Cellular Differentiation of Human Monocytes Is Regulated by Time-Dependent Interleukin-4 Signaling and the Transcriptional Regulator NCOR2.

Human in vitro generated monocyte-derived dendritic cells (moDCs) and macrophages are used clinically, e.g., to induce immunity against cancer. However, their physiological counterparts, ontogeny, transcriptional regulation, and heterogeneity remains largely unknown, hampering their clinical use. High-dimensional techniques were used to elucidate transcriptional, phenotypic, and functional differences between human in vivo and in vitro generated mononuclear phagocytes to facilitate their full potential in the clinic. We demonstrate that monocytes differentiated by macrophage colony-stimulating factor (M-CSF) or granulocyte macrophage colony-stimulating factor (GM-CSF) resembled in vivo inflammatory macrophages, while moDCs resembled in vivo inflammatory DCs. Moreover, differentiated monocytes presented with profound transcriptomic, phenotypic, and functional differences. Monocytes integrated GM-CSF and IL-4 stimulation combinatorically and temporally, resulting in a mode- and time-dependent differentiation relying on NCOR2. Finally, moDCs are phenotypically heterogeneous and therefore necessitate the use of high-dimensional phenotyping to open new possibilities for better clinical tailoring of these cellular therapies.

The translocon protein Sec61 mediates antigen transport from endosomes in the cytosol for cross-presentation to CD8(+) T cells.

The molecular mechanisms regulating antigen translocation into the cytosol for cross-presentation are under controversial debate, mainly because direct data is lacking. Here, we have provided direct evidence that the activity of the endoplasmic reticulum (ER) translocon protein Sec61 is essential for endosome-to-cytosol translocation. We generated a Sec61-specific intrabody, a crucial tool that trapped Sec61 in the ER and prevented its recruitment into endosomes without influencing Sec61 activity and antigen presentation in the ER. Expression of this ER intrabody inhibited antigen translocation and cross-presentation, demonstrating that endosomal Sec61 indeed mediates antigen transport across endosomal membranes. Moreover, we showed that the recruitment of Sec61 toward endosomes, and hence antigen translocation and cross-presentation, is dependent on dendritic cell activation by Toll-like receptor (TLR) ligands. These data shed light on a long-lasting question regarding antigen cross-presentation and point out a role of the ER-associated degradation machinery in compartments distinct from the ER.

Soluble mannose receptor induces proinflammatory macrophage activation and metaflammation.

Proinflammatory activation of macrophages in metabolic tissues is critically important in the induction of obesity-induced metaflammation. Here, we demonstrate that the soluble mannose receptor (sMR) plays a direct functional role in both macrophage activation and metaflammation. We show that sMR binds CD45 on macrophages and inhibits its phosphatase activity, leading to an Src/Akt/NF-κB-mediated cellular reprogramming toward an inflammatory phenotype both in vitro and in vivo. Remarkably, increased serum sMR levels were observed in obese mice and humans and directly correlated with body weight. Importantly, enhanced sMR levels increase serum proinflammatory cytokines, activate tissue macrophages, and promote insulin resistance. Altogether, our results reveal sMR as regulator of proinflammatory macrophage activation, which could constitute a therapeutic target for metaflammation and other hyperinflammatory diseases.

Guidelines for mouse and human DC functional assays.

This article is part of the Dendritic Cell Guidelines article series, which provides a collection of state-of-the-art protocols for the preparation, phenotype analysis by flow cytometry, generation, fluorescence microscopy, and functional characterization of mouse and human dendritic cells (DC) from lymphoid organs and various non-lymphoid tissues. Recent studies have provided evidence for an increasing number of phenotypically distinct conventional DC (cDC) subsets that on one hand exhibit a certain functional plasticity, but on the other hand are characterized by their tissue- and context-dependent functional specialization. Here, we describe a selection of assays for the functional characterization of mouse and human cDC. The first two protocols illustrate analysis of cDC endocytosis and metabolism, followed by guidelines for transcriptomic and proteomic characterization of cDC populations. Then, a larger group of assays describes the characterization of cDC migration in vitro, ex vivo, and in vivo. The final guidelines measure cDC inflammasome and antigen (cross)-presentation activity. While all protocols were written by experienced scientists who routinely use them in their work, this article was also peer-reviewed by leading experts and approved by all co-authors, making it an essential resource for basic and clinical DC immunologists.

Spatial and mechanistic separation of cross-presentation and endogenous antigen presentation.

Antiviral or antitumor immunity requires activation of cytotoxic CD8+ T cells by dendritic cells, which present viral or tumor antigens on major histocompatibility complex (MHC) class I molecules. The intracellular mechanisms facilitating MHC class I-restricted presentation of extracellular antigens ('cross-presentation') are unclear. Here we demonstrate that cross-presentation of soluble antigen occurred in an early endosomal compartment distinct from the endoplasmic reticulum where endogenous antigen is loaded onto MHC class I. Efficient cross-presentation required endotoxin-induced, Toll-like receptor 4- and signaling molecule MyD88-dependent relocation of the transporter associated with antigen processing, essential for loading of MHC class I, to early endosomes. Transport of cross-presented antigen from endosomes to the cell surface was inhibited by primaquine, which blocks endosomal trafficking. Thus, cross-presentation is spatially and mechanistically separated from endogenous MHC class I-restricted antigen presentation and is biased toward antigens containing microbial molecular patterns.

Mannose receptor induces T-cell tolerance via inhibition of CD45 and up-regulation of CTLA-4.

The mannose receptor (MR) is an endocytic receptor involved in serum homeostasis and antigen presentation. Here, we identify the MR as a direct regulator of CD8(+) T-cell activity. We demonstrate that MR expression on dendritic cells (DCs) impaired T-cell cytotoxicity in vitro and in vivo. This regulatory effect of the MR was mediated by a direct interaction with CD45 on the T cell, inhibiting its phosphatase activity, which resulted in up-regulation of cytotoxic T-lymphocyte-associated Protein 4 (CTLA-4) and the induction of T-cell tolerance. Inhibition of CD45 prevented expression of B-cell lymphoma 6 (Bcl-6), a transcriptional inhibitor that directly bound the CTLA-4 promoter and regulated its activity. These data demonstrate that endocytic receptors expressed on DCs contribute to the regulation of T-cell functionality.

The renal microenvironment modifies dendritic cell phenotype.

Renal dendritic cells are a major component of the renal mononuclear phagocytic system. In the renal interstitium, these cells are exposed to an osmotic gradient, mainly sodium, whose concentration progressively increases towards inner medulla. Renal allograft rejection affects predominantly the cortex, suggesting a protective role of the renal medullary micromilieu. Whether osmolar variations can modulate the function of renal dendritic cells is currently undefined. Considering the central role of dendritic cells in promoting allorejection, we tested whether the biophysical micromilieu, particularly the interstitial osmotic gradient, influences their alloreactivity. There was a progressive depletion of leukocytes towards the medulla of homeostatic kidney. Only macrophages opposed this tendency. Flow cytometry of homeostatic and post-transplant medullary dendritic cells revealed a switch towards a macrophage-like phenotype. Similarly, bone marrow-derived dendritic cells developed ex vivo in sodium chloride-enriched medium acquired a M2-like signature. Microarray analysis of allotransplant dendritic cells posed a medullary downregulation of genes mainly involved in alloantigen recognition. Gene expression profiles of both medullary dendritic cells and bone marrow-derived dendritic cells matured in hyperosmolar medium had an overlap with the macrophage M2 signature. Thus, the medullary environment inhibits an alloimmune response by modulating the phenotype and function of dendritic cells.

Expression of type I interferon by splenic macrophages suppresses adaptive immunity during sepsis.

Early during Gram-negative sepsis, excessive release of pro-inflammatory cytokines can cause septic shock that is often followed by a state of immune paralysis characterized by the failure to mount adaptive immunity towards secondary microbial infections. Especially, the early mechanisms responsible for such immune hypo-responsiveness are unclear. Here, we show that TLR4 is the key immune sensing receptor to initiate paralysis of T-cell immunity after bacterial sepsis. Downstream of TLR4, signalling through TRIF but not MyD88 impaired the development of specific T-cell immunity against secondary infections. We identified type I interferon (IFN) released from splenic macrophages as the critical factor causing T-cell immune paralysis. Early during sepsis, type I IFN acted selectively on dendritic cells (DCs) by impairing antigen presentation and secretion of pro-inflammatory cytokines. Our results reveal a novel immune regulatory role for type I IFN in the initiation of septic immune paralysis, which is distinct from its well-known immune stimulatory effects. Moreover, we identify potential molecular targets for therapeutic intervention to overcome impairment of T-cell immunity after sepsis.

Ovalbumin modified with pyrraline, a Maillard reaction product, shows enhanced T-cell immunogenicity.

The Maillard reaction (also referred to as "glycation") takes place between reducing sugars and compounds with free amino groups during thermal processing of foods. In the final stage of the complex reaction cascade, the so-called advanced glycation end products (AGEs) are formed, including proteins with various glycation structures. It has been suggested that some AGEs could have immunostimulatory effects. Here, we aimed to identify specific glycation structure(s) that could influence the T-cell immunogenicity and potential allergenicity of food allergens, using ovalbumin (OVA, an egg white allergen) as a model allergen. OVA was specifically modified with representative glycation structures: N(ε)-carboxymethyl lysine (CM-OVA), N(ε)-carboxyethyl lysine (CE-OVA), pyrraline (Pyr-OVA), or methylglyoxal-derived arginine derivatives (MGO-OVA). As well as AGE-OVA, a crude glycation product in thermal incubation of OVA with glucose, only Pyr-OVA, and not other modified OVAs, was efficiently taken up by bone marrow-derived murine dendritic cells (BMDCs). The uptake of Pyr-OVA was reduced in scavenger receptor class A (SR-A)-deficient BMDCs, but not in cells treated with inhibitors of scavenger receptor class B, galectin-3, or blocking antibodies against CD36, suggesting that pyrraline binds to SR-A. Compared with other modified OVAs, Pyr-OVA induced higher activation of OVA-specific CD4(+) T-cells in co-culture with BMDCs. Furthermore, compared with native OVA, AGE-OVA and Pyr-OVA induced higher IgE production in mice. Pyrraline could induce better allergen uptake by DCs via association with SR-A and subsequently enhance CD4(+) T-cell activation and IgE production. Our findings help us to understand how Maillard reaction enhances the potential allergenicity of food allergens.

The endocannabinoid-CB2 receptor axis protects the ischemic heart at the early stage of cardiomyopathy.

Ischemic heart disease is associated with inflammation, interstitial fibrosis and ventricular dysfunction prior to the development of heart failure. Endocannabinoids and the cannabinoid receptor CB2 have been claimed to be involved, but their potential role in cardioprotection is not well understood. We therefore explored the role of the cannabinoid receptor CB2 during the initial phase of ischemic cardiomyopathy development prior to the onset of ventricular dysfunction or infarction. Wild type and CB2-deficient mice underwent daily brief, repetitive ischemia and reperfusion (I/R) episodes leading to ischemic cardiomyopathy. The relevance of the endocannabinoid-CB2 receptor axis was underscored by the finding that CB2 was upregulated in ischemic wild type cardiomyocytes and that anandamide level was transiently increased during I/R. CB2-deficient mice showed an increased rate of apoptosis, irreversible loss of cardiomyocytes and persistent left ventricular dysfunction 60 days after the injury, whereas wild type mice presented neither morphological nor functional defects. These defects were due to lack of cardiomyocyte protection mechanisms, as CB2-deficient hearts were in contrast to controls unable to induce switch in myosin heavy chain isoforms, antioxidative enzymes and chemokine CCL2 during repetitive I/R. In addition, a prolonged inflammatory response and adverse myocardial remodeling were found in CB2-deficient hearts because of postponed activation of the M2a macrophage subpopulation. Therefore, the endocannabinoid-CB2 receptor axis plays a key role in cardioprotection during the initial phase of ischemic cardiomyopathy development.

Mannose receptor polyubiquitination regulates endosomal recruitment of p97 and cytosolic antigen translocation for cross-presentation.

The molecular mechanisms regulating noncanonical protein transport across cellular membranes are poorly understood. Cross-presentation of exogenous antigens on MHC I molecules by dendritic cells (DCs) generally requires antigen translocation from the endosomal compartment into the cytosol for proteasomal degradation. In this study, we demonstrate that such translocation is controlled by the endocytic receptor and regulated by ubiquitination. Antigens internalized by the mannose receptor (MR), an endocytic receptor that targets its ligands specifically toward cross-presentation, were translocated into the cytosol only after attachment of a lysin48-linked polyubiquitin chain to the cytosolic region of the MR. Furthermore, we identify TSG101 as a central regulator of MR ubiquitination and antigen translocation. Importantly, we demonstrate that MR polyubiquitination mediates the recruitment of p97, a member of the ER-associated degradation machinery that provides the driving force for antigen translocation, toward the endosomal membrane, proving the central role of the endocytic receptor and its ubiquitination in antigen translocation.

The proteoglycan biglycan enhances antigen-specific T cell activation potentially via MyD88 and TRIF pathways and triggers autoimmune perimyocarditis.

Biglycan is a proteoglycan ubiquitously present in extracellular matrix of a variety of organs, including heart, and it was reported to be overexpressed in myocardial infarction. Myocardial infarction may be complicated by perimyocarditis through unknown mechanisms. Our aim was to investigate the capacity of TLR2/TLR4 ligand biglycan to enhance the presentation of specific Ags released upon cardiomyocyte necrosis. In vitro, OVA-pulsed bone marrow-derived dendritic cells from wild-type (WT; C57BL/6) and TLR2-, TLR4-, MyD88-, or TRIF-deficient mice were cotreated with LPS, biglycan, or vehicle and incubated with OVA-recognizing MHC I- or MHC II-restricted T cells. Biglycan enhanced OVA-specific cross-priming by >80% to MHC I-restricted T cells in both TLR2- and TLR4-pathway-dependent manners. Accordingly, biglycan-induced cross-priming by both MyD88- and TRIF-deficient dendritic cells (DCs) was strongly diminished. OVA-specific activation of MHC II-restricted T cells was predominantly TLR4 dependent. Our first in vivo correlate was a model of experimental autoimmune perimyocarditis triggered by injection of cardiac Ag-pulsed DCs (BALB/c). Biglycan-treated DCs triggered perimyocarditis to a comparable extent and intensity as LPS-treated DCs (mean scores 1.3 ± 0.3 and 1.5 ± 0.4, respectively). Substitution with TLR4-deficient DCs abolished this effect. In a second in vivo approach, WT and biglycan-deficient mice were followed 2 wk after induction of myocardial infarction. WT mice demonstrated significantly greater myocardial T lymphocyte infiltration in comparison with biglycan-deficient animals. We concluded that the TLR2/4 ligand biglycan, a component of the myocardial matrix, may enhance Ag-specific T cell priming, potentially via MyD88 and TRIF, and stimulate autoimmune perimyocarditis.

The non-phagocytic route of collagen uptake: a distinct degradation pathway.

The degradation of collagens, the most abundant proteins of the extracellular matrix, is involved in numerous physiological and pathological conditions including cancer invasion. An important turnover pathway involves cellular internalization and degradation of large, soluble collagen fragments, generated by initial cleavage of the insoluble collagen fibers. We have previously observed that in primary mouse fibroblasts, this endocytosis of collagen fragments is dependent on the receptor urokinase plasminogen activator receptor-associated protein (uPARAP)/Endo180. Others have identified additional mechanisms of collagen uptake, with different associated receptors, in other cell types. These receptors include ß1-integrins, being responsible for collagen phagocytosis, and the mannose receptor. We have now utilized a newly developed monoclonal antibody against uPARAP/Endo180, which down-regulates the receptor protein level on treated cells, to examine the role of uPARAP/Endo180 as a mediator of collagen internalization by a wide range of cultured cell types. With the exception of macrophages, all cells that proved capable of efficient collagen internalization were of mesenchymal origin and all of these utilized uPARAP/Endo180 for their collagen uptake process. Macrophages internalized collagen in a process mediated by the mannose receptor, a protein belonging to the same protein family as uPARAP/Endo180. ß1-Integrins were found not to be involved in the endocytosis of soluble collagen, irrespectively of whether this was mediated by uPARAP/Endo180 or the mannose receptor. This further distinguishes these pathways from the phagocytic uptake of particulate collagen.

Design of neo-glycoconjugates that target the mannose receptor and enhance TLR-independent cross-presentation and Th1 polarization.

Cross-presentation is an important mechanism by which DCs present exogenous antigens on MHC-I molecules, and activate CD8(+) T cells, cells that are crucial for the elimination of tumors. We investigated the feasibility of exploiting the capacity of the mannose receptor (MR) to improve both cross-presentation of tumor antigens and Th polarization, processes that are pivotal for the anti-tumor potency of cytotoxic T cells. To this end, we selected two glycan ligands of the MR, 3-sulfo-Lewis(A) and tri-GlcNAc (N-acetylglucosamine), to conjugate to the model antigen OVA and assessed in vitro the effect on antigen presentation and Th differentiation. Our results demonstrate that conjugation of either 3-sulfo-Lewis(A) or tri-GlcNAc specifically directs antigen to the MR. Both neo-glycoconjugates showed, even at low doses, improved uptake as compared with native OVA, resulting in enhanced cross-presentation. Using MR(-/-) and MyD88-TRIFF(-/-) bone marrow-derived DCs (BMDCs), we show that the cross-presentation of the neo-glycoconjugates is dependent on MR and independent of TLR-mediated signaling. Whereas proliferation of antigen-specific CD4(+) T cells was unchanged, stimulation with neo-glycoconjugate-loaded DCs enhanced the generation of IFN-γ-producing T cells. We conclude that modification of antigen with either 3-sulfo-Lewis(A) or tri-GlcNAc enhances cross-presentation and permits Th1 skewing, through specific targeting of the MR, which may be beneficial for DC-based vaccination strategies to treat cancer.

Dynamic regulation of CD8 T cell tolerance induction by liver sinusoidal endothelial cells.

Cross-presentation of soluble Ag on MHC class I molecules to naive CD8 T cells by liver sinusoidal endothelial cells (LSECs) leads to induction of T cell tolerance that requires interaction between coinhibitory B7-H1 on LSECs and programmed cell death-1 on CD8 T cells. In this study, we investigate whether cross-presentation of high as well as low Ag concentrations allowed for LSEC-induced tolerance. Ag concentration directly correlated with the cross-presentation capacity of murine LSECs and thus strength of TCR stimulation. Although LSEC cross-presentation at low-Ag concentrations resulted in tolerance, they induced differentiation into effector T cells (CTL) at high-Ag concentrations. CTL differentiation under these conditions was not caused by increased expression of costimulatory CD80/86 on cross-presenting LSECs but was determined by early IL-2 release from naive CD8 T cells. B7-H1 signals from LSECs and TCR avidity reciprocally controlled early T cell release of IL-2 and CTL differentiation. B7-H1 expression directly correlated with cross-presentation at low- but not high-Ag concentrations, indicating an imbalance between TCR and coinhibitory signals regulating T cell release of IL-2. Exogenous IL-2 overrode coinhibitory B7-H1-mediated signals by LSECs and induced full CTL differentiation. Our results imply that LSEC-mediated T cell tolerance can be broken in situations where T cells bearing high-avidity TCR encounter LSECs cross-presenting high numbers of cognate MHC class I peptide molecules, such as during viral infection of the liver. Furthermore, we attribute a novel costimulatory function to IL-2 acting in a T cell autonomous fashion to promote local induction of immunity in the liver even in the absence of CD80/86 costimulation.

Multiple centrosomes enhance migration and immune cell effector functions of mature dendritic cells.

Centrosomes play a crucial role during immune cell interactions and initiation of the immune response. In proliferating cells, centrosome numbers are tightly controlled and generally limited to one in G1 and two prior to mitosis. Defects in regulating centrosome numbers have been associated with cell transformation and tumorigenesis. Here, we report the emergence of extra centrosomes in leukocytes during immune activation. Upon antigen encounter, dendritic cells pass through incomplete mitosis and arrest in the subsequent G1 phase leading to tetraploid cells with accumulated centrosomes. In addition, cell stimulation increases expression of polo-like kinase 2, resulting in diploid cells with two centrosomes in G1-arrested cells. During cell migration, centrosomes tightly cluster and act as functional microtubule-organizing centers allowing for increased persistent locomotion along gradients of chemotactic cues. Moreover, dendritic cells with extra centrosomes display enhanced secretion of inflammatory cytokines and optimized T cell responses. Together, these results demonstrate a previously unappreciated role of extra centrosomes for regular cell and tissue homeostasis.

Endocytosis mechanisms and the cell biology of antigen presentation.

Recent evidence revealed that presentation of soluble antigens is governed by the endocytosis mechanisms that determine the intracellular routing of the endocytosed antigens. Soluble antigens intended for classical exogenous MHC-II-restricted presentation are internalized into lysosomes. Soluble antigens destined for crosspresentation are taken up by distinct endocytosis mechanisms and are conveyed into stable early endosomes. Particulate antigens enter phagosomes, in which both MHC-I-restricted and MHC-II-restricted presentation is initiated. In this review, we discuss the mechanistical differences in presentation of soluble and particulate antigen, the correlation of various endocytic receptors with antigen routing and presentation, the differential expression of these receptors in antigen-presenting cell subsets with respect to their ability to crosspresent, and implications on the molecular mechanisms controlling cross-presentation.

Increased antigen cross-presentation but impaired cross-priming after activation of peroxisome proliferator-activated receptor gamma is mediated by up-regulation of B7H1.

Dendritic cells are able to take up exogenous Ags and present Ag-derived peptides on MHC class I molecules, a process termed cross-presentation. The mannose receptor (MR), an endocytic receptor expressed on a variety of APCs, has been demonstrated to target soluble Ags exclusively toward cross-presentation. In this study, we investigated the role of the murine nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma), a ligand-activated transcription factor with immunomodulatory properties, in MR-mediated endocytosis and cross-presentation of the model Ag OVA. We could demonstrate both in vitro and in vivo that activation of PPARgamma resulted in increased MR expression, which in consequence led to enhanced MR-mediated endocytosis and elevated cross-presentation of soluble OVA. Concomitantly, activation of PPARgamma in dendritic cells induced up-regulation of the coinhibitory molecule B7H1, which, despite enhanced cross-presentation, caused an impaired activation of naive OVA-specific CD8(+) T cells and the induction of T cell tolerance. These data provide a mechanistic basis for the immunomodulatory action of PPARgamma which might open new possibilities in the development of therapeutic approaches aimed at the control of excessive immune responses, e.g., in T cell-mediated autoimmunity.

Glycation of a food allergen by the Maillard reaction enhances its T-cell immunogenicity: role of macrophage scavenger receptor class A type I and II.

BACKGROUND: The Maillard reaction occurs between reducing sugars and proteins during thermal processing of foods. It produces chemically glycated proteins termed advanced glycation end products (AGEs). The glycation structures of AGEs are suggested to function as pathogenesis-related immune epitopes in food allergy. OBJECTIVE: This study aimed at defining the T-cell immunogenicity of food AGEs by using ovalbumin (OVA) as a model allergen. METHODS: AGE-OVA was prepared by means of thermal processing of OVA in the presence of glucose. Activation of OVA-specific CD4(+) T cells by AGE-OVA was evaluated in cocultures with bone marrow-derived murine myeloid dendritic cells (mDCs) as antigen-presenting cells. The uptake mechanisms of mDCs for AGE-OVA were investigated by using inhibitors of putative cell-surface receptors for AGEs, as well as mDCs deficient for these receptors. RESULTS: Compared with the controls (native OVA and OVA thermally processed without glucose), AGE-OVA enhanced the activation of OVA-specific CD4(+) T cells on coculture with mDCs, indicating that the glycation of OVA enhanced the T-cell immunogenicity of the allergen. The mDC uptake of AGE-OVA was significantly higher than that of the controls. We identified scavenger receptor class A type I and II (SR-AI/II) as a mediator of the AGE-OVA uptake, whereas the receptor for AGEs and galectin-3 were not responsible. Importantly, the activation of OVA-specific CD4(+) T cells by AGE-OVA was attenuated on coculture with SR-AI/II-deficient mDCs. CONCLUSION: SR-AI/II targets AGE-OVA to the MHC class II loading pathway in mDCs, leading to an enhanced CD4(+) T-cell activation. The Maillard reaction might thus play an important role in the T-cell immunogenicity of food allergens.

The Mannose Receptor: From Endocytic Receptor and Biomarker to Regulator of (Meta)Inflammation.

The mannose receptor is a member of the C-type lectin (CLEC) family, which can bind and internalize a variety of endogenous and pathogen-associated ligands. Because of these properties, its role in endocytosis as well as antigen processing and presentation has been studied intensively. Recently, it became clear that the mannose receptor can directly influence the activation of various immune cells. Cell-bound mannose receptor expressed by antigen-presenting cells was indeed shown to drive activated T cells towards a tolerogenic phenotype. On the other hand, serum concentrations of a soluble form of the mannose receptor have been reported to be increased in patients suffering from a variety of inflammatory diseases and to correlate with severity of disease. Interestingly, we recently demonstrated that the soluble mannose receptor directly promotes macrophage proinflammatory activation and trigger metaflammation. In this review, we highlight the role of the mannose receptor and other CLECs in regulating the activation of immune cells and in shaping inflammatory responses.