Lionel Le Bourhis

Antimicrobial activity of mucosal-associated invariant T cells

Mucosal-associated invariant T lymphocytes (MAIT lymphocytes) are characterized by two evolutionarily conserved features: an invariant T cell antigen receptor (TCR) α-chain and restriction by the major histocompatibility complex (MHC)-related protein MR1. Here we show that MAIT cells were activated by cells infected with various strains of bacteria and yeast, but not cells infected with virus, in both humans and mice. This activation required cognate interaction between the invariant TCR and MR1, which can present a bacteria-derived ligand. In humans, we observed considerably fewer MAIT cells in blood from patients with bacterial infections such as tuberculosis. In the mouse, MAIT cells protected against infection by Mycobacterium abscessus or Escherichia coli. Thus, MAIT cells are evolutionarily conserved innate-like lymphocytes that sense and help fight off microbial infection.

Human MAIT cells are xenobiotic-resistant, tissue-targeted, CD161hi IL-17-secreting T cells.

Mucosal-associated invariant T (MAIT) cells are very abundant in humans and have antimicrobial specificity, but their functions remain unclear. MAIT cells are CD161(hi)IL-18Rα(+) and either CD4(-)CD8(-) (DN) or CD8αβ(int) T cells. We now show that they display an effector-memory phenotype (CD45RA(-)CD45RO(+)CD95(hi)CD62L(lo)), and their chemokine receptor expression pattern (CCR9(int)CCR7(-)CCR5(hi)CXCR6(hi)CCR6(hi)) indicates preferential homing to tissues and particularly the intestine and the liver. MAIT cells can represent up to 45% of the liver lymphocytes. They produce interferon-γ and Granzyme-B as well as high levels of interleukin-17 after phorbol myristate acetate + ionomycin stimulation. Most MAIT cells are noncycling cells (< 1% are Ki-67(+)) and express the multidrug resistance transporter (ABCB1). As expected from this phenotype, MAIT cells are more resistant to chemotherapy than other T-cell populations. These features might also allow MAIT cells to resist the xenobiotics potentially secreted by the gut bacteria. We also show that this population does not appear to have antiviral specificity and that CD8 MAIT cells include almost all the ABCB1(+)CD161(hi) CD8 T cells. Together with their already known abundance and antimicrobial specificity, the gut-liver homing characteristics, high expression of ABCB1, and ability to secrete interleukin-17 probably participate in the antibacterial properties of MAIT cells.

Mucosal-associated invariant T cells: unconventional development and function.

Mucosal-associated invariant T (MAIT) cells are a population of T cells that display a semi-invariant T cell receptor (TCR) and are restricted by the evolutionarily conserved major histocompatibility complex related molecule, MR1. Here, we review recent knowledge of this T cell population. MAIT cells are abundant in human blood, gut and liver, and display an effector phenotype. They follow an atypical pathway of development and preferentially locate to peripheral tissues. Human and mouse MAIT cells react to bacterially infected cells in an MR1-dependent manner. They migrate to the infection site and can be protective in experimental infection models. MAIT cells secrete interferon-γ, and interleukin-17 under certain conditions. The species conservation, as well as the wide microbial reactivity, infer an important role for this cell population in immunity.

MAIT cells detect and efficiently lyse bacterially-infected epithelial cells.

Mucosal associated invariant T cells (MAIT) are innate T lymphocytes that detect a large variety of bacteria and yeasts. This recognition depends on the detection of microbial compounds presented by the evolutionarily conserved major-histocompatibility-complex (MHC) class I molecule, MR1. Here we show that MAIT cells display cytotoxic activity towards MR1 overexpressing non-hematopoietic cells cocultured with bacteria. The NK receptor, CD161, highly expressed by MAIT cells, modulated the cytokine but not the cytotoxic response triggered by bacteria infected cells. MAIT cells are also activated by and kill epithelial cells expressing endogenous levels of MRI after infection with the invasive bacteria Shigella flexneri. In contrast, MAIT cells were not activated by epithelial cells infected by Salmonella enterica Typhimurium. Finally, MAIT cells are activated in human volunteers receiving an attenuated strain of Shigella dysenteriae-1 tested as a potential vaccine. Thus, in humans, MAIT cells are the most abundant T cell subset able to detect and kill bacteria infected cells.

MAIT cells, surveyors of a new class of antigen: development and functions

Mucosal associated invariant T (MAIT) cells are evolutionarily conserved T cells that are restricted by the non-classical MHC-1b molecule, MR1. MAIT cells are selected on hematopoietic cells, and exit the thymus with a naïve phenotype before expanding in the periphery and attaining a memory phenotype. MAIT cells represent an abundant oligoclonal population in human blood and liver. MAIT cells react against a newly identified class of antigens: vitamin B metabolites, which are found in most bacteria and yeasts. MAIT cells secrete IFN-γ and IL-17 and their frequencies are modified in several diseases. The specificity, evolutionary conservation and unique features of MAIT cells indicate important functions, either against a ubiquitous pathogen or in gut immune/epithelial homeostasis.

IL-12 drives functional plasticity of human group 2 innate lymphoid cells

Lim et al. show that IL-12 can drive IFN-γ production by human group 2 innate lymphoid cells.

In Vitro and In Vivo Analysis of the Gram-Negative Bacteria–Derived Riboflavin Precursor Derivatives Activating Mouse MAIT Cells

Mucosal-associated invariant T (MAIT) cells recognize microbial compounds presented by the MHC-related 1 (MR1) protein. Although riboflavin precursor derivatives from Gram-positive bacteria have been characterized, some level of ligand heterogeneity has been suggested through the analysis of the MAIT cell TCR repertoire in humans and differential reactivity of human MAIT cell clones according to the bacteria. In this study, using Gram-negative bacteria mutated for the riboflavin biosynthetic pathway, we show a strict correlation between the ability to synthesize the 5-amino-ribityl-uracil riboflavin precursor and to activate polyclonal and quasi-monoclonal mouse MAIT cells. To our knowledge, we show for the first time that the semipurified bacterial fraction and the synthetic ligand activate murine MAIT cells in vitro and in vivo. We describe new MR1 ligands that do not activate MAIT cells but compete with bacterial and synthetic compounds activating MAIT cells, providing the capacity to modulate MAIT cell activation. Through competition experiments, we show that the most active synthetic MAIT cell ligand displays the same functional avidity for MR1 as does the microbial compound. Altogether, these results show that most, if not all, MAIT cell ligands found in Escherichia coli are related to the riboflavin biosynthetic pathway and display very limited heterogeneity.

Double Positive Thymocytes Select Mucosal-Associated Invariant T Cells

NKT and mucosal-associated invariant T (MAIT) cells express semi-invariant TCR and restriction by nonclassical MHC class Ib molecules. Despite common features, the respective development of NKT and MAIT subsets is distinct. NKTs proliferate extensively and acquire effector properties prior to thymic export. MAIT cells exit the thymus as naive cells and acquire an effector/memory phenotype in a process requiring both commensal flora and B cells. During thymic development, NKTs are selected by CD1d-expressing cortical thymocytes; however, the hematopoietic cell type responsible for MAIT cell selection remains unresolved. Using reaggregated thymic organ culture and bone marrow chimeras, we demonstrate that positive selection of mouse iVα19 transgenic and Vβ6 transgenic MAIT cell progenitors requires MHC-related 1–expressing CD4+CD8+ double positive thymocytes, whereas thymic B cells, macrophages, and dendritic cell subsets are dispensable. Preincubation of double positive thymocytes with exogenous bacterial ligand increases MHC-related 1 surface expression and enhances mature MAIT cell activation in the in vitro cocultures. The revelation of a common cell type for the selection of both NKT and MAIT subsets raises questions about the mechanisms underlying acquisition of their specific features.

Mucosal-associated invariant T cell–rich congenic mouse strain allows functional evaluation

Mucosal-associated invariant T cells (MAITs) have potent antimicrobial activity and are abundant in humans (5%-10% in blood). Despite strong evolutionary conservation of the invariant TCR-α chain and restricting molecule MR1, this population is rare in laboratory mouse strains (≈0.1% in lymphoid organs), and lack of an appropriate mouse model has hampered the study of MAIT biology. Herein, we show that MAITs are 20 times more frequent in clean wild-derived inbred CAST/EiJ mice than in C57BL/6J mice. Increased MAIT frequency was linked to one CAST genetic trait that mapped to the TCR-α locus and led to higher usage of the distal Vα segments, including Vα19. We generated a MAIThi congenic strain that was then crossed to a transgenic Rorcgt-GFP reporter strain. Using this tool, we characterized polyclonal mouse MAITs as memory (CD44+) CD4-CD8lo/neg T cells with tissue-homing properties (CCR6+CCR7-). Similar to human MAITs, mouse MAITs expressed the cytokine receptors IL-7R, IL-18Rα, and IL-12Rβ and the transcription factors promyelocytic leukemia zinc finger (PLZF) and RAR-related orphan receptor γ (RORγt). Mouse MAITs produced Th1/2/17 cytokines upon TCR stimulation and recognized a bacterial compound in an MR1-dependent manner. During experimental urinary tract infection, MAITs migrated to the bladder and decreased bacterial load. Our study demonstrates that the MAIThi congenic strain allows phenotypic and functional characterization of naturally occurring mouse MAITs in health and disease.