Enzymatic Activity of HPGD in Treg Cells Suppresses Tconv Cells to Maintain Adipose Tissue Homeostasis and Prevent Metabolic Dysfunction.

Regulatory T cells (Treg cells) are important for preventing autoimmunity and maintaining tissue homeostasis, but whether Treg cells can adopt tissue- or immune-context-specific suppressive mechanisms is unclear. Here, we found that the enzyme hydroxyprostaglandin dehydrogenase (HPGD), which catabolizes prostaglandin E2 (PGE2) into the metabolite 15-keto PGE2, was highly expressed in Treg cells, particularly those in visceral adipose tissue (VAT). Nuclear receptor peroxisome proliferator-activated receptor-γ (PPARγ)-induced HPGD expression in VAT Treg cells, and consequential Treg-cell-mediated generation of 15-keto PGE2 suppressed conventional T cell activation and proliferation. Conditional deletion of Hpgd in mouse Treg cells resulted in the accumulation of functionally impaired Treg cells specifically in VAT, causing local inflammation and systemic insulin resistance. Consistent with this mechanism, humans with type 2 diabetes showed decreased HPGD expression in Treg cells. These data indicate that HPGD-mediated suppression is a tissue- and context-dependent suppressive mechanism used by Treg cells to maintain adipose tissue homeostasis.

Dietary AhR Ligands Regulate AhRR Expression in Intestinal Immune Cells and Intestinal Microbiota Composition.

A diet rich in vegetables and fruit is generally considered healthy because of a high content of phytochemicals, vitamins, and fiber. The phytochemical indole-3-carbinol (I3C), a derivative of glucobrassicin, is sold as a dietary supplement promising diverse health benefits. I3C metabolites act as ligands of the aryl hydrocarbon receptor (AhR), an important sensor for environmental polyaromatic chemicals. Here, we investigated how dietary AhR ligand supplementation influences AhR target gene expression and intestinal microbiota composition. For this, we used AhR repressor (AhRR)-reporter mice as a tool to study AhR activation in the intestine following dietary I3C-supplementation in comparison with AhR ligand-deprived diets, including a high fat diet. AhRR expression in intestinal immune cells was mainly driven by dietary AhR ligands and was independent of microbial metabolites. A lack of dietary AhR ligands caused enhanced susceptibility to dextran sodium sulfate (DSS)-induced colitis and correlated with the expansion of Enterobacteriaceae, whereas Clostridiales, Muribaculaceae, and Rikenellaceae were strongly reduced. I3C supplementation largely reverted this effect. Comparison of I3C-induced changes in microbiota composition using wild-type (WT), AhRR-deficient, and AhR-deficient mice revealed both AhR-dependent and -independent alterations in the microbiome. Overall, our study demonstrates that dietary AhR ligand supplementation has a profound influence on Ahrr expression in intestinal immune cells as well as microbiota composition.

RNA Aptamers Recognizing Murine CCL17 Inhibit T Cell Chemotaxis and Reduce Contact Hypersensitivity In Vivo.

The chemokine CCL17, mainly produced by dendritic cells (DCs) in the immune system, is involved in the pathogenesis of various inflammatory diseases. As a ligand of CCR4, CCL17 induces chemotaxis and facilitates T cell-DC interactions. We report the identification of two novel RNA aptamers, which were validated in vitro and in vivo for their capability to neutralize CCL17. Both aptamers efficiently inhibited the directed migration of the CCR4+ lymphoma line BW5147.3 toward CCL17 in a dose-dependent manner. To study the effect of these aptamers in vivo, we used a murine model of contact hypersensitivity. Systemic application of the aptamers significantly prevented ear swelling and T cell infiltration into the ears of sensitized mice after challenge with the contact sensitizer. The results of this proof-of-principle study establish aptamers as potent inhibitors of CCL17-mediated chemotaxis. Potentially, CCL17-specific aptamers may be used therapeutically in humans to treat or prevent allergic and inflammatory diseases.

CCL17 exerts a neuroimmune modulatory function and is expressed in hippocampal neurons.

Chemokines are important signaling molecules in the immune and nervous system. Using a fluorescence reporter mouse model, we demonstrate that the chemokine CCL17, a ligand of the chemokine receptor CCR4, is produced in the murine brain, particularly in a subset of hippocampal CA1 neurons. We found that basal expression of Ccl17 in hippocampal neurons was strongly enhanced by peripheral challenge with lipopolysaccharide (LPS). LPS-mediated induction of Ccl17 in the hippocampus was dependent on local tumor necrosis factor (TNF) signaling, whereas upregulation of Ccl22 required granulocyte-macrophage colony-stimulating factor (GM-CSF). CCL17 deficiency resulted in a diminished microglia density under homeostatic and inflammatory conditions. Further, microglia from naïve Ccl17-deficient mice possessed a reduced cellular volume and a more polarized process tree as assessed by computer-assisted imaging analysis. Regarding the overall branching, cell surface area, and total tree length, the morphology of microglia from naïve Ccl17-deficient mice resembled that of microglia from wild-type mice after LPS stimulation. In line, electrophysiological recordings indicated that CCL17 downmodulates basal synaptic transmission at CA3-CA1 Schaffer collaterals in acute slices from naïve but not LPS-treated animals. Taken together, our data identify CCL17 as a homeostatic and inducible neuromodulatory chemokine affecting the presence and morphology of microglia and synaptic transmission in the hippocampus.

Liver sinusoidal endothelial cell cross-priming is supported by CD4 T cell-derived IL-2.

BACKGROUND & AIMS: Liver sinusoidal endothelial cells (LSECs) are prominent liver-resident antigen (cross-)presenting cells. LSEC cross-priming of naïve CD8 T cells does not require CD4 T cell help in contrast to priming by dendritic cells (DC) but leads to the formation of memory T cells that is preceded by transient Granzyme B (GzmB) expression. Here we provide evidence for a so far unrecognized CD4 T helper cell function in LSEC-induced CD8 T cell activation. METHODS: Naïve CD8 T cells and differentiated T helper 1 (Th1) cells were stimulated by antigen-presenting LSEC, and GzmB expression in CD8 T cells was determined by flow cytometry. To identify molecular pathways mediating this GzmB expression, mechanistic proof-of-concept experiments were conducted using stimulatory anti-CD3 antibody together with Hyper-IL-6. RESULTS: We demonstrate that LSECs simultaneously function in antigen co-presentation to CD8 and CD4 T cells. Such co-presentation revealed a function of Th1 cells to increase GzmB expression in CD8 T cells after LSEC but not DC cross-priming. IL-2 released from Th1 cells was required but not sufficient for rapid GzmB induction in CD8 T cells. T cell receptor together with IL-6 trans-signaling was necessary for IL-2 to mediate rapid GzmB induction. CONCLUSIONS: Our findings indicate that LSECs can serve as a platform to facilitate CD4-CD8 T cell crosstalk enhancing the immune function of LSECs to cross-prime CD8 T cells. IL-6 trans-signaling-mediated responsiveness for IL-2 inducing sustained GzmB expression in CD8 T cells reveals unique mechanisms of CD4 T cell help and CD8 T cell differentiation through liver-resident antigen-presenting cells. LAY SUMMARY: Our findings demonstrate that LSEC co-present antigen to CD8 and CD4 T cells and thereby enable CD4 T cell help for LSEC-priming of CD8 T cells. This CD4 T cell help selectively enhances the rapid upregulation of GzmB and effector function of LSEC-primed CD8 T cells thereby enhancing functional differentiation towards CD8 effector T cells.

Tumor rejection in Cblb-/- mice depends on IL-9 and Th9 cells.

BACKGROUND: Casitas B lymphoma-b (Cbl-b) is a central negative regulator of cytotoxic T and natural killer (NK) cells and functions as an intracellular checkpoint in cancer. In particular, Th9 cells support mast cell activation, promote dendritic cell recruitment, enhance the cytolytic function of cytotoxic T lymphocytes and NK cells, and directly kill tumor cells, thereby contributing to tumor immunity. However, the role of Cbl-b in the differentiation and antitumor function of Th9 cells is not sufficiently resolved. METHODS: Using Cblb-/- mice, we investigated the effect of knocking out Cblb on the differentiation process and function of different T helper cell subsets, focusing on regulatory T cell (Treg) and Th9 cells. We applied single-cell RNA (scRNA) sequencing of in vitro differentiated Th9 cells to understand how Cbl-b shapes the transcriptome and regulates the differentiation and function of Th9 cells. We transferred tumor-model antigen-specific Cblb-/- Th9 cells into melanoma-bearing mice and assessed tumor control in vivo. In addition, we blocked interleukin (IL)-9 in melanoma cell-exposed Cblb-/- mice to investigate the role of IL-9 in tumor immunity. RESULTS: Here, we provide experimental evidence that Cbl-b acts as a rheostat favoring Tregs at the expense of Th9 cell differentiation. Cblb-/- Th9 cells exert superior antitumor activity leading to improved melanoma control in vivo. Accordingly, blocking IL-9 in melanoma cell-exposed Cblb-/- mice reversed their tumor rejection phenotype. Furthermore, scRNA sequencing of in vitro differentiated Th9 cells from naïve T cells isolated from wildtype and Cblb-/- animals revealed a transcriptomic basis for increased Th9 cell differentiation. CONCLUSION: We established IL-9 and Th9 cells as key antitumor executers in Cblb-/- animals. This knowledge may be helpful for the future improvement of adoptive T cell therapies in cancer.

Balancing intestinal and systemic inflammation through cell type-specific expression of the aryl hydrocarbon receptor repressor.

As a sensor of polyaromatic chemicals the aryl hydrocarbon receptor (AhR) exerts an important role in immune regulation besides its requirement for xenobiotic metabolism. Transcriptional activation of AhR target genes is counterregulated by the AhR repressor (AhRR) but the exact function of the AhRR in vivo is currently unknown. We here show that the AhRR is predominantly expressed in immune cells of the skin and intestine, different from other AhR target genes. Whereas AhRR antagonizes the anti-inflammatory function of the AhR in the context of systemic endotoxin shock, AhR and AhRR act in concert to dampen intestinal inflammation. Specifically, AhRR contributes to the maintenance of colonic intraepithelial lymphocytes and prevents excessive IL-1ß production and Th17/Tc17 differentiation. In contrast, the AhRR enhances IFN-γ-production by effector T cells in the inflamed gut. Our findings highlight the physiologic importance of cell-type specific balancing of AhR/AhRR expression in response to microbial, nutritional and other environmental stimuli.

IL-6 trans-signaling-dependent rapid development of cytotoxic CD8+ T cell function.

Immune control of infections with viruses or intracellular bacteria relies on cytotoxic CD8(+) T cells that use granzyme B (GzmB) for elimination of infected cells. During inflammation, mature antigen-presenting dendritic cells instruct naive T cells within lymphoid organs to develop into effector T cells. Here, we report a mechanistically distinct and more rapid process of effector T cell development occurring within 18 hr. Such rapid acquisition of effector T cell function occurred through cross-presenting liver sinusoidal endothelial cells (LSECs) in the absence of innate immune stimulation and known costimulatory signaling. Rather, interleukin-6 (IL-6) trans-signaling was required and sufficient for rapid induction of GzmB expression in CD8(+) T cells. Such LSEC-stimulated GzmB-expressing CD8(+) T cells further responded to inflammatory cytokines, eliciting increased and protracted effector functions. Our findings identify a role for IL-6 trans-signaling in rapid generation of effector function in CD8(+) T cells that may be beneficial for vaccination strategies.

Liver-primed memory T cells generated under noninflammatory conditions provide anti-infectious immunity.

Development of CD8(+) T cell (CTL) immunity or tolerance is linked to the conditions during T cell priming. Dendritic cells (DCs) matured during inflammation generate effector/memory T cells, whereas immature DCs cause T cell deletion/anergy. We identify a third outcome of T cell priming in absence of inflammation enabled by cross-presenting liver sinusoidal endothelial cells. Such priming generated memory T cells that were spared from deletion by immature DCs. Similar to central memory T cells, liver-primed T cells differentiated into effector CTLs upon antigen re-encounter on matured DCs even after prolonged absence of antigen. Their reactivation required combinatorial signaling through the TCR, CD28, and IL-12R and controlled bacterial and viral infections. Gene expression profiling identified liver-primed T cells as a distinct Neuropilin-1(+) memory population. Generation of liver-primed memory T cells may prevent pathogens that avoid DC maturation by innate immune escape from also escaping adaptive immunity through attrition of the T cell repertoire.