Distinct CCR7 glycosylation pattern shapes receptor signaling and endocytosis to modulate chemotactic responses

Regulation of sialic acid metabolism in cancer

Sialic acid, the terminal structure of cell surface glycans, has essential functions in regulating immune response, cell-to-cell communication, and cell adhesion. More importantly, an increased level of sialic acid, termed hypersialylation, has emerged as a commonly observed phenotype in cancer. Therefore, targeting sialic acid ligands (sialoglycans) and their receptors (Siglecs) may provide a new therapeutic approach for cancer immunotherapy. We highlight the complexity of the sialic acid metabolism and its involvement in malignant transformation within individual cancer subtypes. In this review, we focus on the dysregulation of sialylation, the intricate nature of sialic acid synthesis, and clinical perspective. We aim to provide a brief insight into the mechanism of hypersialylation and how our understanding of these processes can be leveraged for the development of novel therapeutics.

Revisiting the immunopathology of congenital disorders of glycosylation: an updated review

Glycosylation is a critical post-translational modification that plays a pivotal role in several biological processes, such as the immune response. Alterations in glycosylation can modulate the course of various pathologies, such as the case of congenital disorders of glycosylation (CDG), a group of more than 160 rare and complex genetic diseases. Although the link between glycosylation and immune dysfunction has already been recognized, the immune involvement in most CDG remains largely unexplored and poorly understood. In this study, we provide an update on the immune dysfunction and clinical manifestations of the 12 CDG with major immune involvement, organized into 6 categories of inborn errors of immunity according to the International Union of Immunological Societies (IUIS). The immune involvement in phosphomannomutase 2 (PMM2)-CDG - the most frequent CDG - was comprehensively reviewed, highlighting a higher prevalence of immune issues during infancy and childhood and in R141H-bearing genotypes. Finally, using PMM2-CDG as a model, we point to links between abnormal glycosylation patterns in host cells and possibly favored interactions with microorganisms that may explain the higher susceptibility to infection. Further characterizing immunopathology and unusual host-pathogen adhesion in CDG can not only improve immunological standards of care but also pave the way for innovative preventive measures and targeted glycan-based therapies that may improve quality of life for people living with CDG.

Naive T lymphocytes chemotax long distance to CCL21 but not to a source of bioactive S1P

Naive T lymphocytes traffic through the organism in search for antigen, alternating between blood and secondary lymphoid organs. Lymphocyte homing to lymph nodes relies on CCL21 chemokine sensing by CCR7 receptors, while exit into efferent lymphatics relies on sphingolipid S1P sensing by S1PR1 receptors. While both molecules are claimed chemotactic, a quantitative analysis of naive T lymphocyte migration along defined gradients is missing. Here, we used a reductionist approach to study the real-time single-cell response of naive T lymphocytes to CCL21 and serum rich in bioactive S1P. Using microfluidic and micropatterning ad hoc tools, we show that CCL21 triggers stable polarization and long-range chemotaxis of cells, whereas S1P-rich serum triggers a transient polarization only and no significant displacement, potentially representing a brief transmigration step through exit portals. Our in vitro data thus suggest that naive T lymphocyte chemotax long distances to CCL21 but not toward a source of bioactive S1P.

Reducing Sialylation Enhances Electrotaxis of Corneal Epithelial Cells

Corneal wound healing is a complex biological process that integrates a host of different signals to coordinate cell behavior. Upon wounding, there is the generation of an endogenous wound electric field that serves as a powerful cue to guide cell migration. Concurrently, the corneal epithelium reduces sialylated glycoforms, suggesting that sialylation plays an important role during electrotaxis. Here, we show that pretreating human telomerase-immortalized corneal epithelial (hTCEpi) cells with a sialyltransferase inhibitor, P-3FAX-Neu5Ac (3F-Neu5Ac), improves electrotaxis by enhancing directionality, but not speed. This was recapitulated using Kifunensine, which inhibits cleavage of mannoses and therefore precludes sialylation on N-glycans. We also identified that 3F-Neu5Ac enhanced the responsiveness of the hTCEpi cell population to the electric field and that pretreated hTCEpi cells showed increased directionality even at low voltages. Furthermore, when we increased sialylation using N-azidoacetylmannosamine-tetraacylated (Ac4ManNAz), hTCEpi cells showed a decrease in both speed and directionality. Importantly, pretreating enucleated eyes with 3F-Neu5Ac significantly improved re-epithelialization in an ex vivo model of a corneal injury. Finally, we show that in hTCEpi cells, sialylation is increased by growth factor deprivation and reduced by PDGF-BB. Taken together, our results suggest that during corneal wound healing, reduced sialylated glycoforms enhance electrotaxis and re-epithelialization, potentially opening new avenues to promote corneal wound healing.

Chemokine binding to PSGL-1 is controlled by O-glycosylation and tyrosine sulfation

Protein glycosylation influences cellular recognition and regulates protein interactions, but how glycosylation functions alongside other common posttranslational modifications (PTMs), like tyrosine sulfation (sTyr), is unclear. We produced a library of 53 chemoenzymatically synthesized glycosulfopeptides representing N-terminal domains of human and murine P-selectin glycoprotein ligand-1 (PSGL-1), varying in sTyr and O-glycosylation (structure and site). Using these, we identified key roles of PSGL-1 O-glycosylation and sTyr in controlling interactions with specific chemokines. Results demonstrate that sTyr positively affects CCL19 and CCL21 binding to PSGL-1 N terminus, whereas O-glycan branching and sialylation reduced binding. For murine PSGL-1, interference between PTMs is greater, attributed to proximity between the two PTMs. Using fluorescence polarization, we found sTyr is a positive determinant for some chemokines. We showed that synthetic sulfopeptides are potent in decreasing chemotaxis of human dendritic cells toward CCL19 and CCL21. Our results provide new research avenues into the interplay of PTMs regulating leukocyte/chemokine interactions.

Emerging roles of a chemoattractant receptor GPR15 and ligands in pathophysiology

Chemokine receptors play a central role in the maintenance of immune homeostasis and development of inflammation by directing leukocyte migration to tissues. GPR15 is a G protein-coupled receptor (GPCR) that was initially known as a co-receptor for human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV), with structural similarity to other members of the chemoattractant receptor family. Since the discovery of its novel function as a colon-homing receptor of T cells in mice a decade ago, GPR15 has been rapidly gaining attention for its involvement in a variety of inflammatory and immune disorders. The recent identification of its natural ligand C10orf99, a chemokine-like polypeptide strongly expressed in gastrointestinal tissues, has established that GPR15-C10orf99 is a novel signaling axis that controls intestinal homeostasis and inflammation through the migration of immune cells. In addition, it has been demonstrated that C10orf99-independent functions of GPR15 and GPR15-independent activities of C10orf99 also play significant roles in the pathophysiology. Therefore, GPR15 and its ligands are potential therapeutic targets. To provide a basis for the future development of GPR15- or GPR15 ligand-targeted therapeutics, we have summarized the latest advances in the role of GPR15 and its ligands in human diseases as well as the molecular mechanisms that regulate GPR15 expression and functions.

Sialylation: A Cloak for Tumors to Trick the Immune System in the Microenvironment

The tumor microenvironment (TME), where the tumor cells incite the surrounding normal cells to create an immune suppressive environment, reduces the effectiveness of immune responses during cancer development. Sialylation, a type of glycosylation that occurs on cell surface proteins, lipids, and glycoRNAs, is known to accumulate in tumors and acts as a "cloak" to help tumor cells evade immunological surveillance. In the last few years, the role of sialylation in tumor proliferation and metastasis has become increasingly evident. With the advent of single-cell and spatial sequencing technologies, more research is being conducted to understand the effects of sialylation on immunity regulation. This review provides updated insights into recent research on the function of sialylation in tumor biology and summarizes the latest developments in sialylation-targeted tumor therapeutics, including antibody-mediated and metabolic-based sialylation inhibition, as well as interference with sialic acid-Siglec interaction.

Heparin Specifically Interacts with Basic BBXB Motifs of the Chemokine CCL21 to Define CCR7 Signaling

Chemokines are critically involved in controlling directed leukocyte migration. Spatiotemporal secretion together with local retention processes establish and maintain local chemokine gradients that guide directional cell migration. Extracellular matrix proteins, particularly glycosaminoglycans (GAGs), locally retain chemokines through electrochemical interactions. The two chemokines CCL19 and CCL21 guide CCR7-expressing leukocytes, such as antigen-bearing dendritic cells and T lymphocytes, to draining lymph nodes to initiate adaptive immune responses. CCL21-in contrast to CCL19-is characterized by a unique extended C-terminus composed of highly charged residues to facilitate interactions with GAGs. Notably, both chemokines can trigger common, but also ligand-biased signaling through the same receptor. The underlying molecular mechanism of ligand-biased CCR7 signaling is poorly understood. Using a series of naturally occurring chemokine variants in combination with newly designed site-specific chemokine mutants, we herein assessed CCR7 signaling, as well as GAG interactions. We demonstrate that the charged chemokine C-terminus does not fully confer CCL21-biased CCR7 signaling. Besides the positively charged C-terminus, CCL21 also possesses specific BBXB motifs comprising basic amino acids. We show that CCL21 variants where individual BBXB motifs are mutated retain their capability to trigger G-protein-dependent CCR7 signaling, but lose their ability to interact with heparin. Moreover, we show that heparin specifically interacts with CCL21, but not with CCL19, and thereby competes with ligand-binding to CCR7 and prevents signaling. Hence, we provide evidence that soluble heparin, but not the other GAGs, complexes with CCL21 to define CCR7 signaling in a ligand-dependent manner.

"Glyco-sulfo barcodes" regulate chemokine receptor function

Chemokine ligands and receptors regulate the directional migration of leukocytes. Post-translational modifications of chemokine receptors including O-glycosylation and tyrosine sulfation have been reported to regulate ligand binding and resulting signaling. Through in silico analyses, we determined potential conserved O-glycosylation and sulfation sites on human and murine CC chemokine receptors. Glyco-engineered CHO cell lines were used to measure the impact of O-glycosylation on CC chemokine receptor CCR5, while mutation of tyrosine residues and treatment with sodium chlorate were performed to determine the effect of tyrosine sulfation. Changing the glycosylation or tyrosine sulfation on CCR5 reduced the receptor signaling by the more positively charged CCL5 and CCL8 more profoundly compared to the less charged CCL3. The loss of negatively charged sialic acids resulted only in a minor effect on CCL3-induced signal transduction. The enzymes GalNAc-T1 and GalNAc-T11 were shown to be involved in the process of chemokine receptor O-glycosylation. These results indicate that O-glycosylation and tyrosine sulfation are involved in the fine-tuning and recognition of chemokine interactions with CCR5 and the resulting signaling.

Emerging Roles of the Unique Molecular Chaperone Cosmc in the Regulation of Health and Disease

The core-1 β1-3galactosyltransferase-specific chaperone 1 (Cosmc) is a unique molecular chaperone of core-1 β1-3galactosyltransferase(C1GALT1), which typically functions inside the endoplasmic reticulum (ER). Cosmc helps C1GALT1 to fold correctly and maintain activity. It also participates in the synthesis of the T antigen, O-glycan, together with C1GALT1. Cosmc is a multifaceted molecule with a wide range of roles and functions. It involves platelet production and the regulation of immune cell function. Besides that, the loss of function of Cosmc also facilitates the development of several diseases, such as inflammation diseases, immune-mediated diseases, and cancer. It suggests that Cosmc is a critical control point in diseases and that it should be regarded as a potential target for oncotherapy. It is essential to fully comprehend Cosmc's roles, as they may provide critical information about its involvement in disease development and pathogenesis. In this review, we summarize the recent progress in understanding the role of Cosmc in normal development and diseases.

Plasminogen and plasmin can bind to human T cells and generate truncated CCL21 that increases dendritic cell chemotactic responses

Plasmin is a broad-spectrum protease and therefore needs to be tightly regulated. Active plasmin is formed from plasminogen, which is found in high concentrations in the blood and is converted by the plasminogen activators. In the circulation, high levels of α2-antiplasmin rapidly and efficiently inhibit plasmin activity. Certain myeloid immune cells have been shown to bind plasmin and plasminogen on their cell surface via proteins that bind to the plasmin(ogen) kringle domains. Our earlier work showed that T cells can activate plasmin, but that they do not themselves express plasminogen. Here, we demonstrate that T cells express several known plasminogen receptors, and that they bind plasminogen on their cell surface. We show T cell-bound plasminogen was converted to plasmin by plasminogen activators upon T cell activation. To examine functional consequences of plasmin generation by activated T cells, we investigated its effect on the chemokine, C-C Motif Chemokine Ligand 21 (CCL21). Video microscopy and western blotting confirmed that plasmin bound by human T cells cleaves CCL21 and increases the chemotactic response of monocyte-derived dendritic cells towards higher CCL21 concentrations along the concentration gradient by increasing their directional migration and track straightness. These results demonstrate how migrating T cells and potentially other activated immune cells may co-opt a powerful proteolytic system from the plasma towards immune processes in the peripheral tissues, where α2-antiplasmin is more likely to be absent. We propose that plasminogen bound to migrating immune cells may strongly modulate chemokine responses in peripheral tissues.

How molecular modelling can better broaden the understanding of glycosylations

Glycosylations are among the most ubiquitous post-translational modifications (PTMs) in proteins, and the effects of their perturbations are seen in various diseases such as cancers, diabetes and arthritis to name a few. Yet they remain one of the most enigmatic aspects of protein structure and function. On the other hand, molecular modelling techniques have been rapidly bridging this knowledge gap since the last decade. In this review, we discuss how these techniques have proven to be indispensable for a better understanding of the role of glycosylations in glycoprotein structure and function.

ccr7 affects both morphogenesis and differentiation during early Xenopus embryogenesis

Chemokines play important roles in early embryogenesis, including morphogenesis and cell differentiation, before the immune system is established. We characterized Xenopus laevis CC-type chemokine receptor 7 S (ccr7.S) to clarify its role during early development. ccr7 transcripts were detected ubiquitously in early embryos. Dorsal overexpression of ccr7.S inhibited gastrulation, and ccr7.S mRNA-injected embryos had short axes and widely opened neural folds. Because the Keller sandwich explants of the injected embryos elongated well, ccr7.S might affect cell migration, but not convergent extension movements. Ventral ccr7.S overexpression induced secondary axes and chrd.1 upregulation in gastrula-stage embryos. Animal cap assays showed increased expression of neural and cement gland marker genes at later stages. Ccr7.S knockdown reduced chrd.1 expression and inhibited gastrulation at the dorsal side. Our findings suggest that ccr7.S plays important roles in morphogenetic movement and cell differentiation.

Characterising Distinct Migratory Profiles of Infiltrating T-Cell Subsets in Human Glioblastoma

Glioblastoma is the most common and aggressive form of primary brain cancer, with no improvements in the 5-year survival rate of 4.6% over the past three decades. T-cell-based immunotherapies such as immune-checkpoint inhibitors and chimeric antigen receptor T-cell therapy have prolonged the survival of patients with other cancers and have undergone early-phase clinical evaluation in glioblastoma patients. However, a major challenge for T-cell-based immunotherapy of glioblastoma and other solid cancers is T-cell infiltration into tumours. This process is mediated by chemokine-chemokine receptor and integrin-adhesion molecule interactions, yet the specific nature of the molecules that may facilitate T-cell homing into glioblastoma are unknown. Here, we have characterised chemokine receptor and integrin expression profiles of endogenous glioblastoma-infiltrating T cells, and the chemokine expression profile of glioblastoma-associated cells, by single-cell RNA-sequencing. Subsequently, chemokine receptors and integrins were validated at the protein level to reveal enrichment of receptors CCR2, CCR5, CXCR3, CXCR4, CXCR6, CD49a, and CD49d in glioblastoma-infiltrating T-cell populations relative to T cells in matched patient peripheral blood. Complementary chemokine ligand expression was then validated in glioblastoma biopsies and glioblastoma-derived primary cell cultures. Together, enriched expression of homing receptor-ligand pairs identified in this study implicate a potential role in mediating T-cell infiltration into glioblastoma. Importantly, our data characterising the migratory receptors on endogenous tumour-infiltrating T cells could be exploited to enhance the tumour-homing properties of future T-cell immunotherapies for glioblastoma.

Polysialic Acid in the Immune System

Polysialic acid (polySia) is a highly regulated polymer of sialic acid (Sia) with such potent biophysical characteristics that when expressed drastically influences the interaction properties of cells. Although much of what is known of polySia in mammals has been elucidated from the study of its role in the central nervous system (CNS), polySia is also expressed in other tissues, including the immune system where it presents dynamic changes during differentiation, maturation, and activation of different types of immune cells of the innate and adaptive response, being involved in key regulatory mechanisms. At least six polySia protein carriers (CCR7, ESL-1, NCAM, NRP2, ST8Sia 2, and ST8Sia 4) are expressed in different types of immune cells, but there is still much to be explored in regard not only to the regulatory mechanisms that determine their expression and the structure of polySia chains but also to the identification of the cis- and trans- ligands of polySia that establish signaling networks. This review summarizes the current knowledge on polySia in the immune system, addressing its biosynthesis, its tools for identification and structural characterization, and its functional roles and therapeutic implications.

CAL-1 as Cellular Model System to Study CCR7-Guided Human Dendritic Cell Migration

Dendritic cells (DCs) are potent and versatile professional antigen-presenting cells and central for the induction of adaptive immunity. The ability to migrate and transport peripherally acquired antigens to draining lymph nodes for subsequent cognate T cell priming is a key feature of DCs. Consequently, DC-based immunotherapies are used to elicit tumor-antigen specific T cell responses in cancer patients. Understanding chemokine-guided DC migration is critical to explore DCs as cellular vaccines for immunotherapeutic approaches. Currently, research is hampered by the lack of appropriate human cellular model systems to effectively study spatio-temporal signaling and CCR7-driven migration of human DCs. Here, we report that the previously established human neoplastic cell line CAL-1 expresses the human DC surface antigens CD11c and HLA-DR together with co-stimulatory molecules. Importantly, if exposed for three days to GM-CSF, CAL-1 cells induce the endogenous expression of the chemokine receptor CCR7 upon encountering the clinically approved TLR7/8 agonist Resiquimod R848 and readily migrate along chemokine gradients. Further, we demonstrate that CAL-1 cells can be genetically modified to express fluorescent (GFP)-tagged reporter proteins to study and visualize signaling or can be gene-edited using CRISPR/Cas9. Hence, we herein present the human CAL-1 cell line as versatile and valuable cellular model system to effectively study human DC migration and signaling.

The C-terminal peptide of CCL21 drastically augments CCL21 activity through the dendritic cell lymph node homing receptor CCR7 by interaction with the receptor N-terminus

The endogenous chemokines CCL19 and CCL21 signal via their common receptor CCR7. CCL21 is the main lymph node homing chemokine, but a weak chemo-attractant compared to CCL19. Here we show that the 41-amino acid positively charged peptide, released through C-terminal cleavage of CCL21, C21TP, boosts the immune cell recruiting activity of CCL21 by up to 25-fold and the signaling activity via CCR7 by ~ 100-fold. Such boosting is unprecedented. Despite the presence of multiple basic glycosaminoglycan (GAG) binding motifs, C21TP boosting of CCL21 signaling does not involve interference with GAG mediated cell-surface retention. Instead, boosting is directly dependent on O-glycosylations in the CCR7 N-terminus. As dictated by the two-step binding model, the initial chemokine binding involves interaction of the chemokine fold with the receptor N-terminus, followed by insertion of the chemokine N-terminus deep into the receptor binding pocket. Our data suggest that apart from a role in initial chemokine binding, the receptor N-terminus also partakes in a gating mechanism, which could give rise to a reduced ligand activity, presumably through affecting the ligand positioning. Based on experiments that support a direct interaction of C21TP with the glycosylated CCR7 N-terminus, we propose that electrostatic interactions between the positively charged peptide and sialylated O-glycans in CCR7 N-terminus may create a more accessible version of the receptor and thus guide chemokine docking to generate a more favorable chemokine-receptor interaction, giving rise to the peptide boosting effect.

Evolution of the Tumor Microenvironment toward Immune-Suppressive Seclusion during Brain Metastasis of Breast Cancer: Implications for Targeted Therapy

Simple Summary

Brain metastases (BM) of breast cancer (BC) are new targets of immunotherapy, but their characteristics are unclear. Therefore, we analyzed the differential expression profile of the tumor microenvironment (TME) in primary breast cancer brain metastasis (BCBM). In the TME of BCBM, immune-related pathways were downregulated and tumor intrinsic factors were upregulated. Moreover, CD8+ T cells and M1 macrophages with cytotoxic effects were decreased, but M2 cells were increased, in BM. Most tumor-suppressive immune functions ceased after BM with a molecular subtype shift. These results suggest the need for targeted therapy and immunotherapy strategies for BCBM.

Abstract

Breast cancer (BC) is the second most common solid malignant tumor that metastasizes to the brain. Despite emerging therapies such as immunotherapy, whether the tumor microenvironment (TME) in breast cancer brain metastasis (BCBM) has potential as a target of new treatments is unclear. Expression profiling of 770 genes in 12 pairs of primary BC and matched brain metastasis (BM) samples was performed using the NanoString nCounter PanCancer IO360^TM Panel. Immune cell profiles were validated by immunohistochemistry (IHC) in samples from 50 patients with BCBM. Pathway analysis revealed that immune-related pathways were downregulated. Immune cell profiling showed that CD8+ T cells and M1 macrophages were significantly decreased, and M2 macrophages were significantly increased, in BM compared to primary BC samples (p = 0.001, p = 0.021 and p = 0.007, respectively). CCL19 and CCL21, the top differentially expressed genes, were decreased significantly in BM compared to primary BC (p < 0.001, both). IHC showed that the CD8+ count was significantly lower (p = 0.027), and the CD163+ and CD206+ counts were higher, in BM than primary BC (p < 0.001, both). A low CD8+ T cell count, low CD86+ M1 macrophage count, and high M2/M1 macrophage ratio were related to unfavorable clinical outcomes. BC exhibits an immunosuppressive characteristic after metastasis to the brain. These findings will facilitate establishment of a treatment strategy for BCBM based on the TME of metastatic cancer.

Post-translational modifications in T cells in systemic erythematosus lupus

Systemic erythematosus lupus (SLE) is a classic autoimmune disease characterized by multiple autoantibodies and immune-mediated tissue damage. The aetiology of this disease is still unclear. A new drug, belimumab, which acts against the B-lymphocyte stimulator (BLyS), can effectively improve the condition of SLE patients, but it cannot resolve all SLE symptoms. The discovery of novel, precise therapeutic targets is urgently needed. It is well known that abnormal T-cell function is one of the most crucial factors contributing to the pathogenesis of SLE. Protein post-translational modifications (PTMs), including phosphorylation, glycosylation, acetylation, methylation, ubiquitination and SUMOylation have been emphasized for their roles in activating protein activity, maintaining structural stability, regulating protein-protein interactions and mediating signalling pathways, in addition to other biological functions. Summarizing the latest data in this area, this review focuses on the potential roles of diverse PTMs in regulating T-cell function and signalling pathways in SLE pathogenesis, with the goal of identifying new targets for SLE therapy.

Glycosylation of Immune Receptors in Cancer

Evading host immune surveillance is one of the hallmarks of cancer. Immune checkpoint therapy, which aims to eliminate cancer progression by reprogramming the antitumor immune response, currently occupies a solid position in the rapidly expanding arsenal of cancer therapy. As most immune checkpoints are membrane glycoproteins, mounting attention is drawn to asking how protein glycosylation affects immune function. The answers to this fundamental question will stimulate the rational development of future cancer diagnostics and therapeutic strategies.

Tyrosine sulfation and O-glycosylation of chemoattractant receptor GPR15 differentially regulate interaction with GPR15L

GPR15 is a G-protein-coupled receptor (GPCR) that directs lymphocyte homing to the colon and skin. Recent studies have identified a chemokine-like protein GPR15L (also known as C10orf99) as a functional ligand of GPR15. In this study, we examined the structural elements that regulate the GPR15-GPR15L interaction with primary focus on post-translational modifications (PTMs) of receptor N-terminus and on the C-terminus of the ligand. Our findings reveal that the GPR15 receptor is sulfated on the N-terminal tyrosine residue(s) and disruption of tyrosine sulfation inhibits binding of GPR15L. In contrast, the disruption of O-glycosylation on the N-terminal threonine or serine residues, or the removal of α2,3-linked sialic acids from O-glycans, enhances the GPR15L binding. Thus, GPR15 represents a unique chemoattractant receptor in which different N-terminal PTMs regulate its ligand binding in a contrasting manner. We further demonstrate that, unlike canonical chemokines, GPR15L activity critically requires its extreme C-terminal residue and that its hydrophobicity may be a key attribute that facilitates an optimal interaction with the receptor. Our results reveal novel insights into chemoattractant receptor-ligand interaction and provide a valid footing for potential intervention targeting the GPR15-GPR15L axis.

A Versatile Toolkit for Semi-Automated Production of Fluorescent Chemokines to Study CCR7 Expression and Functions

Chemokines guide leukocyte migration in different contexts, including homeostasis, immune surveillance and immunity. The chemokines CCL19 and CCL21 control lymphocyte and dendritic cell migration and homing to lymphoid organs. Thereby they orchestrate adaptive immunity in a chemokine receptor CCR7-dependent manner. Likewise, cancer cells that upregulate CCR7 expression are attracted by these chemokines and metastasize to lymphoid organs. In-depth investigation of CCR7 expression and chemokine-mediated signaling is pivotal to understand their role in health and disease. Appropriate fluorescent probes to track these events are increasingly in demand. Here, we present an approach to cost-effectively produce and fluorescently label CCL19 and CCL21 in a semi-automated process. We established a versatile protocol for the production of recombinant chemokines harboring a small C-terminal S6-tag for efficient and site-specific enzymatic labelling with an inorganic fluorescent dye of choice. We demonstrate that the fluorescently labeled chemokines CCL19-S6^Dy649P1 and CCL21-S6^Dy649P1 retain their full biological function as assessed by their abilities to mobilize intracellular calcium, to recruit β-arrestin to engaged receptors and to attract CCR7-expressing leukocytes. Moreover, we show that CCL19-S6^Dy649P1 serves as powerful reagent to monitor CCR7 internalization by time-lapse confocal video microscopy and to stain CCR7-positive primary human and mouse T cell sub-populations.

Glycans and Glycan-Binding Proteins as Regulators and Potential Targets in Leukocyte Recruitment

Leukocyte recruitment is a highly controlled cascade of interactions between proteins expressed by the endothelium and circulating leukocytes. The involvement of glycans and glycan-binding proteins in the leukocyte recruitment cascade has been well-characterised. However, our understanding of these interactions and their regulation has expanded substantially in recent years to include novel lectins and regulatory pathways. In this review, we discuss the role of glycans and glycan-binding proteins, mediating the interactions between endothelium and leukocytes both directly and indirectly. We also highlight recent findings of key enzymes involved in glycosylation which affect leukocyte recruitment. Finally, we investigate the potential of glycans and glycan binding proteins as therapeutic targets to modulate leukocyte recruitment and transmigration in inflammation.

Glycosylation and raft endocytosis in cancer

Changes in glycosylation on proteins or lipids are one of the hallmarks of tumorigenesis. In many cases, it is still not understood how glycan information is translated into biological function. In this review, we discuss at the example of specific cancer-related glycoproteins how their endocytic uptake into eukaryotic cells is tuned by carbohydrate modifications. For this, we not only focus on overall uptake rates, but also illustrate how different uptake processes-dependent or not on the conventional clathrin machinery-are used under given glycosylation conditions. Furthermore, we discuss the role of certain sugar-binding proteins, termed galectins, to tune glycoprotein uptake by inducing their crosslinking into lattices, or by co-clustering them with glycolipids into raft-type membrane nanodomains from which the so-called clathrin-independent carriers (CLICs) are formed for glycoprotein internalization into cells. The latter process has been termed glycolipid-lectin (GL-Lect) hypothesis, which operates in a complementary manner to the clathrin pathway and galectin lattices.

Post-Translational Modifications of G Protein-Coupled Receptors Control Cellular Signaling Dynamics in Space and Time

G protein-coupled receptors (GPCRs) are a large family comprising >800 signaling receptors that regulate numerous cellular and physiologic responses. GPCRs have been implicated in numerous diseases and represent the largest class of drug targets. Although advances in GPCR structure and pharmacology have improved drug discovery, the regulation of GPCR function by diverse post-translational modifications (PTMs) has received minimal attention. Over 200 PTMs are known to exist in mammalian cells, yet only a few have been reported for GPCRs. Early studies revealed phosphorylation as a major regulator of GPCR signaling, whereas later reports implicated a function for ubiquitination, glycosylation, and palmitoylation in GPCR biology. Although our knowledge of GPCR phosphorylation is extensive, our knowledge of the modifying enzymes, regulation, and function of other GPCR PTMs is limited. In this review we provide a comprehensive overview of GPCR post-translational modifications with a greater focus on new discoveries. We discuss the subcellular location and regulatory mechanisms that control post-translational modifications of GPCRs. The functional implications of newly discovered GPCR PTMs on receptor folding, biosynthesis, endocytic trafficking, dimerization, compartmentalized signaling, and biased signaling are also provided. Methods to detect and study GPCR PTMs as well as PTM crosstalk are further highlighted. Finally, we conclude with a discussion of the implications of GPCR PTMs in human disease and their importance for drug discovery. SIGNIFICANCE STATEMENT: Post-translational modification of G protein-coupled receptors (GPCRs) controls all aspects of receptor function; however, the detection and study of diverse types of GPCR modifications are limited. A thorough understanding of the role and mechanisms by which diverse post-translational modifications regulate GPCR signaling and trafficking is essential for understanding dysregulated mechanisms in disease and for improving and refining drug development for GPCRs.

CXCL12-PLGA/Pluronic Nanoparticle Internalization Abrogates CXCR4-Mediated Cell Migration

Chemokine-induced chemotaxis mediates physiological and pathological immune cell trafficking, as well as several processes involving cell migration. Among them, the role of CXCL12/CXCR4 signaling in cancer and metastasis is well known, and CXCR4 has been often targeted with small molecule-antagonists or short CXCL12-derived peptides to limit the pathological processes of cell migration and invasion. To reduce CXCR4-mediated chemotaxis, we adopted a different approach. We manufactured poly(lactic acid-co-glycolic acid) (PLGA)/Pluronic F127 nanoparticles through microfluidics-assisted nanoprecipitation and functionalized them with streptavidin to docking a biotinylated CXCL12 to be exposed on the nanoparticle surface. Our results show that CXCL12-decorated nanoparticles are non-toxic and do not induce inflammatory cytokine release in THP-1 monocytes cultured in fetal bovine and human serum-supplemented media. The cell internalization of our chemokine receptor-targeting particles increases in accordance with CXCR4 expression in FBS/medium. We demonstrated that CXCL12-decorated nanoparticles do not induce cell migration on their own, but their pre-incubation with THP-1 significantly decreases CXCR4⁺-cell migration, thereby antagonizing the chemotactic action of CXCL12. The use of biodegradable and immune-compatible chemokine-mimetic nanoparticles to reduce cell migration opens the way to novel antagonists with potential application in cancer treatments and inflammation.

CCL20 is a novel ligand for the scavenging atypical chemokine receptor 4

The chemokine CCL20 is broadly produced by endothelial cells in the liver, the lung, in lymph nodes and mucosal lymphoid tissues, and recruits CCR6 expressing leukocytes, particularly dendritic cells, mature B cells, and subpopulations of T cells. How CCL20 is systemically scavenged is currently unknown. Here, we identify that fluorescently labeled human and mouse CCL20 are efficiently taken-up by the atypical chemokine receptor ACKR4. CCL20 shares ACKR4 with the homeostatic chemokines CCL19, CCL21, and CCL25, although with a lower affinity. We demonstrate that all 4 human chemokines recruit β-arrestin1 and β-arrestin2 to human ACKR4. Similarly, mouse CCL19, CCL21, and CCL25 equally activate the human receptor. Interestingly, at the same chemokine concentration, mouse CCL20 did not recruit β-arrestins to human ACKR4. Further cross-species analysis suggests that human ACKR4 preferentially takes-up human CCL20, whereas mouse ACKR4 similarly internalizes mouse and human CCL20. Furthermore, we engineered a fluorescently labeled chimeric chemokine consisting of the N-terminus of mouse CCL25 and the body of mouse CCL19, termed CCL25_19, which interacts with and is taken-up by human and mouse ACKR4.

Cosmc controls B cell homing

The molecular mechanisms regulating lymphocyte homing into lymph nodes are only partly understood. Here, we report that B cell-specific deletion of the X-linked gene, Cosmc, and the consequent decrease of protein O-glycosylation, induces developmental blocks of mouse B cells. After transfer into wild-type recipient, Cosmc-null B cells fail to home to lymph nodes as well as non-lymphoid organs. Enzymatic desialylation of wild-type B cells blocks their migration into lymph nodes, indicating a requirement of sialylated O-glycans for proper trafficking. Mechanistically, Cosmc-deficient B cells have normal rolling and firm arrest on high endothelium venules (HEV), thereby attributing their inefficient trafficking to alterations in the subsequent transendothelial migration step. Finally, Cosmc-null B cells have defective chemokine signaling responses. Our results thus demonstrate that Cosmc and its effects on O-glycosylation are important for controlling B cell homing.

The Roles of CCR7 for the Homing of Memory CD8+ T Cells into Their Survival Niches

Memory CD8+ T cells in the immune system are responsible for the removal of external Ags for a long period of time to protect against re-infection. Naïve to memory CD8+ T cell differentiation and memory CD8+ T cell maintenance require many different factors including local environmental factors. Thus, it has been suggested that the migration of memory CD8+ T cells into specific microenvironments alters their longevity and functions. In this review, we have summarized the subsets of memory CD8+ T cells based on their migratory capacities and described the niche hypothesis for their survival. In addition, the basic roles of CCR7 in conjunction with the migration of memory CD8+ T cells and recent understandings of their survival niches have been introduced. Finally, the applications of altering CCR7 signaling have been discussed.

ACKR4 Recruits GRK3 Prior to β-Arrestins but Can Scavenge Chemokines in the Absence of β-Arrestins

Chemokines are essential for guiding cell migration. Atypical chemokine receptors (ACKRs) contribute to the cell migration process by binding, internalizing and degrading local chemokines, which enables the formation of confined gradients. ACKRs are heptahelical membrane spanning molecules structurally related to G-protein coupled receptors (GPCRs), but seem to be unable to signal through G-proteins upon ligand binding. ACKR4 internalizes the chemokines CCL19, CCL21, and CCL25 and is best known for shaping functional CCL21 gradients. Ligand binding to ACKR4 has been shown to recruit β-arrestins that has led to the assumption that chemokine scavenging relies on β-arrestin-mediated ACKR4 trafficking, a common internalization route taken by class A GPCRs. Here, we show that CCL19, CCL21, and CCL25 readily recruited β-arrestin1 and β-arrestin2 to human ACKR4, but found no evidence for β-arrestin-dependent or independent ACKR4-mediated activation of the kinases Erk1/2, Akt, or Src. However, we demonstrate that β-arrestins interacted with ACKR4 in the steady-state and contributed to the spontaneous trafficking of the receptor in the absence of chemokines. Deleting the C-terminus of ACKR4 not only interfered with the interaction of β-arrestins, but also with the uptake of fluorescently labeled cognate chemokines. We identify the GPCR kinase GRK3, and to a lesser extent GRK2, but not GRK4, GRK5, and GRK6, to be recruited to chemokine-stimulated ACKR4. We show that GRK3 recruitment proceded the recruitment of β-arrestins upon ACKR4 engagement and that GRK2/3 inhibition partially interfered with steady-state interaction and chemokine-driven recruitment of β-arrestins to ACKR4. Overexpressing β-arrestin2 accelerated the uptake of fluorescently labeled CCL19, indicating that β-arrestins contribute to the chemokine scavenging activity of ACKR4. By contrast, cells lacking β-arrestins were still capable to take up fluorescently labeled CCL19 demonstrating that β-arrestins are dispensable for chemokine scavenging by ACKR4.

Structural Features of an Extended C-Terminal Tail Modulate the Function of the Chemokine CCL21

The chemokines CCL21 and CCL19, through binding of their cognate receptor CCR7, orchestrate lymph node homing of dendritic cells and naïve T cells. CCL21 differs from CCL19 via an unstructured 32 residue C-terminal domain. Previously described roles for the CCL21 C-terminus include GAG-binding, spatial localization to lymphatic vessels, and autoinhibitory modulation of CCR7-mediated chemotaxis. While truncation of the C-terminal tail induced chemical shift changes in the folded chemokine domain, the structural basis for its influence on CCL21 function remains largely unexplored. CCL21 concentration-dependent NMR chemical shifts revealed weak, nonphysiological self-association that mimics the truncation of the C-terminal tail. We generated a series of C-terminal truncation variants to dissect the C-terminus influence on CCL21 structure and receptor activation. Using NMR spectroscopy, we found that CCL21 residues 80-90 mediate contacts with the chemokine domain. In cell-based assays for CCR7 and ACKR4 activation, we also found that residues 92-100 reduced CCL21 potency in calcium flux, cAMP inhibition, and β-arrestin recruitment. Taken together, these structure-function studies support a model wherein intramolecular interactions with specific residues of the flexible C-terminus stabilize a less active monomer conformation of the CCL21. We speculate that the autoinhibitory intramolecular contacts between the C-terminal tail and chemokine body are disrupted by GAG binding and/or interactions with the CCR7 receptor to ensure optimal functionality.

Human T cell glycosylation and implications on immune therapy for cancer

Glycosylation is an important post-translational modification, giving rise to a diverse and abundant repertoire of glycans on the cell surface, collectively known as the glycome. When focusing on immunity, glycans are indispensable in virtually all signaling and cell-cell interactions. More specifically, glycans have been shown to regulate key pathophysiological steps within T cell biology such as T cell development, thymocyte selection, T cell activity and signaling as well as T cell differentiation and proliferation. They are of major importance in determining the interaction of human T cells with tumor cells. In this review, we will describe the role of glycosylation of human T cells in more depth, elaborate on the importance of glycosylation in the interaction of human T cells with tumor cells and discuss the potential of cancer immunotherapies that are based on manipulating the glycome functions at the tumor immune interface.^1,2

G Protein-Coupled Receptors in the Sweet Spot: Glycosylation and other Post-translational Modifications

Post-translational modifications (PTMs) are a fundamental phenomenon across all classes of life and several hundred different types have been identified. PTMs contribute widely to the biological functions of proteins and greatly increase their diversity. One important class of proteins regulated by PTMs, is the cell surface expressed G protein-coupled receptors (GPCRs). While most PTMs have been shown to exert distinct biological functions, we are only beginning to approach the complexity that the potential interplay between different PTMs may have on biological functions and their regulation. Importantly, PTMs and their potential interplay represent an appealing mechanism for cell and tissue specific regulation of GPCR function and may partially contribute to functional selectivity of some GPCRs. In this review we highlight examples of PTMs located in GPCR extracellular domains, with special focus on glycosylation and the potential interplay with other close-by PTMs such as tyrosine sulfation, proteolytic cleavage, and phosphorylation.

Immune and Stroma Related Genes in Breast Cancer: A Comprehensive Analysis of Tumor Microenvironment Based on the Cancer Genome Atlas (TCGA) Database

Background: Tumor microenvironment is essential for breast cancer progression and metastasis. Our study sets out to examine the genes affecting stromal and immune infiltration in breast cancer progression and prognosis.

Materials and Methods: This work provides an approach for quantifying stromal and immune scores by using ESTIMATE algorithm based on gene expression matrix of breast cancer patients in TCGA database. We found differentially expressed genes (DEGs) through limma R package. Functional enrichments were accessed through Gene Ontology (GO) analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis. Besides, we constructed a protein-protein network, identified several hub genes in Cytoscape, and discovered functionally similar genes in GeneMANIA. Hub genes were validated with prognostic data by Kaplan-Meier analysis both in The Cancer Genome Atlas (TCGA) database and Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) database and a meta-analysis of hub genes prognosis data was utilized in multiple databases. Furthermore, their relationship with infiltrating immune cells was evaluated by Tumor IMmune Estimation Resource (TIMER) web tool. Cox regression was utilized for overall survival (OS) and recurrence-free survival (RFS) in TCGA database and OS in METABRIC database in order to evaluate the impact of stromal and immune scores on patients prognosis.

Results: One thousand and eighty-five breast cancer patients were investigated and 480 differentiated expressed genes (DEGs) were found based on the analysis of mRNA expression profiles. Functional analysis of DEGs revealed their potential functions in immune response and extracellular interaction. Protein-protein interaction network gave evidence of 10 hub genes. Some of the hub genes could be used as predictive markers for patients prognosis. In this study, we found that tumor purity and specific immune cells infiltration varied in response to hub genes expression. The multivariate cox regression highlighted the fact that immune score played a detrimental role in overall survival (HR = 0.45, 95% CI: 0.27–0.74, p = 0.002) and recurrence-free survival (HR = 0.41, 95% CI: 0.22–0.77, p = 0.006) in TCGA database. These result was confirmed in METABRIC database that immune score was a protector of OS (HR = 0.88, 95% CI: 0.77–0.99, p = 0.039).

Conclusions: Our findings promote a better understanding of the potential genes behind the regulation of tumor microenvironment and cells infiltration. Immune score should be considered as a prognostic factor for patients' survival.

A degradatory fate for CCR4 suggests a primary role in Th2 inflammation

CCR4 is the sole receptor for the chemokines CCL22 and CCL17. Clinical studies of asthmatic airways have shown levels of both ligands and CCR4⁺ Th2 cells to be elevated, suggestive of a role in disease. Consequently, CCR4 has aroused much interest as a potential therapeutic target and an understanding of how its cell surface expression is regulated is highly desirable. To this end, receptor expression, receptor endocytosis, and chemotaxis were assessed using transfectants expressing CCR4, CCR4⁺ human T cell lines, and human Th2 cells polarized in vitro. CCL17 and CCL22 drove rapid endocytosis of CCR4 in a dose-dependent manner. Replenishment at the cell surface was slow and sensitive to cycloheximide, suggestive of de novo synthesis of CCR4. Constitutive CCR4 endocytosis was also observed, with the internalized CCR4 found to be significantly degraded over a 6-h incubation. Truncation of the CCR4 C-terminus by 40 amino acids had no effect on cell surface expression, but resulted in significant impairment of ligand-induced endocytosis. Consequently, migration to both CCL17 and CCL22 was significantly enhanced. In contrast, truncation of CCR4 did not impair constitutive endocytosis or degradation, suggesting the use of alternative receptor motifs in these processes. We conclude that CCR4 cell surface levels are tightly regulated, with a degradative fate for endocytosed receptor. We postulate that this strict control is desirable, given that Th2 cells recruited by CCR4 can induce the further expression of CCR4 ligands in a positive feedback loop, thereby enhancing allergic inflammation.

CDG and immune response: From bedside to bench and back

Glycosylation is an essential biological process that adds structural and functional diversity to cells and molecules, participating in physiological processes such as immunity. The immune response is driven and modulated by protein-attached glycans that mediate cell-cell interactions, pathogen recognition and cell activation. Therefore, abnormal glycosylation can be associated with deranged immune responses. Within human diseases presenting immunological defects are congenital disorders of glycosylation (CDG), a family of around 130 rare and complex genetic diseases. In this review, we have identified 23 CDG with immunological involvement, characterized by an increased propensity to-often life-threatening-infection. Inflammatory and autoimmune complications were found in 7 CDG types. CDG natural history(ies) and the mechanisms behind the immunological anomalies are still poorly understood. However, in some cases, alterations in pathogen recognition and intracellular signaling (eg, TGF-β1, NFAT, and NF-κB) have been suggested. Targeted therapies to restore immune defects are only available for PGM3-CDG and SLC35C1-CDG. Fostering research on glycoimmunology may elucidate the involved pathophysiological mechanisms and open new therapeutic avenues, thus improving CDG patients' quality of life.

CCR5: Established paradigms and new frontiers for a 'celebrity' chemokine receptor

Because of the level of attention it received due to its role as the principal HIV coreceptor, CCR5 has been described as a 'celebrity' chemokine receptor. Here we describe the development of CCR5 inhibitory strategies that have been developed for HIV therapy and which are now additionally being considered for use in HIV prevention and cure. The wealth of CCR5-related tools that have been developed during the intensive investigation of CCR5 as an HIV drug target can now be turned towards the study of CCR5 as a model chemokine receptor. We also summarize what is currently known about the cell biology and pharmacology of CCR5, providing an update on new areas of investigation that have emerged in recent research. Finally, we discuss the potential of CCR5 as a drug target for diseases other than HIV, discussing the evidence linking CCR5 and its natural chemokine ligands with inflammatory diseases, particularly neuroinflammation, and certain cancers. These pathologies may provide new uses for the strategies for CCR5 blockade originally developed to combat HIV/AIDS.

Biased Signaling of CCL21 and CCL19 Does Not Rely on N-Terminal Differences, but Markedly on the Chemokine Core Domains and Extracellular Loop 2 of CCR7

Chemokine receptors play important roles in the immune system and are linked to several human diseases. Targeting chemokine receptors have so far shown very little success owing to, to some extent, the promiscuity of the immune system and the high degree of biased signaling within it. CCR7 and its two endogenous ligands display biased signaling and here we investigate the differences between the two ligands, CCL21 and CCL19, with respect to their biased activation of CCR7. We use bystander bioluminescence resonance energy transfer (BRET) based signaling assays and Transwell migration assays to determine (A) how swapping of domains between the two ligands affect their signaling patterns and (B) how receptor mutagenesis impacts signaling. Using chimeric ligands we find that the chemokine core domains are central for determining signaling outcome as the lack of β-arrestin-2 recruitment displayed by CCL21 is linked to its core domain and not N-terminus. Through a mutagenesis screen, we identify the extracellular domains of CCR7 to be important for both ligands and show that the two chemokines interact differentially with extracellular loop 2 (ECL-2). By using in silico modeling, we propose a link between ECL-2 interaction and CCR7 signal transduction. Our mutagenesis study also suggests a lysine in the top of TM3, K130^3.26, to be important for G protein signaling, but not β-arrestin-2 recruitment. Taken together, the bias in CCR7 between CCL19 and CCL21 relies on the chemokine core domains, where interactions with ECL-2 seem particularly important. Moreover, TM3 selectively regulates G protein signaling as found for other chemokine receptors.

Altered Cell Surface N-Glycosylation of Resting and Activated T Cells in Systemic Lupus Erythematosus

Altered cell surface glycosylation in congenital and acquired diseases has been shown to affect cell differentiation and cellular responses to external signals. Hence, it may have an important role in immune regulation; however, T cell surface glycosylation has not been studied in systemic lupus erythematosus (SLE), a prototype of autoimmune diseases. Analysis of the glycosylation of T cells from patients suffering from SLE was performed by lectin-binding assay, flow cytometry, and quantitative real-time PCR. The results showed that resting SLE T cells presented an activated-like phenotype in terms of their glycosylation pattern. Additionally, activated SLE T cells bound significantly less galectin-1 (Gal-1), an important immunoregulatory lectin, while other lectins bound similarly to the controls. Differential lectin binding, specifically Gal-1, to SLE T cells was explained by the increased gene expression ratio of sialyltransferases and neuraminidase 1 (NEU1), particularly by elevated ST6 beta-galactosamide alpha-2,6-sialyltranferase 1 (ST6GAL1)/NEU1 and ST3 beta-galactoside alpha-2,3-sialyltransferase 6 (ST3GAL6)/NEU1 ratios. These findings indicated an increased terminal sialylation. Indeed, neuraminidase treatment of cells resulted in the increase of Gal-1 binding. Altered T cell surface glycosylation may predispose the cells to resistance to the immunoregulatory effects of Gal-1, and may thus contribute to the pathomechanism of SLE.

The Role of the CXCL12/CXCR4/ACKR3 Axis in Autoimmune Diseases

Chemokine receptors are members of the G protein-coupled receptor superfamily. These receptors are intimately involved in cell movement, and thus play a critical role in several physiological and pathological situations that require the precise regulation of cell positioning. CXCR4 is one of the most studied chemokine receptors and is involved in many functions beyond leukocyte recruitment. During embryogenesis, it plays essential roles in vascular development, hematopoiesis, cardiogenesis, and nervous system organization. It has been also implicated in tumor progression and autoimmune diseases and, together with CD4, is one of the co-receptors used by the HIV-1 virus to infect immune cells. In contrast to other chemokine receptors that are characterized by ligand promiscuity, CXCR4 has a unique ligand-stromal cell-derived factor-1 (SDF1, CXCL12). However, this ligand also binds ACKR3, an atypical chemokine receptor that modulates CXCR4 functions and is overexpressed in multiple cancer types. The CXCL12/CXCR4/ACKR3 axis constitutes a potential therapeutic target for a wide variety of inflammatory diseases, not only by interfering with cell migration but also by modulating immune responses. Thus far, only one antagonist directed against the ligand-binding site of CXCR4, AMD3100, has demonstrated clinical relevance. Here, we review the role of this ligand and its receptors in different autoimmune diseases.

Fluorescently Tagged CCL19 and CCL21 to Monitor CCR7 and ACKR4 Functions

Chemokines are essential guidance cues orchestrating cell migration in health and disease. Cognate chemokine receptors sense chemokine gradients over short distances to coordinate directional cell locomotion. The chemokines CCL19 and CCL21 are essential for recruiting CCR7-expressing dendritic cells bearing pathogen-derived antigens and lymphocytes to lymph nodes, where the two cell types meet to launch an adaptive immune response against the invading pathogen. CCR7-expressing cancer cells are also recruited by CCL19 and CCL21 to metastasize in lymphoid organs. In contrast, atypical chemokine receptors (ACKRs) do not transmit signals required for cell locomotion but scavenge chemokines. ACKR4 is crucial for internalizing and degrading CCL19 and CCL21 to establish local gradients, which are sensed by CCR7-expressing cells. Here, we describe the production of fluorescently tagged chemokines by fusing CCL19 and CCL21 to monomeric red fluorescent protein (mRFP). We show that purified CCL19-mRFP and CCL21-mRFP are versatile and powerful tools to study CCR7 and ACKR4 functions, such as receptor trafficking and chemokine scavenging, in a spatiotemporal fashion. We demonstrate that fluorescently tagged CCL19 and CCL21 permit the visualization and quantification of chemokine gradients in real time, while CCR7-expressing leukocytes and cancer cells sense the guidance cues and migrate along the chemokine gradients.

CCL19 with CCL21-tail displays enhanced glycosaminoglycan binding with retained chemotactic potency in dendritic cells

CCL19 is more potent than CCL21 in inducing chemotaxis of human dendritic cells (DC). This difference is attributed to 1) a stronger interaction of the basic C-terminal tail of CCL21 with acidic glycosaminoglycans (GAGs) in the environment and 2) an autoinhibitory function of this C-terminal tail. Moreover, different receptor docking modes and tissue expression patterns of CCL19 and CCL21 contribute to fine-tuned control of CCR7 signaling. Here, we investigate the effect of the tail of CCL21 on chemokine binding to GAGs and on CCR7 activation. We show that transfer of CCL21-tail to CCL19 (CCL19^CCL21-tail ) markedly increases binding of CCL19 to human dendritic cell surfaces, without impairing CCL19-induced intracellular calcium release or DC chemotaxis, although it causes reduced CCR7 internalization. The more potent chemotaxis induced by CCL19 and CCL19^CCL21-tail compared to CCL21 is not transferred to CCL21 by replacing its N-terminus with that of CCL19 (CCL21^CCL19-N-term ). Measurements of cAMP production in CHO cells uncover that CCL21-tail transfer (CCL19^CCL21-tail ) negatively affects CCL19 potency, whereas removal of CCL21-tail (CCL21^tailless ) increases signaling compared to full-length CCL21, indicating that the tail negatively affects signaling via cAMP. Similar to chemokine-driven calcium mobilization and chemotaxis, the potency of CCL21 in cAMP is not improved by transfer of the CCL19 N-terminus to CCL21 (CCL21^CCL19-N-term ). Together these results indicate that ligands containing CCL21 core and C-terminal tail (CCL21 and CCL21^CCL19-N-term ) are most restricted in their cAMP signaling; a phenotype attributed to a stronger GAG binding of CCL21 and defined structural differences between CCL19 and CCL21.

Insight on the regulation of chemokine activities

The activity of chemokines is regulated by several mechanisms that control the final cellular response. The present review discusses the complexity of the regulation of the chemokine system, and the novel findings describing how in persistent infections, the expression of chemokine receptors on the surface of T cells does not correlate with their homing potential. Thanks to the latest advances in our comprehension of the chemokine system, novel approaches targeting chemokines, chemokine receptors, or protein of their signaling pathway should be considered in order to achieve a personalized therapy.

A unique signal sequence of the chemokine receptor CCR7 promotes package into COPII vesicles for efficient receptor trafficking

Chemokine receptors are considered to belong to the group of G protein-coupled receptors that use the first transmembrane domain as signal anchor sequence for membrane insertion instead of a cleavable N-terminal signal sequence. Chemokine recognition is determined by the N-termini of chemokine receptors. Here, we show that the chemokine receptor CCR7, which is essential for directed migration of adaptive immune cells, possesses a 24 amino acids long N-terminal signal sequence that is unique among chemokine receptors. This sequence is cleaved off the mature human and mouse protein. Introducing single point mutations in the hydrophobic core h-region or in the polar C-terminal segment (c-region) of the signal sequence to interfere with its cleavage retained CCR7 in the ER and prevented its surface expression. Furthermore, we demonstrate the correct topology of the 35 amino acids short extracellular N-tail of CCR7 in a deletion mutant lacking the natural signal sequence. This signal sequence deletion mutant of CCR7 is fully functional as it efficiently binds its ligand, elicits chemokine-induced calcium mobilization, and directs cell migration. However, we show that the signal sequence promotes efficient recruitment of the GPCR to ER exit sites, thereby controlling efficient ER to Golgi trafficking of CCR7 on its way to reach the plasma membrane.