Human MAIT cells show metabolic quiescence with rapid glucose-dependent upregulation of granzyme B upon stimulation.

Mucosal-associated invariant T (MAIT) cells are a well-characterized innate-like T cell population abundant in the human liver, peripheral tissues and blood. MAIT cells serve in the first line of defense against infections, through engagement of their T cell receptor, which recognizes microbial metabolites presented on MR1, and through cytokine-mediated triggering. Typically, they show a quiescent memory phenotype but can undergo rapid upregulation of effector functions including cytolysis upon stimulation. T cells profoundly change their cellular metabolism during their maturation and activation. We sought to determine how MAIT cell metabolism may facilitate both the long-term memory phase in tissue and the transition to rapid effector function. Here, we show, by flow cytometric metabolism assays and extracellular flux analysis that, despite an effector-memory profile, human MAIT cells are metabolically quiescent in a resting state comparable to naïve and central memory T cells. Upon stimulation, they rapidly increase uptake of glucose and show a concomitant upregulation of the effector molecules notably granzyme B, which is impaired by inhibition of glycolysis with 2-deoxyglucose. These findings suggest that MAIT cells share some metabolic characteristics of both resting and effector T cell subsets, with a rapid transition upon triggering. Metabolic programming of this cell type may be of interest in understanding and modulating their function in infectious diseases and cancer.

Analysis of CD161 expression on human CD8+ T cells defines a distinct functional subset with tissue-homing properties.

CD8(+) T lymphocytes play a key role in host defense, in particular against important persistent viruses, although the critical functional properties of such cells in tissue are not fully defined. We have previously observed that CD8(+) T cells specific for tissue-localized viruses such as hepatitis C virus express high levels of the C-type lectin CD161. To explore the significance of this, we examined CD8(+)CD161(+) T cells in healthy donors and those with hepatitis C virus and defined a population of CD8(+) T cells with distinct homing and functional properties. These cells express high levels of CD161 and a pattern of molecules consistent with type 17 differentiation, including cytokines (e.g., IL-17, IL-22), transcription factors (e.g., retinoic acid-related orphan receptor gamma-t, P = 6 x 10(-9); RUNX2, P = 0.004), cytokine receptors (e.g., IL-23R, P = 2 x 10(-7); IL-18 receptor, P = 4 x 10(-6)), and chemokine receptors (e.g., CCR6, P = 3 x 10(-8); CXCR6, P = 3 x 10(-7); CCR2, P = 4 x 10(-7)). CD161(+)CD8(+) T cells were markedly enriched in tissue samples and coexpressed IL-17 with high levels of IFN-gamma and/or IL-22. The levels of polyfunctional cells in tissue was most marked in those with mild disease (P = 0.0006). These data define a T cell lineage that is present already in cord blood and represents as many as one in six circulating CD8(+) T cells in normal humans and a substantial fraction of tissue-infiltrating CD8(+) T cells in chronic inflammation. Such cells play a role in the pathogenesis of chronic hepatitis and arthritis and potentially in other infectious and inflammatory diseases of man.

Single-cell transcriptome analysis of CD8 + T-cell memory inflation.

Background: Persistent viruses such as murine cytomegalovirus (MCMV) and adenovirus-based vaccines induce strong, sustained CD8 + T-cell responses, described as memory "inflation". These retain functionality, home to peripheral organs and are associated with a distinct transcriptional program. Methods: To further define the nature of the transcriptional mechanisms underpinning memory inflation at different sites we used single-cell RNA sequencing of tetramer-sorted cells from MCMV-infected mice, analyzing transcriptional networks in virus-specific populations in the spleen and gut intra-epithelial lymphocytes (IEL). Results: We provide a transcriptional map of T-cell memory and define a module of gene expression, which distinguishes memory inflation in spleen from resident memory T-cells (T RM) in the gut. Conclusions: These data indicate that CD8 + T-cell memory in the gut epithelium induced by persistent viruses and vaccines has a distinct quality from both conventional memory and "inflationary" memory which may be relevant to protection against mucosal infections.

Expression of lectin-like transcript-1 in human tissues.

Background: Receptor-ligand pairs of C-type lectin-like proteins have been shown to play an important role in cross talk between lymphocytes, as well as in immune responses within concrete tissues and structures, such as the skin or the germinal centres. The CD161-Lectin-like Transcript 1 (LLT1) pair has gained particular attention in recent years, yet a detailed analysis of LLT1 distribution in human tissue is lacking. One reason for this is the limited availability and poor characterisation of anti-LLT1 antibodies. Methods: We assessed the staining capabilities of a novel anti-LLT1 antibody clone (2H7), both by immunohistochemistry and flow cytometry, showing its efficiency at LLT1 recognition in both settings. We then analysed LLT1 expression in a wide variety of human tissues. Results: We found LLT1 expression in circulating B cells and monocytes, but not in lung and liver-resident macrophages. We found strikingly high LLT1 expression in immune-privileged sites, such as the brain, placenta and testes, and confirmed the ability of LLT1 to inhibit NK cell function. Conclusions: Overall, this study contributes to the development of efficient tools for the study of LLT1. Moreover, its expression in different healthy human tissues and, particularly, in immune-privileged sites, establishes LLT1 as a good candidate as a regulator of immune responses.

Mucosal-associated invariant T-cells: new players in anti-bacterial immunity.

Mucosal-associated invariant T (MAIT) cells are an innate-like T-cell population involved in anti-bacterial immunity. In human beings, MAIT cells are abundant, comprising ~10% of the CD8(+) T-cell compartment in blood. They are enriched at mucosal sites and are particularly prevalent within the liver. MAIT cells are defined by the expression of a semi-invariant T-cell receptor (Vα7.2-Jα33/12/20) and are restricted by the non-polymorphic, highly evolutionarily conserved MHC class Ib molecule, MHC-related protein (MR)1. MR1 has recently been shown to present an unstable pyrimidine intermediate derived from a biosynthetic precursor of riboflavin; riboflavin biosynthesis occurs in many bacteria but not in human beings. Consistent with this, MAIT cells are responsive to riboflavin-metabolizing bacteria, including Salmonella. In mouse models, MAIT cells have been shown to play a non-redundant role in anti-bacterial immunity, including against Escherichia coli, Klebsiella pneumoniae, and Mycobacterium bovis BCG. In human beings, MAIT cells are decreased in frequency in the blood of patients with tuberculosis or pneumonia, and their frequency has been inversely correlated with the risk of subsequent systemic bacterial infection in patients in intensive care. Intriguingly, MAIT cells are also depleted from the blood early in HIV infection and fail to recover with anti-retroviral therapy, which may contribute to the susceptibility of patients infected with HIV to certain bacterial infections, including non-typhoidal Salmonella. In this review, we will discuss what is currently known about MAIT cells, the role that Salmonella has played in elucidating MAIT cell restriction and function, and the role MAIT cells might play in the control of Salmonella infection.

CD161(+)CD4(+) T cells are enriched in the liver during chronic hepatitis and associated with co-secretion of IL-22 and IFN-γ.

Hepatitis C virus infection is a major cause of chronic liver disease. CD4(+) T cells play a key role in disease outcome. However, the critical functions and associated phenotypes of intrahepatic CD4(+) T cells are not well defined. We have previously shown that CD8(+) T cells expressing the C type lectin CD161 are highly enriched in the human liver, especially during chronic hepatitis. These cells are associated with a type 17 differentiation pattern and express cytokines including IL-17A, IL-22, and IFN-γ. We therefore analyzed expression of CD161 on CD4(+) T cells in blood and liver and addressed the relevant phenotype and functional capacity of these populations. We observed marked enrichment of CD161(+)CD4(+) T cells in the liver during chronic hepatitis such that they are the dominant subtype (mean 55% of CD4(+) T cells). IL-22 and IL-17 secreting CD4(+) T cells were readily found in the livers of HCV(+) and NASH donors, although not enriched compared to blood. There was, however, specific enrichment of a novel subset of IL-22/IFN-γ dual secretors (p = 0.02) compared to blood, a result reconfirmed with direct ex vivo analyses. These data indicate the dominance of CD161(+) expressing lymphocyte populations within the hepatic infiltrate, associated with a distinct cytokine profile. Given their documented roles as antiviral and hepatoprotective cytokines respectively, the impact of co-secretion of IFN-γ and IL-22 in the liver may be particularly significant.