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.

Dendritic cell-derived exosomes (Dex) are small extracellular vesicles secreted by viable dendritic cells. In the two phase-I trials that we conducted using the first generation of Dex (IFN-γ-free) in end-stage cancer, we reported that Dex exerted natural killer (NK) cell effector functions in patients. A second generation of Dex (IFN-γ-Dex) was manufactured with the aim of boosting NK and T cell immune responses. We carried out a phase II clinical trial testing the clinical benefit of IFN-γ-Dex loaded with MHC class I- and class II-restricted cancer antigens as maintenance immunotherapy after induction chemotherapy in patients bearing inoperable non-small cell lung cancer (NSCLC) without tumor progression. The primary endpoint was to observe at least 50% of patients with progression-free survival (PFS) at 4 mo after chemotherapy cessation. Twenty-two patients received IFN-γ-Dex. One patient exhibited a grade three hepatotoxicity. The median time to progression was 2.2 mo and median overall survival (OS) was 15 mo. Seven patients (32%) experienced stabilization of >4 mo. The primary endpoint was not reached. An increase in NKp30-dependent NK cell functions were evidenced in a fraction of these NSCLC patients presenting with defective NKp30 expression. Importantly, MHC class II expression levels of the final IFN-γ-Dex product correlated with expression levels of the NKp30 ligand BAG6 on Dex, and with NKp30-dependent NK functions, the latter being associated with longer progression-free survival. This phase II trial confirmed the capacity of Dex to boost the NK cell arm of antitumor immunity in patients with advanced NSCLC.

The quantification and characterization of circulating immune cells provide key indicators of human health and disease. To identify the relative effects of environmental and genetic factors on variation in the parameters of innate and adaptive immune cells in homeostatic conditions, we combined standardized flow cytometry of blood leukocytes and genome-wide DNA genotyping of 1,000 healthy, unrelated people of Western European ancestry. We found that smoking, together with age, sex and latent infection with cytomegalovirus, were the main non-genetic factors that affected variation in parameters of human immune cells. Genome-wide association studies of 166 immunophenotypes identified 15 loci that showed enrichment for disease-associated variants. Finally, we demonstrated that the parameters of innate cells were more strongly controlled by genetic variation than were those of adaptive cells, which were driven by mainly environmental exposure. Our data establish a resource that will generate new hypotheses in immunology and highlight the role of innate immunity in susceptibility to common autoimmune diseases.