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Abstract Cell death mechanisms in T lymphocytes vary according to their developmental stage, cell subset and activation status. The cell death control mechanisms of mucosal-associated invariant T (MAIT) cells, a specialized T cell population, are largely unknown. Here we report that MAIT cells express key necroptotic machinery; receptor-interacting protein kinase 3 (RIPK3) and mixed lineage kinase domain-like (MLKL) protein, in abundance. Despite this, we discovered that the loss of RIPK3, but not necroptotic effector MLKL or apoptotic caspase-8, specifically increased MAIT cell abundance at steady-state in the thymus, spleen, liver and lungs, in a cell-intrinsic manner. In contrast, over the course of infection with Francisella tularensis, RIPK3 deficiency did not impact the magnitude of the expansion nor contraction of MAIT cell pools. These findings suggest that, distinct from conventional T cells, the accumulation of MAIT cells is restrained by RIPK3 signalling, likely prior to thymic egress, in a manner independent of canonical apoptotic and necroptotic cell death pathways.

IntroductionMucosal-associated invariant T (MAIT) cells are a population of innate-like T cells, which mediate host immunity to microbial infection by recognizing metabolite antigens derived from microbial riboflavin synthesis presented by the MHC-I-related protein 1 (MR1). Namely, the potent MAIT cell antigens, 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU) and 5-(2-oxoethylideneamino)-6-D-ribitylaminouracil (5-OE-RU), form via the condensation of the riboflavin precursor 5-amino-6-D-ribitylaminouracil (5-A-RU) with the reactive carbonyl species (RCS) methylglyoxal (MG) and glyoxal (G), respectively. Although MAIT cells are abundant in humans, they are rare in mice, and increasing their abundance using expansion protocols with antigen and adjuvant has been shown to facilitate their study in mouse models of infection and disease. MethodsHere, we outline three methods to increase the abundance of MAIT cells in C57BL/6 mice using a combination of inflammatory stimuli, 5-A-RU and MG.ResultsOur data demonstrate that the administration of synthetic 5-A-RU in combination with one of three different inflammatory stimuli is sufficient to increase the frequency and absolute numbers of MAIT cells in C57BL/6 mice. The resultant boosted MAIT cells are functional and can provide protection against a lethal infection of Legionella longbeachae.ConclusionThese results provide alternative methods for expanding MAIT cells with high doses of commercially available 5-A-RU (± MG) in the presence of various danger signals.

MAIT cells are abundant in the lungs and confer protection against bacterial pathogens. Whilst activation of these cells has been described during viral infections, here van Wilgenburg and colleagues show that in a murine model MAIT cells contribute to the protective host immune response to influenza virus infection.

Mucosal associated invariant T (MAIT) cells have been implicated in antibacterial responses. Here the authors show MAIT cells confer IFN-γ-mediated protection from lethal infection in a mouse model of Legionella infection, which can be enhanced by synthetic MR1 ligands.

Summary: Mucosal-associated invariant T (MAIT) cells are MR1-restricted innate-like T cells conserved across mammalian species, including mice and humans. By sequencing RNA from sorted MR1-5-OP-RU tetramer+ cells derived from either human blood or murine lungs, we define the basic transcriptome of an activated MAIT cell in both species and demonstrate how this profile changes during the resolution of infection and during reinfection. We observe strong similarities between MAIT cells in humans and mice. In both species, activation leads to strong expression of pro-inflammatory cytokines and chemokines as well as a strong tissue repair signature, recently described in murine commensal-specific H2-M3-restricted T cells. Transcriptomes of MAIT cells and H2-M3-specific CD8+ T cells displayed the most similarities to invariant natural killer T (iNKT) cells when activated, but to γδ T cells after the resolution of infection. These data define the requirements for and consequences of MAIT cell activation, revealing a tissue repair phenotype expressed upon MAIT cell activation in both species. : Mucosal-associated invariant T (MAIT) cells are implicated in antibacterial and antiviral immunity. Using RNA sequencing of human MAIT cells stimulated with their cognate ligand and murine MAIT cells stimulated by acute Legionella infection, Hinks et al. report that activation leads to expression of a strong tissue repair signature in both species. Keywords: mucosal-associated invariant T cell, T cell, transcriptome, MHC-related protein 1, activation, lung, human, mouse, riboflavin, tissue repair