolites produced by bacteria and fungi. Consistent with their ability to sense ligands derived from bacterial sources, MAIT cells have been associated with the immune response to a variety of bacterial infections, such as Mycobacterium spp., Salmonella spp. and Escherichia coli. To date, MAIT cells have been studied in humans, non-human primates and mice. However, they have only been putatively identified in cattle by PCR based methods; no phenotypic or functional analyses have been performed. Here, we identified a MAIT cell population in cattle utilizing MR1 tetramers and high-throughput TCR sequencing. Phenotypic analysis of cattle MAIT cells revealed features highly analogous to those of MAIT cells in humans and mice, including expression of an orthologous TRAV1-TRAJ33 TCR α chain, an effector memory phenotype irrespective of tissue localization, and expression of the transcription factors PLZF and EOMES. We determined the frequency of MAIT cells in peripheral blood and multiple tissues, finding that cattle MAIT cells are enriched in mucosal tissues as well as in the mesenteric lymph node. Cattle MAIT cells were responsive to stimulation by 5-OP-RU and riboflavin biosynthesis competent bacteria in vitro. Furthermore, MAIT cells in milk increased in frequency in cows with mastitis. Following challenge with virulent Mycobacterium bovis, a causative agent of bovine tuberculosis and a zoonosis, peripheral blood MAIT cells expressed higher levels of perforin. Thus, MAIT cells are implicated in the immune response to two major bacterial infections in cattle. These data suggest that MAIT cells are functionally highly conserved and that cattle are an excellent large animal model to study the role of MAIT cells in important zoonotic infections.

Highlights

Mucosal-associated invariant T (MAIT) cells represent the largest α/β T cell population specific for a single antigen in humans, comprising up to 10% of all T cells in the periphery and 50% in the liver [1,2,3,4]
Due to the high level of MHC related protein 1 (MR1) [15, 16, 18, 36] and MAIT T cell receptor (TCR) α chain [15, 16, 21, 24,25,26] conservation between species, we hypothesized that human MR1 tetramers would likely cross react with cattle MAIT cells
We isolated peripheral blood mononuclear cells (PBMC) from a cohort of cattle (n = 17) of varying age (3 to 56 months) and staining with MR1-5-OP-RU tetramers identified a clear population of CD3+ tetramer+ ‘putative’ MAIT cells (Figures 1A,B) with a mean frequency of 0.6% amongst CD3+ cells, which was comparable to previous qPCR estimates of MAIT cell frequency in cattle

Summary

Introduction

Mucosal-associated invariant T (MAIT) cells represent the largest α/β T cell population specific for a single antigen in humans, comprising up to 10% of all T cells in the periphery and 50% in the liver [1,2,3,4]. MAIT cells are unconventional T cells that emerge from the thymus in a “pre-primed” state and express an effector memory phenotype (in humans CD45RA−CD45RO+CD95hiCD62Llo) [10]. This facilitates rapid innate-like responses to bacterial infections, such as Francisella tularensis [11], Legionella longbeachae [12] and Mycobacterium tuberculosis (TB) [13, 14]. MAIT cells express an essentially invariant T cell receptor (TCR) α chain (TRA), composed in humans of the variable gene segment 1-2 (TRAV1-2) ( known as Vα7.2 in Arden nomenclature) rearranged with the TRA joining gene segment 33 (TRAJ33) ( known as Jα33), or less commonly TRAJ12 (Jα12) and TRAJ20 (Jα20), paired with TCR β chains (TRB) featuring dominant usage of specific TRBV gene segments [13, 20,21,22,23]. Whilst mice have provided valuable information on MAIT cells in protection against infections [1, 12, 29, 30], pathology [31] and tissue repair [32], this might not always be fully representative of MAIT cell function in humans

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Results

Conclusion