The immune system has evolved to mount an effective defense against pathogens and to minimize deleterious immune-mediated inflammation caused by commensal microorganisms, immune responses against self and environmental antigens, and metabolic inflammatory disorders. Regulatory T (Treg) cell–mediated suppression serves as a vital mechanism of negative regulation of immune-mediated inflammation and features prominently in autoimmune and autoinflammatory disorders, allergy, acute and chronic infections, cancer, and metabolic inflammation. The discovery that Foxp3 is the transcription factor that specifies the Treg cell lineage facilitated recent progress in understanding the biology of regulatory T cells. In this review, we discuss cellular and molecular mechanisms in the differentiation and function of these cells.

Regulatory T Cells: Mechanisms of Differentiation and Function

The fate of developing T cells is specified by the interaction of their antigen receptors with self-peptide-MHC complexes that are displayed by thymic antigen-presenting cells (APCs). Various subsets of thymic APCs are strategically positioned in particular thymic microenvironments and they coordinate the selection of a functional and self-tolerant T cell repertoire. In this Review, we discuss the different strategies that these APCs use to sample and process self antigens and to thereby generate partly unique, 'idiosyncratic' peptide-MHC ligandomes. We discuss how the particular composition of the peptide-MHC ligandomes that are presented by specific APC subsets not only shapes the T cell repertoire in the thymus but may also indelibly imprint the behaviour of mature T cells in the periphery.

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Positive and negative selection of the T cell repertoire: what thymocytes see (and don't see).

https://www.cell.com/cell/pdf/S0092-8674(15)01025-9.pdf

Phosphoenolpyruvate Is a Metabolic Checkpoint of Anti-tumor T Cell Responses.

INTRODUCTION Primary liver tumors and liver metastasis currently represent the leading cause of cancer-related deaths. The liver intimately cross-talks with the gut and performs many essential functions related to digestion, metabolism of nutrients, and clearance of bacterial metabolites. Diseased livers are often associated with altered gut bacterial composition, or dysbiosis, and it has been suggested that gut bacterial products contribute to malignant transformation of hepatocytes. The liver is exposed to the gut microbiome through the portal vein and is an immunological organ that is heavily populated by immune cells. Emerging studies have shown that gut commensal bacteria are important regulators of antitumor immunity. Although it has been established that the gut microbiome influences the efficacy of cancer immunotherapy, the role of gut bacteria in antitumor surveillance in the liver is poorly understood. RATIONALE The liver is exposed to gut bacterial metabolites and products by way of blood from the intestine, which comprises 70% of the whole liver blood supply. Changes in the gut microbiome may affect immune cell function in the liver, and commensal bacteria can mediate the metabolism of primary into secondary bile acids, which recirculate back into the liver through the enterohepatic circulation. Given that bile acids are known to be involved in liver cancer development, we focused on the role of bile acids in immunosurveillance of tumors growing in the liver. We altered gut bacteria and examined changes of hepatic immune cells and antitumor immunity directed against liver tumors. Uncovering how the gut microbiome uses bile acids to shape immunity to liver cancer may have future therapeutic applications. RESULTS Using one primary liver model and three liver metastasis models, we found that altering commensal gut bacteria induced a liver-selective antitumor effect. A selective increase of hepatic CXCR6 + natural killer T (NKT) cells was observed, independent of mouse strain, gender, or presence of liver tumors. The accumulated hepatic NKT cells showed an activated phenotype and produced more interferon-γ upon antigen stimulation. In vivo studies using both antibody-mediated cell depletion and NKT-deficient mice confirmed that NKT cells mediated the inhibition of tumor growth in the liver. Further investigation showed that NKT cell accumulation was regulated by the expression of CXCL16, the solo ligand for CXCR6, on liver sinusoidal endothelial cells, which form the lining of liver capillaries and the first barrier for the blood coming from the gut entering the liver. Primary bile acids increased CXCL16 expression, whereas secondary bile acids showed the opposite effect. Removing gram-positive bacteria by antibiotic treatment with vancomycin, which contains the bacteria mediating primary-to-secondary bile acid conversion, was sufficient to induce hepatic NKT cell accumulation and decrease liver tumor growth. Feeding secondary bile acids or colonization of bile acid–metabolizing bacteria, reversed both NKT cell accumulation and inhibition of liver tumor growth in mice with altered gut commensal bacteria. In nontumor liver tissue from human patients with primary liver cancer, primary bile acid chenodeoxycholic acid (CDCA) levels correlated with CXCL16 expression, whereas an inverse correlation was observed with secondary bile acid glycolithocholate (GLCA), suggesting that the finding may apply to humans. CONCLUSION We describe a mechanism by which the gut microbiome uses bile acids as messengers to control a chemokine-dependent accumulation of hepatic NKT cells and antitumor immunity in the liver, against both primary and metastatic liver tumors. These findings not only have possible implications for future cancer therapeutic studies but also provide a link between the gut microbiome, its metabolites, and immune responses in the liver.

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Gut microbiome–mediated bile acid metabolism regulates liver cancer via NKT cells

A balance between pro- and anti-inflammatory mechanisms at mucosal interfaces, which are sites of constitutive exposure to microbes and non-microbial foreign substances, allows for efficient protection against pathogens yet prevents adverse inflammatory responses associated with allergy, asthma and intestinal inflammation 1 . Regulatory T (Treg) cells prevent systemic and tissuespecific autoimmunity and inflammatory lesions at mucosal interfaces. These cells are generated in the thymus (tTreg cells) and in the periphery (induced (i)Treg cells), and their dual origin implies a division of labour between tTreg and iTreg cells in immune homeostasis. Here we show that a highly selective blockage in differentiation of iTreg cells in mice did not lead to unprovoked multiorgan autoimmunity, exacerbation of induced tissue-specific autoimmune pathology, or increased pro-inflammatory responses of T helper 1 (TH1) and TH17 cells. However, mice deficient in iTreg cells spontaneously developed pronounced TH2-type pathologies at mucosal sites—in the gastrointestinal tract and lungs—with hallmarks of allergic inflammation and asthma. Furthermore, iTreg-cell deficiency altered gut microbial communities. These results suggest that whereas Treg cells generated in the thymus appear sufficient for control of systemic and tissue-specific autoimmunity, extrathymic differentiation of Treg cells affects commensal microbiota composition and serves a distinct, essential function in restraint of allergic-type inflammation at mucosal interfaces. Exquisitely balanced control mechanisms operating at mucosal sites are able to accommodate potent immune defences and the need to prevent tissue damage resulting from inflammatory responses caused by commensal microorganisms, food and environmental antigens, allergens, and noxious substances 1 .

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Extrathymically generated regulatory T cells control mucosal T H 2 inflammation

The ability of antigen receptors to engage self-ligands with varying affinity is crucial for lymphocyte development. To further explore this concept, we generated transgenic mice expressing GFP from the immediate early gene Nr4a1 (Nur77) locus. GFP was up-regulated in lymphocytes by antigen receptor stimulation but not by inflammatory stimuli. In T cells, GFP was induced during positive selection, required major histocompatibility complex for maintenance, and directly correlated with the strength of T cell receptor (TCR) stimulus. Thus, our results define a novel tool for studying antigen receptor activation in vivo. Using this model, we show that regulatory T cells (Treg cells) and invariant NKT cells (iNKT cells) perceived stronger TCR signals than conventional T cells during development. However, although Treg cells continued to perceive strong TCR signals in the periphery, iNKT cells did not. Finally, we show that Treg cell progenitors compete for recognition of rare stimulatory TCR self-ligands.

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T cell receptor signal strength in Treg and iNKT cell development demonstrated by a novel fluorescent reporter mouse

Mature invariant natural killer T cells (iNKT cells) are an early innate source of cytokines. Hogquist and colleagues classify three lineages of iNKT cells by their distinct transcription factors and cytokine profiles.

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Steady-state production of IL-4 modulates immunity in mouse strains and is determined by lineage diversity of iNKT cells

CD1d-reactive invariant NKT cells (iNKT) play a vital role in determining the characteristics of immune responses to infectious agents. Previous reports suggest that iNKT cell activation during infection can be: 1) solely driven by cytokines from innate immune cells, 2) require microbial Ag, or 3) require self-Ag. In this study, we examined the role of Ag receptor stimulation in iNKT cells during several bacterial and viral infections. To test for Ag receptor signaling, Nur77gfp BAC transgenic mice, which upregulate GFP in response to Ag receptor but not inflammatory signals, were analyzed. iNKT cells in the reporter mice infected with mouse CMV produced IFN-γ but did not upregulate GFP, consistent with their reported CD1d-independent activation. However, two bacteria known to produce lipid Ags for iNKT cells induced GFP expression and cytokine production. In contrast, although Salmonella typhimurium was proposed to induce the presentation of a self-lipid, iNKT cells produced IFN-γ but did not upregulate GFP postinfection in vivo. Even in CD1d-deficient hosts, iNKT cells were still able to produce IFN-γ after S. typhimurium infection. Furthermore, although it has been proposed that endogenous lipid presentation is a result of TLR stimulation of APCs, injection of different TLR agonists led to iNKT cell IFN-γ but not increased GFP expression. These data indicate that robust iNKT cell responses to bacteria, as well as viruses, can be obtained in the absence of antigenic stimulation.

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Antigen-dependent versus -independent activation of invariant NKT cells during infection.

Corrigendum: Steady-state production of IL-4 modulates immunity in mouse strains and is determined by lineage diversity of i NKT cells

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Erratum: Steady-state production of IL-4 modulates immunity in mouse strains and is determined by lineage diversity of iNKT cells( Nat Immunol. (2013) 14 (1146-1154))

Keli L. Holzapfel

Papers

T cell receptor signal strength in Treg and iNKT cell development demonstrated by a novel fluorescent reporter mouse

Steady-state production of IL-4 modulates immunity in mouse strains and is determined by lineage diversity of iNKT cells

Antigen-dependent versus -independent activation of invariant NKT cells during infection.

Erratum: Steady-state production of IL-4 modulates immunity in mouse strains and is determined by lineage diversity of iNKT cells( Nat Immunol. (2013) 14 (1146-1154))