Mucosal-associated invariant T (MAIT) cells are numerous in humans and are implicated in various pathological settings. As such, understanding their biology is essential, and will lead to innovative therapeutic strategies. MAIT cells are selected on double positive thymocytes leading to memory phenotype and immediate effector functions. This original selection process targets MAIT cells to mucosal tissues in which they establish long term residency. MAIT cells are also found in secondary lymphoid organs, possibly providing B and T cell help during immune responses. Recent data suggest that MAIT cells migrate between mucosal tissues and their draining lymph nodes (LNs) at steady state and following inflammation. However, the role and dynamics of MAIT cells in LNs remain poorly understood. A comprehensive phenotypic and functional characterization of MAIT cells in various LNs and the corresponding organs is needed to decipher the specific role of LN MAIT cells within immune responses. Assessing the mechanisms involved in cell pool establishment and exchange will also help characterizing MAIT cell biology. We hypothesize that MAIT cells circulate back and forth between tissues and draining LNs, hence being poised in LNs to tissue-specific immune responses. Following tissue inflammation, this migration may ensure both adaptation of the LN response and mobilization of MAIT cells for rapid effector function in the tissue. Here we will test these hypotheses by 1) characterizing MAIT cells in mesenteric, mediastinal and inguinal/brachial LNs and in the drained tissues; 2) analyzing MAIT cell dynamics between different LNs and tissues at steady state, including pool establishment and mechanisms of migration and retention in LNs; 3) assessing MAIT cell effector functions following various triggers in different LNs and trafficking during inflammation. Building on long term expertise, specifically developed animal models and tools, and state-of-the-art techniques, this project will characterize the dynamics and functions of LN MAIT cells, to provide a conceptual framework that could be broaden to other unconventional T cells and resident memory cells.

Mucosal associated invariant T cells (MAITs) represent an abundant subset in humans with unique specificity for microbial metabolites. MAIT conservation along evolution indicates important, non-redundant functions. Despite changes in MAIT frequency and phenotype in several infectious and non-infectious diseases, their functions are still unclear. By contrast with mainstream CD4+ or CD8+ T cells, MAITs differentiate into effector cells during thymic development. How the differentiation program imparted in the thymus is modified by tissue and environmental cues to determine MAIT functions in tissues is not known. Our preliminary data support a new function for MAITs in boosting tissue repair in skin wound healing. In this project, we will assess how the differentiation program generated in the thymus is modified by tissue and environmental cues to determine the positioning and effector functions of MAITs in the skin at steady state. This will provide the grounds to determine the way MAITs are protective in skin wound healing using new genetic tools, gnotobiotic experiments and new chemical compounds. The three aims of this project are: 1. MAIT development in the thymus versus tissue and environmental cues determining the positioning and properties of MAIT cells in the skin at steady state: What are the transcription factors or genes involved in MAIT1 and MAIT17 commitment and trafficking/retention in the skin? What are the integrins/chemokines or other mediators involved in MAIT positioning in the skin? What are the roles of MR1 and MAIT ligand and bacteria in seeding and maintaining MAIT subsets in the skin? This will provide the grounds to genetically manipulate the functions of MAITs in the following aims to decipher the mechanisms involved in the protective effect of MAITs during skin wound healing. 2. To study the effector functions of MAITs leading to protection during skin wound healing: Is the protective effect mediated directly by MAITs or through the recruitment of other cell types? What are the mediators secreted or expressed by MAITs? Which MAIT subsets are involved? When and where the tissue repair functional effector modules are expressed by MAITs during the different phases of skin wound healing? 3. To study the way MAIT protective functions are triggered during skin wound healing: Are MAITs activated through cognate (TCR/Antigen) or non-cognate (lymphokine) cues? Are MAITs recruited or do they proliferate in situ during tissue repair? Is activating or inhibiting MAITs beneficial or deleterious during skin wound healing? We will also test the therapeutic potential of activating MAITs in humans.

Macrophages play key roles in tissue regeneration and inflammation at steady-state, two processes that are essential to maintain tissue homeostasis and, if dis-regulated, can lead to pathology. We have recently discovered a peculiar population of macrophages in the distal colon that is needed to locally maintain the survival of epithelial cells in homeostasis. These macrophages are localized in the stroma, but insert “balloon-like protrusions (BLPs)” through the basement membrane in between epithelial cells. BLPs sample the fluids that are absorbed through the epithelium and instruct epithelial cells to stop absorption if fluids are overloaded with fungal toxins. We here propose a project aiming to identify the key players involved in the tripartite interaction between macrophages, epithelial cells and fungi (Aim 1), unraveling the underlying molecular and cellular mechanisms (Aim 2) and evaluating whether and how failure in this “ménage à trois” can lead to colon pathology (Aim 3).

The process of aging represents the gradual deterioration of function with time, which affects almost all tissues and organs. In humans, it is the most prevailing risk factor for the morbidity and mortality attributable to numerous acute and chronic diseases. However, the cellular and molecular bases for these phenomena remain largely unknown. Our preliminary work has uncovered a striking build-up of tissue macrophages that carry numerous markers of senescence in multiple organs of aged mice, implicating loss or alteration of macrophage function as a potential driver of aging phenotype including inflammaging. In this project, we will uncover the mechanisms underlying age- and inflammation-induced senescence/senescence-like state of tissue macrophages, we will determine how this shapes tissue inflammation and immune response during aging, and we will explore how modulation of macrophage senescence could be used to impact the aging process.

Adoptive cellular immunotherapy has emerged as a fascinating approach to prevent and/or limit graft-versus-host disease (GVHD), a major source of morbidity and mortality following allogeneic transplantation. To date however, the most appropriate regulatory population to use remains elusive. Human mucosal associated invariant T cells (MAITs) are unconventional T cells very abundant in barrier tissues and blood, which express a semi-invariant TCR recognizing microbial-derived riboflavin derivatives presented by the highly conserved MR1 molecule. MAITs exhibit potent TCR-dependent and -independent effector functions in response to homeostatic perturbations. Recent studies have also revealed the tissue repair and regulatory capacities of MAITs, but the mechanisms involved in these new functions are hardly explored. We recently demonstrated that MAITs lack alloreactive potential and do not contribute to GVHD tissue lesions, prompting us to exploit their regulatory potential for adoptive cell therapy in allogeneic settings. Our program has three specific objectives: 1/ to characterize the MAIT regulatory mechanisms in vitro in a mixed lymphocyte reaction (MLR) model using transcriptomic, deep phenotyping and live microscopy, 2/ to confirm the mechanisms sustaining the protective role of human MAITs in vivo in a xenogenic GVHD model, and 3/ to validate the identified molecule(s) using gene-edited MAITs or specific antagonists. A main challenge of our project will be to determine if MAITs perform dual effector and regulatory functions depending on the inflammatory context. Our results should allow considering human MAITs as universal, off-the-shelf therapeutic tools to control GVHD or other inflammatory mucosal diseases.