Recent Advances in MAIT Cells Biology

Mucosal-Associated Invariant T (MAIT) cells are unique in their ability to recognize microbial-derived metabolites presented by the MHC class I-like molecule MR1 (MHC-Related 1). These metabolites are highly conserved in a large and diverse group of bacteria and fungi, and as such may be considered as microbial-associated molecular pattern-like molecules. Upon activation, MAIT cells rapidly mediate a protective response through the release of a large panel of effector cytokines and their ability to kill infected target cells; they also interact with antigen-presenting cells to increase their maturation. Ideally positioned at mucosal sites and in the liver, MAIT cells represent a bridge between the immediate inflammatory response and delayed antigen-specific immunity. Indeed, an increasing body of literature describes their involvement as a defense mechanism in bacterial, fungal, as well as viral infections, and fast-growing interest in MAIT-based therapeutic interventions is emerging. However, because MAIT cells are abundant in humans but scarce in mice, a majority of published work on MAIT cells is only descriptive and does not answer many important questions pertaining to their biology. For example, MAIT cells are positively selected in an MR1-dependent manner in the thymus, but the identity of the elusive antigen(s) driving this process remains unknown. Similarly, it was shown that MAIT cells exit the thymus as naïve cells, and undergo a progressive maturation into memory cells, probably accompanied by some level of clonal expansion, through a process involving the microbiota. However, the interplay between the microbiota, MR1-expressing cells, and possible other factors, in the process driving MAIT cells maturation has not been explored yet. More generally, our knowledge of MAIT cells ontogeny, homeostasis, and functions is still in its infancy. It is crucial to better understand the biology of these cells if we want to manipulate them for therapeutic purposes.

In this Research Topic, we encourage authors to submit Original research, Reviews and Opinion articles highlighting all aspects of MAIT cells biology, both in human and animal models focusing on but not limited to the following subtopics:

- MAIT cell ontogeny
- MR1 presentation and antigens
- MAIT cells and the microbiota
- MAIT cell migration and tissue residency
- MAIT cell homeostasis
- Plasticity and regulation of MAIT cell functions

Mechanistic analyses of MAIT cells in animal models of disease are welcome, but purely descriptive studies will be out of scope. Specifically, MAIT cell activation in patients with cancer or infectious diseases is largely covered in recent Research Topics and will not be considered here.