Secreted IgM modulates IL-10 expression in B cells

IL-10+ B cells are critical for immune homeostasis and restraining immune responses in infection, cancer, and inflammation; however, the signals that govern IL-10+ B cell differentiation are ill-defined. Here we find that IL-10+ B cells expand in mice lacking secreted IgM ((s)IgM-/-) up to 10-fold relative to wildtype (WT) among all major B cell and regulatory B cell subsets. The IL-10+ B cell increase is polyclonal and presents within 24 hours of birth. In WT mice, sIgM is produced prenatally and limits the expansion of IL-10+ B cells. Lack of the high affinity receptor for sIgM, FcμR, in B cells translates into an intermediate IL-10+ B cell phenotype relative to WT or sIgM-/- mice. Our study thus shows that sIgM regulates IL-10 programming in B cells in part via B cell-expressed FcμR, thereby revealing a function of sIgM in regulating immune homeostasis.

Chemerin triggers migration of a CD8 T cell subset with natural killer cell functions

The recruitment of cells with effector functions into the tumor microenvironment holds potential for delaying cancer progression. We show that subsets of human CD28-effector CD8 T cells, CCR7- CD45RO+ effector memory, and CCR7- CD45RO- effector memory RA phenotypes, express the chemerin receptor CMKLR1 and bind chemerin via the receptor. CMKLR1-expressing human CD8 effector memory T cells present gene, protein, and cytotoxic features of NK cells. Active chemerin promotes chemotaxis of CMKLR1-expressing CD8 effector memory cells and triggers activation of the α4β1 integrin. In an experimental prostate tumor mouse model, chemerin expression is downregulated in the tumor microenvironment, which is associated with few tumor-infiltrating CD8+ T cells, while forced overexpression of chemerin by mouse prostate cancer cells leads to an accumulation of intra-tumor CD8+ T cells. Furthermore, α4 integrin blockade abrogated the chemerin-dependent recruitment of CD8+ T effector memory cells into implanted prostate tumors in vivo. The results identify a role for chemerin:CMKLR1 in defining a specialized NK-like CD8 T cell, and suggest the use of chemerin-dependent modalities to target effector CMKLR1-expressing T cells to the tumor microenvironment for immunotherapeutic purposes.

Transcription factor induction of vascular blood stem cell niches in vivo

The hematopoietic niche is a supportive microenvironment composed of distinct cell types, including specialized vascular endothelial cells that directly interact with hematopoietic stem and progenitor cells (HSPCs). The molecular factors that specify niche endothelial cells and orchestrate HSPC homeostasis remain largely unknown. Using multi-dimensional gene expression and chromatin accessibility analyses in zebrafish, we define a conserved gene expression signature and cis-regulatory landscape that are unique to sinusoidal endothelial cells in the HSPC niche. Using enhancer mutagenesis and transcription factor overexpression, we elucidate a transcriptional code that involves members of the Ets, Sox, and nuclear hormone receptor families and is sufficient to induce ectopic niche endothelial cells that associate with mesenchymal stromal cells and support the recruitment, maintenance, and division of HSPCs in vivo. These studies set forth an approach for generating synthetic HSPC niches, in vitro or in vivo, and for effective therapies to modulate the endogenous niche.

The small and large intestine contain related mesenchymal subsets that derive from embryonic Gli1+ precursors

The intestinal lamina propria contains a diverse network of fibroblasts that provide key support functions to cells within their local environment. Despite this, our understanding of the diversity, location and ontogeny of fibroblasts within and along the length of the intestine remains incomplete. Here we show that the small and large intestinal lamina propria contain similar fibroblast subsets that locate in specific anatomical niches. Nevertheless, we find that the transcriptional profile of similar fibroblast subsets differs markedly between the small intestine and colon suggesting region specific functions. We perform in vivo transplantation and lineage-tracing experiments to demonstrate that adult intestinal fibroblast subsets, smooth muscle cells and pericytes derive from Gli1-expressing precursors present in embryonic day 12.5 intestine. Trajectory analysis of single cell RNA-seq datasets of E12.5 and adult mesenchymal cells suggest that adult smooth muscle cells and fibroblasts derive from distinct embryonic intermediates and that adult fibroblast subsets develop in a linear trajectory from CD81⁺ fibroblasts. Finally, we provide evidence that colonic subepithelial PDGFRα^hi fibroblasts comprise several functionally distinct populations that originate from an Fgfr2-expressing fibroblast intermediate. Our results provide insights into intestinal stromal cell diversity, location, function, and ontogeny, with implications for intestinal development and homeostasis.

An NKX-COUP-TFII morphogenetic code directs mucosal endothelial addressin expression

Immunoglobulin family and carbohydrate vascular addressins encoded by Madcam1 and St6gal1 control lymphocyte homing into intestinal tissues, regulating immunity and inflammation. The addressins are developmentally programmed to decorate endothelial cells lining gut post-capillary and high endothelial venules (HEV), providing a prototypical example of organ- and segment-specific endothelial specialization. We identify conserved NKX-COUP-TFII composite elements (NCCE) in regulatory regions of Madcam1 and St6gal1 that bind intestinal homeodomain protein NKX2-3 cooperatively with venous nuclear receptor COUP-TFII to activate transcription. The Madcam1 element also integrates repressive signals from arterial/capillary Notch effectors. Pan-endothelial COUP-TFII overexpression induces ectopic addressin expression in NKX2-3+ capillaries, while NKX2-3 deficiency abrogates expression by HEV. Phylogenetically conserved NCCE are enriched in genes involved in neuron migration and morphogenesis of the heart, kidney, pancreas and other organs. Our results define an NKX-COUP-TFII morphogenetic code that targets expression of mucosal vascular addressins.

Development of follicular dendritic cells in lymph nodes depends on retinoic acid-mediated signaling

Specialized stromal cells occupy and help define B- and T-cell domains, which are crucial for proper functioning of our immune system. Signaling through lymphotoxin and TNF receptors is crucial for the development of different stromal subsets, which are thought to arise from a common precursor. However, mechanisms that control the selective generation of the different stromal phenotypes are not known. Using in vitro cultures of embryonic mouse stromal cells, we show that retinoic acid-mediated signaling is important for the differentiation of precursors towards the Cxcl13pos follicular dendritic cell (FDC) lineage, and also blocks lymphotoxin-mediated Ccl19pos fibroblastic reticular cell lineage differentiation. Accordingly, at the day of birth we observe the presence of Cxcl13posCcl19neg/low and Cxcl13neg/lowCcl19pos cells within neonatal lymph nodes. Furthermore, ablation of retinoic acid receptor signaling in stromal precursors early after birth reduces Cxcl13 expression, and complete blockade of retinoic acid signaling prevents the formation of FDC networks in lymph nodes.

The aryl hydrocarbon receptor regulates expression of mucosal trafficking receptor GPR15

GPR15 is a chemoattractant receptor that facilitates colon homing of regulatory and effector CD4+ T cells in health and colitis. The molecular mechanisms that control GPR15 expression are not fully known. Here we report the presence of two highly conserved aryl hydrocarbon receptor (AHR) binding sequences in a 3' enhancer of GPR15, leading us to investigate AHR function in regulating GPR15 expression. Using luciferase reporter assays, we show that AHR activation increased GPR15 expression and requires both the AHR binding sites. Consistent with a transcriptional regulatory role, treatment with AHR agonists induce GPR15 expression on human CD4+ T cells. Using AHR-deficient mice, we demonstrate that the lack of AHR signaling drastically reduces GPR15 expression on effector/memory and Foxp3+ CD4+ T cells. In mixed bone marrow chimeras of AHR-deficient and wildtype cells, GPR15 expression was similarly diminished on AHR-deficient CD4+ effector/memory and regulatory T cells in the colon and small intestine. Furthermore, administration of AHR agonists upregulated GPR15 expression on CD4+ effector/memory T cells and increased their homing capability, especially to the colon. Collectively, our studies reveal a novel function of the AHR in regulation of GPR15 expression and increased colon trafficking of CD4+ T cells expressing GPR15.

A CD22-Shp1 phosphatase axis controls integrin β7 display and B cell function in mucosal immunity

The integrin α4β7 selectively regulates lymphocyte trafficking and adhesion in the gut and gut-associated lymphoid tissue (GALT). Here, we describe unexpected involvement of the tyrosine phosphatase Shp1 and the B cell lectin CD22 (Siglec-2) in the regulation of α4β7 surface expression and gut immunity. Shp1 selectively inhibited β7 endocytosis, enhancing surface α4β7 display and lymphocyte homing to GALT. In B cells, CD22 associated in a sialic acid-dependent manner with integrin β7 on the cell surface to target intracellular Shp1 to β7. Shp1 restrained plasma membrane β7 phosphorylation and inhibited β7 endocytosis without affecting β1 integrin. B cells with reduced Shp1 activity, lacking CD22 or expressing CD22 with mutated Shp1-binding or carbohydrate-binding domains displayed parallel reductions in surface α4β7 and in homing to GALT. Consistent with the specialized role of α4β7 in intestinal immunity, CD22 deficiency selectively inhibited intestinal antibody and pathogen responses.

Single-Cell Analysis of Blood-Brain Barrier Response to Pericyte Loss

RATIONALE

Pericytes are capillary mural cells playing a role in stabilizing newly formed blood vessels during development and tissue repair. Loss of pericytes has been described in several brain disorders, and genetically induced pericyte deficiency in the brain leads to increased macromolecular leakage across the blood-brain barrier (BBB). However, the molecular details of the endothelial response to pericyte deficiency remain elusive.

OBJECTIVE

To map the transcriptional changes in brain endothelial cells resulting from lack of pericyte contact at single-cell level and to correlate them with regional heterogeneities in BBB function and vascular phenotype.

METHODS AND RESULTS

We reveal transcriptional, morphological, and functional consequences of pericyte absence for brain endothelial cells using a combination of methodologies, including single-cell RNA sequencing, tracer analyses, and immunofluorescent detection of protein expression in pericyte-deficient adult Pdgfbret/ret mice. We find that endothelial cells without pericyte contact retain a general BBB-specific gene expression profile, however, they acquire a venous-shifted molecular pattern and become transformed regarding the expression of numerous growth factors and regulatory proteins. Adult Pdgfbret/ret brains display ongoing angiogenic sprouting without concomitant cell proliferation providing unique insights into the endothelial tip cell transcriptome. We also reveal heterogeneous modes of pericyte-deficient BBB impairment, where hotspot leakage sites display arteriolar-shifted identity and pinpoint putative BBB regulators. By testing the causal involvement of some of these using reverse genetics, we uncover a reinforcing role for angiopoietin 2 at the BBB.

CONCLUSIONS

By elucidating the complexity of endothelial response to pericyte deficiency at cellular resolution, our study provides insight into the importance of brain pericytes for endothelial arterio-venous zonation, angiogenic quiescence, and a limited set of BBB functions. The BBB-reinforcing role of ANGPT2 (angiopoietin 2) is paradoxical given its wider role as TIE2 (TEK receptor tyrosine kinase) receptor antagonist and may suggest a unique and context-dependent function of ANGPT2 in the brain.

Retinoic Acid Promotes Endothelial Cell Cycle Early G1 State to Enable Human Hemogenic Endothelial Cell Specification

Development of blood-forming (hemogenic) endothelial cells that give rise to hematopoietic stem and progenitor cells (HSPCs) is critical during embryogenesis to generate the embryonic and postnatal hematopoietic system. We previously demonstrated that the specification of murine hemogenic endothelial cells is promoted by retinoic acid (RA) signaling and requires downstream endothelial cell cycle control. Whether this mechanism is conserved in human hemogenic endothelial cell specification is unknown. Here, we present a protocol to derive primordial endothelial cells from human embryonic stem cells and promote their specification toward hemogenic endothelial cells. Furthermore, we demonstrate that RA treatment significantly increases human hemogenic endothelial cell specification. That is, RA promotes endothelial cell cycle arrest to enable RA-induced instructive signals to upregulate the genes needed for hematopoietic transition. These insights provide guidance for the ex vivo generation of autologous human hemogenic endothelial cells that are needed to produce human HSPCs for regenerative medicine applications.

Endothelial HIF-2α as a Key Endogenous Mediator Preventing Emphysema

Rationale: Endothelial injury may provoke emphysema, but molecular pathways of disease development require further discernment. Emphysematous lungs exhibit decreased expression of HIF-2α (hypoxia-inducible factor-2α)-regulated genes, and tobacco smoke decreases pulmonary HIF-2α concentrations. These findings suggest that decreased HIF-2α expression is important in the development of emphysema.Objectives: The objective of this study was to evaluate the roles of endothelial-cell (EC) HIF-2α in the pathogenesis of emphysema in mice.Methods: Mouse lungs were examined for emphysema after either the loss or the overexpression of EC Hif-2α. In addition, SU5416, a VEGFR2 inhibitor, was used to induce emphysema. Lungs were evaluated for HGF (hepatocyte growth factor), a protein involved in alveolar development and homeostasis. Lungs from patients with emphysema were measured for endothelial HIF-2α expression.Measurements and Main Results: EC Hif-2α deletion resulted in emphysema in association with fewer ECs and pericytes. After SU5416 exposure, EC Hif-2α-knockout mice developed more severe emphysema, whereas EC Hif-2α-overexpressing mice were protected. EC Hif-2α-knockout mice demonstrated lower levels of HGF. Human emphysema lung samples exhibited reduced EC HIF-2α expression.Conclusions: Here, we demonstrate a unique protective role for pulmonary endothelial HIF-2α and how decreased expression of this endogenous factor causes emphysema; its pivotal protective function is suggested by its ability to overcome VEGF antagonism. HIF-2α may maintain alveolar architecture by promoting vascular survival and associated HGF production. In summary, HIF-2α may be a key endogenous factor that prevents the development of emphysema, and its upregulation has the potential to foster lung health in at-risk patients.

A molecular map of murine lymph node blood vascular endothelium at single cell resolution

Blood vascular endothelial cells (BECs) control the immune response by regulating blood flow and immune cell recruitment in lymphoid tissues. However, the diversity of BEC and their origins during immune angiogenesis remain unclear. Here we profile transcriptomes of BEC from peripheral lymph nodes and map phenotypes to the vasculature. We identify multiple subsets, including a medullary venous population whose gene signature predicts a selective role in myeloid cell (vs lymphocyte) recruitment to the medulla, confirmed by videomicroscopy. We define five capillary subsets, including a capillary resident precursor (CRP) that displays stem cell and migratory gene signatures, and contributes to homeostatic BEC turnover and to neogenesis of high endothelium after immunization. Cell alignments show retention of developmental programs along trajectories from CRP to mature venous and arterial populations. Our single cell atlas provides a molecular roadmap of the lymph node blood vasculature and defines subset specialization for leukocyte recruitment and vascular homeostasis.

Retinoic Acid and Lymphotoxin Signaling Promote Differentiation of Human Intestinal M Cells

BACKGROUND & AIMS

Intestinal microfold (M) cells are a unique subset of intestinal epithelial cells in the Peyer's patches that regulate mucosal immunity, serving as portals for sampling and uptake of luminal antigens. The inability to efficiently develop human M cells in cell culture has impeded studies of the intestinal immune system. We aimed to identify signaling pathways required for differentiation of human M cells and establish a robust culture system using human ileum enteroids.

METHODS

We analyzed transcriptome data from mouse Peyer's patches to identify cell populations in close proximity to M cells. We used the human enteroid system to determine which cytokines were required to induce M-cell differentiation. We performed transcriptome, immunofluorescence, scanning electron microscope, and transcytosis experiments to validate the development of phenotypic and functional human M cells.

RESULTS

A combination of retinoic acid and lymphotoxin induced differentiation of glycoprotein 2-positive human M cells, which lack apical microvilli structure. Upregulated expression of innate immune-related genes within M cells correlated with a lack of viral antigens after rotavirus infection. Human M cells, developed in the enteroid system, internalized and transported enteric viruses, such as rotavirus and reovirus, across the intestinal epithelium barrier in the enteroids.

CONCLUSIONS

We identified signaling pathways required for differentiation of intestinal M cells, and used this information to create a robust culture method to develop human M cells with capacity for internalization and transport of viruses. Studies of this model might increase our understanding of antigen presentation and the systemic entry of enteric pathogens in the human intestine.

A Single-Cell Transcriptional Roadmap of the Mouse and Human Lymph Node Lymphatic Vasculature

Single-cell transcriptomics promise to revolutionize our understanding of the vasculature. Emerging computational methods applied to high-dimensional single-cell data allow integration of results between samples and species and illuminate the diversity and underlying developmental and architectural organization of cell populations. Here, we illustrate these methods in the analysis of mouse lymph node (LN) lymphatic endothelial cells (LEC) at single-cell resolution. Clustering identifies five well-delineated subsets, including two medullary sinus subsets not previously recognized as distinct. Nearest neighbor alignments in trajectory space position the major subsets in a sequence that recapitulates the known features and suggests novel features of LN lymphatic organization, providing a transcriptional map of the lymphatic endothelial niches and of the transitions between them. Differences in gene expression reveal specialized programs for (1) subcapsular ceiling endothelial interactions with the capsule connective tissue and cells; (2) subcapsular floor regulation of lymph borne cell entry into the LN parenchyma and antigen presentation; and (3) pathogen interactions and (4) LN remodeling in distinct medullary subsets. LEC of the subcapsular sinus floor and medulla, which represent major sites of cell entry and exit from the LN parenchyma respectively, respond robustly to oxazolone inflammation challenge with enriched signaling pathways that converge on both innate and adaptive immune responses. Integration of mouse and human single-cell profiles reveals a conserved cross-species pattern of lymphatic vascular niches and gene expression, as well as specialized human subsets and genes unique to each species. The examples provided demonstrate the power of single-cell analysis in elucidating endothelial cell heterogeneity, vascular organization, and endothelial cell responses. We discuss the findings from the perspective of LEC functions in relation to niche formations in the unique stromal and highly immunological environment of the LN.

Neutrophils Recirculate through Lymph Nodes to Survey Tissues for Pathogens

The adaptive immune function of lymph nodes is dependent on constant recirculation of lymphocytes. In this article, we identify neutrophils present in the lymph node at steady state, exhibiting the same capacity for recirculation. In germ-free mice, neutrophils still recirculate through lymph nodes, and in mice cohoused with wild microbiome mice, the level of neutrophils in lymph nodes increases significantly. We found that at steady state, neutrophils enter the lymph node entirely via L-selectin and actively exit via efferent lymphatics via an S1P dependent mechanism. The small population of neutrophils in the lymph node can act as reconnaissance cells to recruit additional neutrophils in the event of bacterial dissemination to the lymph node. Without these reconnaissance cells, there is a delay in neutrophil recruitment to the lymph node and a reduction in swarm formation following Staphylococcus aureus infection. This ability to recruit additional neutrophils by lymph node neutrophils is initiated by LTB4. This study establishes the capacity of neutrophils to recirculate, much like lymphocytes via L-selectin and high endothelial venules in lymph nodes and demonstrates how the presence of neutrophils at steady state fortifies the lymph node in case of an infection disseminating through lymphatics.

Exploration of Cell Development Pathways through High-Dimensional Single Cell Analysis in Trajectory Space

High-dimensional single cell profiling coupled with computational modeling is emerging as a powerful tool to elucidate developmental programs directing cell lineages. We introduce tSpace, an algorithm based on the concept of "trajectory space", in which cells are defined by their distance along nearest neighbor pathways to every other cell in a population. Graphical mapping of cells in trajectory space allows unsupervised reconstruction and exploration of complex developmental sequences. Applied to flow and mass cytometry data, the method faithfully reconstructs thymic T cell development and reveals development and trafficking regulation of tonsillar B cells. Applied to the single cell transcriptome of mouse intestine and C. elegans, the method recapitulates development from intestinal stem cells to specialized epithelial phenotypes more faithfully than existing algorithms and orders C. elegans cells concordantly to the associated embryonic time. tSpace profiling of complex populations is well suited for hypothesis generation in developing cell systems.

Endothelial Hypoxia-Inducible Factor-2α Is Required for the Maintenance of Airway Microvasculature

BACKGROUND

Hypoxia-inducible factors (HIFs), especially HIF-1α and HIF-2α, are key mediators of the adaptive response to hypoxic stress and play essential roles in maintaining lung homeostasis. Human and animal genetics studies confirm that abnormal HIF correlates with pulmonary vascular pathology and chronic lung diseases, but it remains unclear whether endothelial cell HIF production is essential for microvascular health. The large airway has an ideal circulatory bed for evaluating histological changes and physiology in genetically modified rodents.

METHODS

The tracheal microvasculature of mice, with conditionally deleted or overexpressed HIF-1α or HIF-2α, was evaluated for anatomy, perfusion, and permeability. Angiogenic signaling studies assessed vascular changes attributable to dysregulated HIF expression. An orthotopic tracheal transplantation model further evaluated the contribution of individual HIF isoforms in airway endothelial cells.

RESULTS

The genetic deletion of Hif-2α but not Hif-1α caused tracheal endothelial cell apoptosis, diminished pericyte coverage, reduced vascular perfusion, defective barrier function, overlying epithelial abnormalities, and subepithelial fibrotic remodeling. HIF-2α promoted microvascular integrity in airways through endothelial angiopoietin-1/TIE2 signaling and Notch activity. In functional tracheal transplants, HIF-2α deficiency in airway donors accelerated graft microvascular loss, whereas HIF-2α or angiopoietin-1 overexpression prolonged transplant microvascular perfusion. Augmented endothelial HIF-2α in transplant donors promoted airway microvascular integrity and diminished alloimmune inflammation.

CONCLUSIONS

Our findings reveal that the constitutive expression of endothelial HIF-2α is required for airway microvascular health.

Single-Cell Survey of Human Lymphatics Unveils Marked Endothelial Cell Heterogeneity and Mechanisms of Homing for Neutrophils

Lymphatic vessels form a critical component in the regulation of human health and disease. While their functional significance is increasingly being recognized, the comprehensive heterogeneity of lymphatics remains uncharacterized. Here, we report the profiling of 33,000 lymphatic endothelial cells (LECs) in human lymph nodes (LNs) by single-cell RNA sequencing. Unbiased clustering revealed six major types of human LECs. LECs lining the subcapsular sinus (SCS) of LNs abundantly expressed neutrophil chemoattractants, whereas LECs lining the medullary sinus (MS) expressed a C-type lectin CD209. Binding of a carbohydrate Lewis X (CD15) to CD209 mediated neutrophil binding to the MS. The neutrophil-selective homing by MS LECs may retain neutrophils in the LN medulla and allow lymph-borne pathogens to clear, preventing their spread through LNs in humans. Our study provides a comprehensive characterization of LEC heterogeneity and unveils a previously undefined role for medullary LECs in human immunity.

Aged blood impairs hippocampal neural precursor activity and activates microglia via brain endothelial cell VCAM1

An aged circulatory environment can activate microglia, reduce neural precursor cell activity, and impair cognition in mice. We hypothesized that brain endothelial cells (BECs) mediate at least some of these effects. We observe BECs in the aged mouse hippocampus express an inflammatory transcriptional profile with focal upregulation of Vascular Cell Adhesion Molecule 1 (VCAM1), a protein that facilitates vascular-immune cell interactions. Concomitantly, the shed, soluble form of VCAM1 is prominently increased in plasma of aged humans and mice, and their plasma is sufficient to increase VCAM1 expression in cultured BECs and young mouse hippocampi. Systemic anti-VCAM1 antibody or genetic ablation of VCAM1 in BECs counteracts the detrimental effects of aged plasma on young brains and reverses aging aspects including microglial reactivity and cognitive deficits in old mouse brains. Together, these findings establish brain endothelial VCAM1 at the blood-brain barrier (BBB) as a possible target to treat age-related neurodegeneration.

Chemerin Suppresses Breast Cancer Growth by Recruiting Immune Effector Cells Into the Tumor Microenvironment

Infiltration of immune cells into the tumor microenvironment (TME) can regulate growth and survival of neoplastic cells, impacting tumorigenesis and tumor progression. Correlations between the number of effector immune cells present in a tumor and clinical outcomes in many human tumors, including breast, have been widely described. Current immunotherapies utilizing checkpoint inhibitors or co-stimulatory molecule agonists aim to activate effector immune cells. However, tumors often lack adequate effector cell numbers within the TME, resulting in suboptimal responses to these agents. Chemerin (RARRES2) is a leukocyte chemoattractant widely expressed in many tissues and is known to recruit innate leukocytes. CMKLR1 is a chemotactic cellular receptor for chemerin and is expressed on subsets of dendritic cells, NK cells, and macrophages. We have previously shown that chemerin acts as a tumor suppressive cytokine in mouse melanoma models by recruiting innate immune defenses into the TME. Chemerin/RARRES2 is down-regulated in many tumors, including breast, compared to normal tissue counterparts. Here, using a syngeneic orthotopic EMT6 breast carcinoma model, we show that forced overexpression of chemerin by tumor cells results in significant recruitment of NK cells and T cells within the TME. While chemerin secretion by EMT6 cells did not alter their phenotypic behavior in vitro, it did significantly suppress tumor growth in vivo. To define the cellular effectors required for this anti-tumor phenotype, we depleted NK cells or CD8+ T cells and found that either cell type is required for chemerin-dependent suppression of EMT6 tumor growth. Finally, we show significantly reduced levels of RARRES2 mRNA in human breast cancer samples compared to matched normal tissues. Thus, for the first time we have shown that increasing chemerin expression within the breast carcinoma TME can suppress growth by recruitment of NK and T cells, thereby supporting this approach as a promising immunotherapeutic strategy.

Collagenase-based Single Cell Isolation of Primary Murine Brain Endothelial Cells Using Flow Cytometry

The brain endothelium is a highly specialized vascular structure that maintains the activity and integrity of the central nervous system (CNS). Previous studies have reported that the integrity of the brain endothelium is compromised in a plethora of neuropathologies. Therefore, it is of particular interest to establish a method that enables researchers to investigate and understand the molecular changes in CNS endothelial cells and underlying mechanisms in conjunction with murine models of disease. In the past, approaches to isolate endothelial cells have either involved the use of transgenic reporter mice or suffered from insufficiently pure cell populations and poor yield. This protocol here is based on well-established protocols that were modified and combined to allow single cell isolation of highly pure brain endothelial cell populations using fluorescence activated cell sorting (FACS). Briefly, after careful removal of the meninges and dissection of the cortex/hippocampus, the brain tissue is mechanically homogenized and enzymatically digested in two steps resulting in a single cell suspension. Cells are stained with a cocktail of fluorochrome-conjugated antibodies identifying not only brain endothelial cells, but also potentially contaminating cell types such as pericytes, astrocytes, and lineage cells. Using flow cytometry, cell populations are separated and sorted directly into either RNA lysis buffer for bulk RNA analyses (e.g., RNA microarray and RNA-Seq) or in pure fetal bovine serum to preserve viability for other downstream applications such as single cell RNA-Seq and Assay for Transposase-Accessible Chromatin using sequencing (ATAC-Seq). The protocol does not require the expression of a transgene to label brain endothelial cells and thus, may be applied to any mouse model. In our hands, the protocol has been highly reproducible with an average yield of 3 × 105 cells from a pool of four adult mice.

Gut-Selective Integrin-Targeted Therapies for Inflammatory Bowel Disease

Integrins are cell surface receptors with bidirectional signalling capabilities that can bind to adhesion molecules in order to mediate homing of leukocytes to peripheral tissues. Gut-selective leukocyte homing is facilitated by interactions between α4β7 and its ligand, mucosal addressin cellular adhesion molecule-1 [MAdCAM-1], while retention of lymphocytes in mucosal tissues is mediated by αEβ7 binding to its ligand E-cadherin. Therapies targeting gut-selective trafficking have shown efficacy in inflammatory bowel disease [IBD], confirming the importance of leukocyte trafficking in disease pathobiology. This review will provide an overview of integrin structure, function and signalling, and highlight the role that these molecules play in leukocyte homing and retention. Anti-integrin therapeutics, including gut-selective antibodies against the β7 integrin subunit [etrolizumab] and the α4β7 integrin heterodimer [vedolizumab and abrilumab], and the non-gut selective anti-α4 integrin [natalizumab], will be discussed, as well as novel targeting approaches using small molecules.

Single-cell analysis of early progenitor cells that build coronary arteries

Arteries and veins are specified by antagonistic transcriptional programs. However, during development and regeneration, new arteries can arise from pre-existing veins through a poorly understood process of cell fate conversion. Here, using single-cell RNA sequencing and mouse genetics, we show that vein cells of the developing heart undergo an early cell fate switch to create a pre-artery population that subsequently builds coronary arteries. Vein cells underwent a gradual and simultaneous switch from venous to arterial fate before a subset of cells crossed a transcriptional threshold into the pre-artery state. Before the onset of coronary blood flow, pre-artery cells appeared in the immature vessel plexus, expressed mature artery markers, and decreased cell cycling. The vein-specifying transcription factor COUP-TF2 (also known as NR2F2) prevented plexus cells from overcoming the pre-artery threshold by inducing cell cycle genes. Thus, vein-derived coronary arteries are built by pre-artery cells that can differentiate independently of blood flow upon the release of inhibition mediated by COUP-TF2 and cell cycle factors.

Mass Cytometric Analysis of HIV Entry, Replication, and Remodeling in Tissue CD4+ T Cells

To characterize susceptibility to HIV infection, we phenotyped infected tonsillar T cells by single-cell mass cytometry and created comprehensive maps to identify which subsets of CD4+ T cells support HIV fusion and productive infection. By comparing HIV-fused and HIV-infected cells through dimensionality reduction, clustering, and statistical approaches to account for viral perturbations, we identified a subset of memory CD4+ T cells that support HIV entry but not viral gene expression. These cells express high levels of CD127, the IL-7 receptor, and are believed to be long-lived lymphocytes. In HIV-infected patients, CD127-expressing cells preferentially localize to extrafollicular lymphoid regions with limited viral replication. Thus, CyTOF-based phenotyping, combined with analytical approaches to distinguish between selective infection and receptor modulation by viruses, can be used as a discovery tool.

A Chimeric Antibody against ACKR3/CXCR7 in Combination with TMZ Activates Immune Responses and Extends Survival in Mouse GBM Models

Glioblastoma (GBM) is the least treatable type of brain tumor, afflicting over 15,000 people per year in the United States. Patients have a median survival of 16 months, and over 95% die within 5 years. The chemokine receptor ACKR3 is selectively expressed on both GBM cells and tumor-associated blood vessels. High tumor expression of ACKR3 correlates with poor prognosis and potential treatment resistance, making it an attractive therapeutic target. We engineered a single chain FV-human FC-immunoglobulin G1 (IgG₁) antibody, X7Ab, to target ACKR3 in human and mouse GBM cells. We used hydrodynamic gene transfer to overexpress the antibody, with efficacy in vivo. X7Ab kills GBM tumor cells and ACKR3-expressing vascular endothelial cells by engaging the cytotoxic activity of natural killer (NK) cells and complement and the phagocytic activity of macrophages. Combining X7Ab with TMZ allows the TMZ dosage to be lowered, without compromising therapeutic efficacy. Mice treated with X7Ab and in combination with TMZ showed significant tumor reduction by MRI and longer survival overall. Brain-tumor-infiltrating leukocyte analysis revealed that X7Ab enhances the activation of M1 macrophages to support anti-tumor immune response in vivo. Targeting ACKR3 with immunotherapeutic monoclonal antibodies (mAbs) in combination with standard of care therapies may prove effective in treating GBM.

Papain-based Single Cell Isolation of Primary Murine Brain Endothelial Cells Using Flow Cytometry

Brain endothelial cells (BECs) form the integral component of the blood-brain barrier (BBB) which separates the systemic milieu from the brain parenchyma and protects the brain from pathogens and circulating factors. In order to study BEC biology, it was of particular interest to establish a method that enables researchers to investigate and understand the underlying molecular mechanisms regulating their function during homeostasis, aging and disease. Furthermore, due to the heterogeneity of the cerebrovasculature and different vessel types that comprise the BBB, it is of particular interest to isolate primary BECs for single cell analysis from various subregions of the brain, such as the neurogenic and highly vascularized hippocampus and to enrich for specific vessel types. In the past, approaches to isolate endothelial cells were dependent on transgenic mice and often resulted in insufficiently pure cell populations and poor yield. This protocol describes a technique that allows single-cell isolation of highly pure brain endothelial cell populations using fluorescence activated cell sorting (FACS). Briefly, after perfusion and careful removal of the meninges, and dissection of the cortex/hippocampus, the brain tissue is mechanically homogenized and enzymatically digested resulting in a single cell suspension. Cells are stained with fluorochrome-conjugated antibodies identifying CD31⁺ brain endothelial cells, as well as CD45⁺CD11b⁺ myeloid cells for exclusion. Using flow cytometry, cell populations are separated and CD31⁺BECs are sorted in bulk into RNA later or as single cells directly into either RNA lysis buffer for single or bulk RNA-Seq analyses. The protocol does not require the expression of a transgene to label brain endothelial cells and thus, may be applied to any mouse model. In our hands, the protocol has been highly reproducible with an average yield of 1 × 10⁵ cells isolated from an adult mouse cortex/hippocampus.

Human CD22 Inhibits Murine B Cell Receptor Activation in a Human CD22 Transgenic Mouse Model

CD22, a sialic acid-binding Ig-type lectin (Siglec) family member, is an inhibitory coreceptor of the BCR with established roles in health and disease. The restricted expression pattern of CD22 on B cells and most B cell lymphomas has made CD22 a therapeutic target for B cell-mediated diseases. Models to better understand how in vivo targeting of CD22 translates to human disease are needed. In this article, we report the development of a transgenic mouse expressing human CD22 (hCD22) in B cells and assess its ability to functionally substitute for murine CD22 (mCD22) for regulation of BCR signaling, Ab responses, homing, and tolerance. Expression of hCD22 on transgenic murine B cells is comparable to expression on human primary B cells, and it colocalizes with mCD22 on the cell surface. Murine B cells expressing only hCD22 have identical calcium (Ca²⁺) flux responses to anti-IgM as mCD22-expressing wild-type B cells. Furthermore, hCD22 transgenic mice on an mCD22^-/- background have restored levels of marginal zone B cells and Ab responses compared with deficiencies observed in CD22^-/- mice. Consistent with these observations, hCD22 transgenic mice develop normal humoral responses in a peanut allergy oral sensitization model. Homing of B cells to Peyer's patches was partially rescued by expression of hCD22 compared with CD22^-/- B cells, although not to wild-type levels. Notably, Siglec-engaging antigenic liposomes formulated with an hCD22 ligand were shown to prevent B cell activation, increase cell death, and induce tolerance in vivo. This hCD22 transgenic mouse will be a valuable model for investigating the function of hCD22 and preclinical studies targeting hCD22.

A Mucosal and Cutaneous Chemokine Ligand for the Lymphocyte Chemoattractant Receptor GPR15

Chemoattractants control lymphocyte recruitment from the blood, contributing to the systemic organization of the immune system. The G protein-linked receptor GPR15 mediates lymphocyte homing to the large intestines and skin. Here we show that the 9 kDa CC-motif containing cationic polypeptide AP57/colon-derived sushi containing domain-2 binding factor (CSBF), encoded by C10orf99 in the human and 2610528A11Rik in the mouse, functions as a chemokine ligand for GPR15 (GPR15L). GPR15L binds GPR15 and attracts GPR15-expressing T cells including lymphocytes in colon-draining lymph nodes and Vγ3⁺ thymic precursors of dermal epithelial T cells. Patterns of GPR15L expression by epithelial cells in adult mice and humans suggest a homeostatic role for the chemokine in lymphocyte localization to the large intestines, as well as a role in homing to the epidermis during wound healing or inflammation. GPR15L is also significantly expressed in squamous mucosa of the oral cavity and esophagus with still poorly defined regulation. Identification of the chemotactic activity of GPR15L adds to its reported antibacterial and tumor cell growth regulatory functions and suggests the potential of targeting GPR15L–GPR15 interactions for modulation of mucosal and cutaneous inflammation.

Vitamin D immunoregulation through dendritic cells

Vitamin D (VD3) has been linked to immunological processes, and its supplementation may have a role in treatment or prevention of diseases with underlying autoimmune or pro‐inflammatory states. As initiators of the immune responses, dendritic cells (DC) are a potential target of VD3 to dampen autoimmunity and inflammation, but the role of DC in VD3‐mediated immunomodulation in vivo is not understood. In addition to being targets of VD3, DC can provide a local source of bioactive VD3 for regulation of T‐cell responses. Here we review existing studies that describe the tolerogenic potential of VD3 on DC, and discuss them in the context of current understanding of DC development and function. We speculate on mechanisms that might account for the potent but poorly understood tolerogenic activities of VD3 and the role of DC as both targets and sources of this hormone.

Trafficking receptor signatures define blood plasmablasts responding to tissue-specific immune challenge

Antibody-secreting cells are generated in regional lymphoid tissues and traffic as plasmablasts (PBs) via lymph and blood to target sites for local immunity. We used multiparameter flow cytometry to define PB trafficking programs (TPs, combinations of adhesion molecules and chemoattractant receptors) and their imprinting in patients in response to localized infection or immune insults. TPs enriched after infection or autoimmune inflammation of mucosae correlate with sites of immune response or symptoms, with different TPs imprinted during small intestinal, colon, throat, and upper respiratory immune challenge. PBs induced after intramuscular or intradermal influenza vaccination, including flu-specific antibody-secreting cells, display TPs characterized by the lack of mucosal homing receptors. PBs of healthy donors display diverse mucosa-associated TPs, consistent with homeostatic immune activity. Identification of TP signatures of PBs may facilitate noninvasive monitoring of organ-specific immune responses.

Leukocyte Trafficking to the Small Intestine and Colon

Leukocyte trafficking to the small and large intestines is tightly controlled to maintain intestinal immune homeostasis, mediate immune responses, and regulate inflammation. A wide array of chemoattractants, chemoattractant receptors, and adhesion molecules expressed by leukocytes, mucosal endothelium, epithelium, and stromal cells controls leukocyte recruitment and microenvironmental localization in intestine and in the gut-associated lymphoid tissues (GALTs). Naive lymphocytes traffic to the gut-draining mesenteric lymph nodes where they undergo antigen-induced activation and priming; these processes determine their memory/effector phenotypes and imprint them with the capacity to migrate via the lymph and blood to the intestines. Mechanisms of T-cell recruitment to GALT and of T cells and plasmablasts to the small intestine are well described. Recent advances include the discovery of an unexpected role for lectin CD22 as a B-cell homing receptor GALT, and identification of the orphan G-protein-coupled receptor 15 (GPR15) as a T-cell chemoattractant/trafficking receptor for the colon. GPR15 decorates distinct subsets of T cells in mice and humans, a difference in species that could affect translation of the results of mouse colitis models to humans. Clinical studies with antibodies to integrin α4β7 and its vascular ligand mucosal vascular addressin cell adhesion molecule 1 are proving the value of lymphocyte trafficking mechanisms as therapeutic targets for inflammatory bowel diseases. In contrast to lymphocytes, cells of the innate immune system express adhesion and chemoattractant receptors that allow them to migrate directly to effector tissue sites during inflammation. We review the mechanisms for innate and adaptive leukocyte localization to the intestinal tract and GALT, and discuss their relevance to human intestinal homeostasis and inflammation.

Generation and transcriptional programming of intestinal dendritic cells: essential role of retinoic acid

The vitamin A metabolite retinoic acid (RA) regulates adaptive immunity in the intestines, with well-characterized effects on IgA responses, Treg induction, and gut trafficking of T- and B-effector cells. It also controls the generation of conventional dendritic cell (cDC) precursors in the bone marrow and regulates cDC subset representation, but its roles in the specialization of intestinal cDC subsets are understudied. Here we show that RA acts cell intrinsically in developing gut-tropic pre-mucosal dendritic cell (pre-μDC) to effect the differentiation and drive the specialization of intestinal CD103(+)CD11b(-) (cDC1) and of CD103(+)CD11b(+) (cDC2). Systemic deficiency or DC-restricted antagonism of RA signaling resulted in altered phenotypes of intestinal cDC1 and cDC2, and reduced numbers of cDC2. Effects of dietary deficiency were most apparent in the proximal small intestine and were rapidly reversed by reintroducing vitamin A. In cultures of pre-μDC with Flt3L and granulocyte-macrophage colony-stimulating factor (GM-CSF), RA induced cDC with characteristic phenotypes of intestinal cDC1 and cDC2 by controlling subset-defining cell surface receptors, regulating subset-specific transcriptional programs, and suppressing proinflammatory nuclear factor-κB-dependent gene expression. Thus, RA is required for transcriptional programming and maturation of intestinal cDC, and with GM-CSF and Flt3L provides a minimal environment for in vitro generation of intestinal cDC1- and cDC2-like cDC from specialized precursors.

Evaluation of Tumor-infiltrating Leukocyte Subsets in a Subcutaneous Tumor Model

Specialized immune cells that infiltrate the tumor microenvironment regulate the growth and survival of neoplasia.  Malignant cells must elude or subvert anti-tumor immune responses in order to survive and flourish. Tumors take advantage of a number of different mechanisms of immune "escape," including the recruitment of tolerogenic DC, immunosuppressive regulatory T cells (Tregs), and myeloid-derived suppressor cells (MDSC) that inhibit cytotoxic anti-tumor responses. Conversely, anti-tumor effector immune cells can slow the growth and expansion of malignancies: immunostimulatory dendritic cells, natural killer cells which harbor innate anti-tumor immunity, and cytotoxic T cells all can participate in tumor suppression. The balance between pro- and anti-tumor leukocytes ultimately determines the behavior and fate of transformed cells; a multitude of human clinical studies have borne this out. Thus, detailed analysis of leukocyte subsets within the tumor microenvironment has become increasingly important. Here, we describe a method for analyzing infiltrating leukocyte subsets present in the tumor microenvironment in a mouse tumor model. Mouse B16 melanoma tumor cells were inoculated subcutaneously in C57BL/6 mice. At a specified time, tumors and surrounding skin were resected en bloc and processed into single cell suspensions, which were then stained for multi-color flow cytometry. Using a variety of leukocyte subset markers, we were able to compare the relative percentages of infiltrating leukocyte subsets between control and chemerin-expressing tumors. Investigators may use such a tool to study the immune presence in the tumor microenvironment and when combined with traditional caliper size measurements of tumor growth, will potentially allow them to elucidate the impact of changes in immune composition on tumor growth. Such a technique can be applied to any tumor model in which the tumor and its microenvironment can be resected and processed.

Role and species-specific expression of colon T cell homing receptor GPR15 in colitis

Lymphocyte recruitment maintains intestinal immune homeostasis but also contributes to inflammation. The orphan chemoattractant receptor GPR15 mediates regulatory T cell homing and immunosuppression in the mouse colon. We show that GPR15 is also expressed by mouse T_H17 and T_H1 effector cells, and is required for colitis in a model that depends on their trafficking to the colon. In humans GPR15 is expressed by effector cells including pathogenic T_H2 cells in ulcerative colitis, but is not expressed by regulatory T (T_reg) cells. The T_H2 transcriptional activator GATA-3 and the T_reg–associated transcriptional repressor FOXP3 robustly bind human, but not mouse, GPR15 enhancer sequences, correlating with expression. Our results highlight species differences in GPR15 regulation, and suggest it as a potential therapeutic target for colitis.

A novel CMKLR1 small molecule antagonist suppresses CNS autoimmune inflammatory disease

Therapies that target leukocyte trafficking pathways can reduce disease activity and improve clinical outcomes in multiple sclerosis (MS). Experimental autoimmune encephalomyelitis (EAE) is a widely studied animal model that shares many clinical and histological features with MS. Chemokine-like receptor-1 (CMKLR1) is a chemoattractant receptor that is expressed by key effector cells in EAE and MS, including macrophages, subsets of dendritic cells, natural killer cells and microglia. We previously showed that CMKLR1-deficient (CMKLR1 KO) mice develop less severe clinical and histological EAE than wild-type mice. In this study, we sought to identify CMKLR1 inhibitors that would pharmaceutically recapitulate the CMKLR1 KO phenotype in EAE. We identified 2-(α-naphthoyl) ethyltrimethylammonium iodide (α-NETA) as a CMKLR1 small molecule antagonist that inhibits chemerin-stimulated β-arrestin2 association with CMKLR1, as well as chemerin-triggered CMKLR1+ cell migration. α-NETA significantly delayed the onset of EAE induced in C57BL/6 mice by both active immunization with myelin oligodendrocyte glycoprotein peptide 35-55 and by adoptive transfer of encephalitogenic T cells. In addition, α-NETA treatment significantly reduced mononuclear cell infiltrates within the CNS. This study provides additional proof-of-concept data that targeting CMKLR1:chemerin interactions may be beneficial in preventing or treating MS.

Leukocyte chemoattractant receptors in human disease pathogenesis

Combinations of leukocyte attractant ligands and cognate heptahelical receptors specify the systemic recruitment of circulating cells by triggering integrin-dependent adhesion to endothelial cells, supporting extravasation, and directing specific intratissue localization via gradient-driven chemotaxis. Chemoattractant receptors also control leukocyte egress from lymphoid organs and peripheral tissues. In this article, we summarize the fundamental mechanics of leukocyte trafficking, from the evolution of multistep models of leukocyte recruitment and navigation to the regulation of chemoattractant availability and function by atypical heptahelical receptors. To provide a more complete picture of the migratory circuits involved in leukocyte trafficking, we integrate a number of nonchemokine chemoattractant receptors into our discussion. Leukocyte chemoattractant receptors play key roles in the pathogenesis of autoimmune diseases, allergy, inflammatory disorders, and cancer. We review recent advances in our understanding of chemoattractant receptors in disease pathogenesis, with a focus on genome-wide association studies in humans and the translational implications of mechanistic studies in animal disease models.

Absence of Nkx2-3 homeodomain transcription factor reprograms the endothelial addressin preference for lymphocyte homing in Peyer's patches

Although the homing of lymphocytes to GALT has been extensively studied, little is known about how high endothelial venules (HEVs) within Peyer's patches (PPs) are patterned to display dominantly mucosal addressin cell adhesion molecule 1 (MAdCAM-1). In this study, we report that Nkx2-3-deficient mice show gradual loss of MAdCAM-1 in PPs postnatally and increased levels of mRNA for peripheral lymph node addressin (PNAd) backbone proteins as well as enhanced expression of MECA79 sulfated glycoepitope at the luminal aspect of HEVs, thus replacing MAdCAM-1 with PNAd. Induction of PNAd in mutant PPs requires lymphotoxin β receptor activity, and its upregulation needs the presence of mature T and B cells. Furthermore, treatment with MECA-79 anti-PNAd mAb in vivo effectively blocks lymphocyte homing to mutant PPs. Despite the replacement of MAdCAM-1 by PNAd in HEV endothelia, lymphocytes could efficiently home to PPs in mutant mice. We conclude that although Nkx2-3 activity controls the addressin balance of HEVs in GALT, the general HEV functionality is preserved independently from Nkx2-3, indicating a substantial plasticity in the specification of GALT HEV endothelium.

Comparative transcriptional and functional profiling defines conserved programs of intestinal DC differentiation in humans and mice

Dendritic cells (DCs) that orchestrate mucosal immunity have been studied in mice. Here we characterize human gut DC populations, and define their relationship to previously studied human and mouse DCs. CD103⁺Sirpα⁻ DCs were related to human blood CD141⁺ and to mouse intestinal CD103⁺CD11b⁻ DCs and expressed markers of cross-presenting DCs. CD103⁺Sirpα⁺ DCs aligned with human blood CD1c⁺ DCs and mouse intestinal CD103⁺CD11b⁺ DCs and supported regulatory T cell induction. Both CD103⁺ DC subsets induced T_H17 cells, while CD103⁻Sirpα⁺ DCs induced T_H1 cells. Comparative transcriptomics revealed conserved transcriptional programs among CD103⁺ DC subsets and uncovered a selective role for Bcl-6 and Blimp-1 in CD103⁺Sirpα⁻ and intestinal CD103⁺CD11b⁺ DC specification, respectively. These results highlight evolutionarily conserved and divergent programming of intestinal DCs.

Thymus-homing dendritic cells in central tolerance

Central tolerance is critical in establishing a peripheral T-cell repertoire purged of functional autoreactive T cells. One of the major requirements for effective central tolerance is the presentation of self and other innocuous antigens (Ags), including food, gut flora, or airway allergens, to developing T cells in the thymus. This seemingly challenging task can be mediated in some cases by ectopic expression of tissue-specific Ags by thymic epithelial cells or by entry of systemic blood-borne Ags into the thymus. More recently, thymic homing peripheral dendritic cells (DCs) have been proposed as cellular transporters of peripheral tissue-specific Ags or foreign innocuous Ags. The aim of this viewpoint is to discuss the three principal thymic DC populations and their trafficking properties in the context of central tolerance. We will first discuss the importance of peripheral DC trafficking to the thymus and then compare and contrast the three DC subsets. We will describe how they were characterized, describe their trafficking to and their microenvironmental positioning in the thymus, and discuss the functional consequence of thymic trafficking and localization on thymic selection events.

Canonical and noncanonical Wnt proteins program dendritic cell responses for tolerance

Ag-presenting dendritic cells (DCs) interpret environmental signals to orchestrate local and systemic immune responses. They govern the balance between tolerance and inflammation at epithelial surfaces, where the immune system must provide robust pathogen responses while maintaining tolerance to commensal flora and food Ags. The Wnt family of secreted proteins, which control epithelial and hematopoietic development and homeostasis, is emerging as an important regulator of inflammation. In this study, we show that canonical and noncanonical Wnts directly stimulate murine DC production of anti-inflammatory cytokines. Wnt3A triggers canonical β-catenin signaling and preferentially induces DC TGF-β and VEGF production, whereas Wnt5A induces IL-10 through alternative pathways. The Wnts also alter DC responses to microbe- or pathogen-associated molecular patterns, inhibiting proinflammatory cytokine induction in response to TLR ligands and promoting DC generation of Foxp3(+) regulatory T cells. Moreover, although both Wnts suppress proinflammatory responses to bacterial endotoxin and to TLR1/2, TLR7, and TLR9 ligands, Wnt5A, but not Wnt3A, inhibits IL-6 production in response to the viral mimic, polyinosinic:polycytidylic acid. Thus, Wnt family members directly and differentially regulate DC functions, an ability that may contribute to the balance between tolerance and inflammation at epithelial sites of exposure to microbes and environmental Ags.

Plasmacytoid dendritic cells promote rotavirus-induced human and murine B cell responses

B cell-dependent immunity to rotavirus, an important intestinal pathogen, plays a significant role in viral clearance and protects against reinfection. Human in vitro and murine in vivo models of rotavirus infection were used to delineate the role of primary plasmacytoid DCs (pDCs) in initiating B cell responses. Human pDCs were necessary and sufficient for B cell activation induced by rotavirus. Type I IFN recognition by B cells was essential for rotavirus-mediated B cell activation in vitro and murine pDCs and IFN-α/β-mediated B cell activation after in vivo intestinal rotavirus infection. Furthermore, rotavirus-specific serum and mucosal antibody responses were defective in mice lacking functional pDCs at the time of infection. These data demonstrate that optimal B cell activation and virus-specific antibody secretion following mucosal infection were a direct result of pDC-derived type I IFN. Importantly, viral shedding significantly increased in pDC-deficient mice, suggesting that pDC-dependent antibody production influences viral clearance. Thus, mucosal pDCs critically influence the course of rotavirus infection through rotavirus recognition and subsequent IFN production and display powerful adjuvant properties to initiate and enhance humoral immunity.

Chemerin is an antimicrobial agent in human epidermis

Chemerin, a chemoattractant ligand for chemokine-like receptor 1 (CMKLR1) is predicted to share similar tertiary structure with antibacterial cathelicidins. Recombinant chemerin has antimicrobial activity. Here we show that endogenous chemerin is abundant in human epidermis, and that inhibition of bacteria growth by exudates from organ cultures of primary human skin keratinocytes is largely chemerin-dependent. Using a panel of overlapping chemerin-derived synthetic peptides, we demonstrate that the antibacterial activity of chemerin is primarily mediated by Val⁶⁶-Pro⁸⁵, which causes direct bacterial lysis. Therefore, chemerin is an antimicrobial agent in human skin.

Plasmablast frequency and trafficking receptor expression are altered in pediatric ulcerative colitis

BACKGROUND

The incidence of pediatric ulcerative colitis (UC), a chronic autoinflammatory disease of the colon, is on the rise. Although an increased infiltration of B cells from the peripheral blood into the colon occurs in UC, B-cell trafficking is understudied. We hypothesized that the frequency of circulating plasmablasts (PBs) and their trafficking receptor (TR) expression may be indicative of the location and degree of pathology in pediatric UC.

METHODS

We conducted multicolor flow cytometry analyses of circulating IgA(+/-) PBs and IgA(+) memory B cells (MBCs) in pediatric UC patients with remission, mild, moderate, and severe state of disease (n = 12), and healthy pediatric (n = 2) and adult donors (n = 11).

RESULTS

Compared to healthy donors the average frequency of PBs among total peripheral blood lymphocytes is increased 30-fold during severe UC activity, and positively correlates with Pediatric Ulcerative Colitis Activity Index score, C-reactive protein level, and erythrocyte sedimentation rate. A greater percent of PBs in severe patients express the gut-homing receptors α4β7 and CCR10, and the inflammatory homing molecule P-selectin ligand (P-sel lig). The percent of IgA(+) MBCs expressing α4β7, however, is reduced. Furthermore, expression of the small intestine TR CCR9 is decreased on α4β7(high) PBs, and on α4β7(high) /CCR10(high) PBs and MBCs in these patients, consistent with preferential cell targeting to the colon.

CONCLUSIONS

Peripheral blood PBs with a colon-homing phenotype (α4β7/CCR10/P-sel lig) are elevated in children with severe UC. Screening this B-cell subset may provide a complementary approach in monitoring disease activity or therapeutic efficacy in pediatric UC.

The chemoattractant chemerin suppresses melanoma by recruiting natural killer cell antitumor defenses

Chemerin recruits NK cells to suppress melanoma growth.

Infiltration of specialized immune cells regulates the growth and survival of neoplasia. Here, in a survey of public whole genome expression datasets we found that the gene for chemerin, a widely expressed endogenous chemoattractant protein, is down-regulated in melanoma as well as other human tumors. Moreover, high chemerin messenger RNA expression in tumors correlated with improved outcome in human melanoma. In experiments using the B16 transplantable mouse melanoma, tumor-expressed chemerin inhibited in vivo tumor growth without altering in vitro proliferation. Growth inhibition was associated with an altered profile of tumor-infiltrating cells with an increase in natural killer (NK) cells and a relative reduction in myeloid-derived suppressor cells and putative immune inhibitory plasmacytoid dendritic cells. Tumor inhibition required host expression of CMKLR1 (chemokine-like receptor 1), the chemoattractant receptor for chemerin, and was abrogated by NK cell depletion. Intratumoral injection of chemerin also inhibited tumor growth, suggesting the potential for therapeutic application. These results show that chemerin, whether expressed by tumor cells or within the tumor environment, can recruit host immune defenses that inhibit tumorigenesis and suggest that down-regulation of chemerin may be an important mechanism of tumor immune evasion.

Janus-like opposing roles of CD47 in autoimmune brain inflammation in humans and mice

CD47 exerts different effects on disease in distinct cell types and locations and during different stages of experimental autoimmune encephalomyelitis.

Comparison of transcriptomic and proteomic data from pathologically similar multiple sclerosis (MS) lesions reveals down-regulation of CD47 at the messenger RNA level and low abundance at the protein level. Immunohistochemical studies demonstrate that CD47 is expressed in normal myelin and in foamy macrophages and reactive astrocytes within active MS lesions. We demonstrate that CD47^−/− mice are refractory to experimental autoimmune encephalomyelitis (EAE), primarily as the result of failure of immune cell activation after immunization with myelin antigen. In contrast, blocking with a monoclonal antibody against CD47 in mice at the peak of paralysis worsens EAE severity and enhances immune activation in the peripheral immune system. In vitro assays demonstrate that blocking CD47 also promotes phagocytosis of myelin and that this effect is dependent on signal regulatory protein α (SIRP-α). Immune regulation and phagocytosis are mechanisms for CD47 signaling in autoimmune neuroinflammation. Depending on the cell type, location, and disease stage, CD47 has Janus-like roles, with opposing effects on EAE pathogenesis.

Expression, regulation, and function of atypical chemerin receptor CCRL2 on endothelial cells

Chemokine (CC motif) receptor-like 2 (CCRL2) binds leukocyte chemoattractant chemerin and can regulate local levels of the attractant, but does not itself support cell migration. In this study, we show that CCRL2 and VCAM-1 are upregulated on cultured human and mouse vascular endothelial cells (EC) and cell lines by proinflammatory stimuli. CCRL2 induction is dependent on NF-κB and JAK/STAT signaling pathways, and activated endothelial cells specifically bind chemerin. In vivo, CCRL2 is constitutively expressed at high levels by lung endothelial cells and at lower levels by liver endothelium; and liver but not lung EC respond to systemic LPS injection by further upregulation of the receptor. Plasma levels of total chemerin are elevated in CCRL2(-/-) mice and are significantly enhanced after systemic LPS treatment in CCRL2(-/-) mice compared with wild-type mice. Following acute LPS-induced pulmonary inflammation in vivo, chemokine-like receptor 1 (CMKLR1)(+) NK cell recruitment to the airways is significantly impaired in CCRL2(-/-) mice compared with wild-type mice. In vitro, chemerin binding to CCRL2 on endothelial cells triggers robust adhesion of CMKLR1(+) lymphoid cells through an α(4)β(1) integrin/VCAM-1-dependent mechanism. In conclusion, CCRL2 is expressed by EC in a tissue- and activation-dependent fashion, regulates circulating chemerin levels and its bioactivity, and enhances chemerin- and CMKLR1-dependent lymphocyte/EC adhesion in vitro and recruitment to inflamed airways in vivo. Its expression and/or induction on EC by proinflammatory stimuli provide a novel and specific mechanism for the local enrichment of chemerin at inflammatory sites, regulating the recruitment of CMKLR1(+) cells.

Plasmacytoid dendritic cells transport peripheral antigens to the thymus to promote central tolerance

Central tolerance can be mediated by peripheral dendritic cells (DCs) that transport innocuous antigens (Ags) to the thymus for presentation to developing T cells, but the responsible DC subsets remained poorly defined. Immature plasmacytoid DCs (pDCs) express CCR9, a chemokine receptor involved in migration of T cell precursors to the thymus. We show here that CCR9 mediated efficient thymic entry of endogenous or i.v. transfused pDCs. pDCs activated by Toll-like receptor (TLR) ligands downregulated CCR9 and lost their ability to home to the thymus. Moreover, endogenous pDCs took up subcutaneously injected fluorescent Ag and, in the absence of TLR signals, transported Ag to the thymus in a CCR9-dependent fashion. Injected, Ag-loaded pDCs effectively deleted Ag-specific thymocytes, and this thymic clonal deletion required CCR9-mediated homing and was prevented by infectious signals. Thus, peripheral pDCs can contribute to immune tolerance through CCR9-dependent transport of peripheral Ags and subsequent deletion of Ag-reactive thymocytes.

Chemokine-like receptor 1 regulates skeletal muscle cell myogenesis

The chemokine-like receptor-1 (CMKLR1) is a G protein-coupled receptor that is activated by chemerin, a secreted plasma leukocyte attractant and adipokine. Previous studies identified that CMKLR1 is expressed in skeletal muscle in a stage-specific fashion during embryogenesis and in adult mice; however, its function in skeletal muscle remains unclear. Based on the established function of CMKLR1 in cell migration and differentiation, we investigated the hypothesis that CMKLR1 regulates the differentiation of myoblasts into myotubes. In C(2)C(12) mouse myoblasts, CMKLR1 expression increased threefold with differentiation into multinucleated myotubes. Decreasing CMKLR1 expression by adenoviral-delivered small-hairpin RNA (shRNA) impaired the differentiation of C(2)C(12) myoblasts into mature myotubes and reduced the mRNA expression of myogenic regulatory factors myogenin and MyoD while increasing Myf5 and Mrf4. At embryonic day 12.5 (E12.5), CMKLR1 knockout (CMKLR1(-/-)) mice appeared developmentally delayed and displayed significantly lower wet weights and a considerably diminished myotomal component of somites as revealed by immunolocalization of myosin heavy chain protein compared with wild-type (CMKLR1(+/+)) mouse embryos. These changes were associated with increased Myf5 and decreased MyoD protein expression in the somites of E12.5 CMKLR1(-/-) mouse embryos. Adult male CMKLR1(-/-) mice had significantly reduced bone-free lean mass and weighed less than the CMKLR1(+/+) mice. We conclude that CMKLR1 is essential for myogenic differentiation of C(2)C(12) cells in vitro, and the CMKLR1 null mice have a subtle skeletal muscle deficit beginning from embryonic life that persists during postnatal life.

Disruption of the chemokine-like receptor-1 (CMKLR1) gene is associated with reduced adiposity and glucose intolerance

Adipose tissue secretes a variety of bioactive signaling molecules, termed adipokines, which regulate numerous biological functions including appetite, energy balance, glucose homeostasis, and inflammation. Chemerin is a novel adipokine that regulates adipocyte differentiation and metabolism by binding to and activating the G protein-coupled receptor, chemokine like receptor-1 (CMKLR1). In the present study, we investigated the impact of CMKLR1 deficiency on adipose development, glucose homeostasis, and inflammation in vivo. Herein we report that regardless of diet (low or high fat), CMKLR1(-/-) mice had lower food consumption, total body mass, and percent body fat compared with wild-type controls. CMKLR1(-/-) mice also exhibited decreased hepatic and white adipose tissue TNFα and IL-6 mRNA levels coincident with decreased hepatic dendritic cell infiltration, decreased adipose CD3+ T cells, and increased adipose natural killer cells. CMKLR1(-/-) mice were glucose intolerant compared with wild-type mice, and this was associated with decreased glucose stimulated insulin secretion as well as decreased skeletal muscle and white adipose tissue glucose uptake. Collectively these data provide compelling evidence that CMKLR1 influences adipose tissue development, inflammation, and glucose homeostasis and may contribute to the metabolic derangement characteristic of obesity and obesity-related diseases.

A disintegrin and metalloproteinase 10 regulates antibody production and maintenance of lymphoid architecture

A disintegrin and metalloproteinase 10 (ADAM10) is a zinc-dependent proteinase related to matrix metalloproteinases. ADAM10 has emerged as a key regulator of cellular processes by cleaving and shedding extracellular domains of multiple transmembrane receptors and ligands. We have developed B cell-specific ADAM10-deficient mice (ADAM10(B-/-)). In this study, we show that ADAM10 levels are significantly enhanced on germinal center B cells. Moreover, ADAM10(B-/-) mice had severely diminished primary and secondary responses after T-dependent immunization. ADAM10(B-/-) displayed impaired germinal center formation, had fewer follicular Th cells, decreased follicular dendritic cell networks, and altered chemokine expression in draining lymph nodes (LNs). Interestingly, when spleen and LN structures from immunized mice were analyzed for B and T cell localization, tissues structure was aberrant in ADAM10(B-/-) mice. Importantly, when ADAM10-deficient B cells were stimulated in vitro, they produced comparable Ab as wild type B cells. This result demonstrates that the defects in humoral responses in vivo result from inadequate B cell activation, likely because of the decrease in follicular Th cells and the changes in structure. Thus, ADAM10 is essential for the maintenance of lymphoid structure after Ag challenge.