Mucosal-associated invariant T (MAIT) cells IntroductionMainstream T cells display a very diverse repertoire of antigen receptors, which enable these cells to respond to a wide variety of antigens presented by polymorphic major histocompatibility complex (MHC) molecules. During development, interactions between the T cells and selecting MHC class II and I molecules on thymic epithelial cells lead to the specific features of CD4 and CD8 T-cell lineage. However, 2 "innate-like" T lymphocytes, the natural killer T (NKT) and mucosal-associated invariant T (MAIT) cells, follow different ontogenic pathways with selection by nonpolymorphic MHC class Ib molecules. 1 Human MAIT cells express the semi-invariant T-cell receptor (TCR; iV␣7.2-J␣33) and are selected by the MHC class Ib molecule, MR1 on hematopoietic cells, which confer peculiar features. 2,3 Indeed, MAIT cells are enriched in the intestinal lamina propria and are numerous in the peripheral blood of healthy subjects. They are present in cord blood in small numbers with a naive phenotype, whereas in adults they represent approximately 10% of mature CD8 or CD4 Ϫ CD8 Ϫ (DN) T cells and display an effector-memory phenotype. We recently showed that MAIT cells specifically react against antigen-presenting cells (APCs), fed with a wide variety of bacteria and yeasts but not viruses, and display protective activity in experimental bacterial infection models. 4,5 In humans, they are defined as CD161 hi IL-18R␣ ϩ V␣7.2 ϩ ␥␦ Ϫ CD3 ϩ lymphocytes. 3,4 Either CD161 or IL-18R␣ expression at the cell surface, together with the V␣7.2 segment, allows for the unequivocal identification of MAIT cells in both peripheral blood and tissues. 3,4 CD161 (KLRB1, NKRP1A) is a C-type lectin family member, part of the NK complex. 6 Ligands for CD161 have been described only recently, and blocking or cross-linking CD161 may modulate T-cell functions. 7-9 CD161 is also expressed by the majority of NK cells and diverse subsets of T lymphocytes that include TCR-␥␦ T cells and most NKT cells. CD161 expression is found on a significant proportion of tissue-infiltrating T cells, such as the intestine and liver. [10][11][12] The frequency of these cells varies in certain pathologic conditions, such as cancer and viral or autoimmune diseases. [12][13][14][15] Recently, it has been demonstrated that a subset of naive cord blood CD4 T cells express CD161 and are precursors of peripheral, mature, Th17 T cells. 16 CD161 expression seems to correlate with interleukin-17 (IL-17) secretion by CD4 T cells. 17 Among TCR-␣␤ T cells, most of the CD161 ϩ T cells belong to the CD4 subset. However, CD8 T cells expressing CD161 can also be found, but their nature has just begun to be explored. CD161 ϩ CD8 T cells can be split into 2 different populations expressing intermediate or high levels of CD161. Two recent reports independently raised new findings on this latter subset. The first shows that CD161 hi CD8 T cells represent self-renewing memory cells, which survive chemotherapy. 18 These cells are absent from...

Mucosal-associated invariant T lymphocytes (MAIT lymphocytes) are characterized by two evolutionarily conserved features: an invariant T cell antigen receptor (TCR) alpha-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.