Chemokines as novel and versatile reagents for flow cytometry and cell sorting

Extensive flow cytometric immunophenotyping of human PBMC incorporating detection of chemokine receptors, cytokines and tetramers

Characterization of immune cells is essential to advance our understanding of immunology and flow cytometry is an important tool in this context. Addressing both cellular phenotype and antigen-specific functional responses of the same cells is valuable to achieve a more integrated understanding of immune cell behavior and maximizes information obtained from precious samples. Until recently, panel size was limiting, resulting in panels generally focused on either deep immunophenotyping or functional readouts. Ongoing developments in the field of (spectral) flow cytometry have made panels of 30⁺ markers more accessible, opening up possibilities for advanced integrated analyses. Here, we optimized immune phenotyping by co-detection of markers covering chemokine receptors, cytokines and specific T cell/peptide tetramer interaction using a 32-color panel. Such panels enable integrated analysis of cellular phenotypes and markers assessing the quality of immune responses and will contribute to our understanding of the immune system.

CCR7+ dendritic cells sorted by binding of CCL19 show enhanced Ag-presenting capacity and antitumor potency

Dendritic cell therapy has been a promising addition to the current armory of therapeutic options in cancer for more than 20 years but has not yet achieved breakthrough success. To successfully initiate immunity, dendritic cells have to enter the lymph nodes. However, experience to date of therapeutic dendritic cell administration indicates that this is frequently an extremely inefficient process. The major regulator of dendritic cell migration to the lymph nodes is the chemokine receptor CCR7 and in vitro generated dendritic cells typically display heterogeneous expression of this receptor. Here we demonstrate that positive selection for the dendritic cell subpopulation expressing CCR7, using a chemically-synthesized ligand:CCL19, enriches for cells with enhanced lymph node migration and Ag presentation competence as well as a chemokine expression profile indicative of improved interactions with T cells. This enhanced lymph node homing capacity of enriched CCR7+ cells is seen in comparison to a population of unsorted dendritic cells containing an equivalent number of CCR7+ dendritic cells. Importantly, this indicates that separating the CCR7+ dendritic cells from the CCR7- cells, rather than simple CCL19 exposure, is required to affect the enhanced lymph node migration of the CCR7+ cells. In models of both subcutaneous and metastatic melanoma, we demonstrate that the dendritic cells sorted for CCR7 expression trigger enhanced CD8 T-cell driven antitumor immune responses which correlate with reduced tumor burden and increased survival. Finally, we demonstrate that this approach is directly translatable to human dendritic cell therapy using the same reagents coupled with clinical-grade flow-cytometric sorting.

Development of novel reagents to chicken FLT3, XCR1 and CSF2R for the identification and characterization of avian conventional dendritic cells

Conventional dendritic cells (cDC) are bone marrow-derived immune cells that play a central role in linking innate and adaptive immunity. cDCs efficiently uptake, process and present antigen to naïve T cells, driving clonal expansion of antigen-specific T-cell responses. In chicken, vital reagents are lacking for the efficient and precise identification of cDCs. In this study, we have developed several novel reagents for the identification and characterization of chicken cDCs. Chicken FLT3 cDNA was cloned and a monoclonal antibody to cell surface FLT3 was generated. This antibody identified a distinct FLT3^HI splenic subset which lack expression of signature markers for B cells, T cells or monocyte/macrophages. By combining anti-FLT3 and CSF1R-eGFP transgenic expression, three major populations within the mononuclear phagocyte system were identified in the spleen. The cDC1 subset of mammalian cDCs express the chemokine receptor XCR1. To characterize chicken cDCs, a synthetic chicken chemokine (C motif) ligand (XCL1) peptide conjugated to Alexa Fluor 647 was developed (XCL1^AF647 ). Flow cytometry staining of XCL1^AF647 on splenocytes showed that all chicken FLT3^HI cells exclusively express XCR1, supporting the hypothesis that this population comprises bona fide chicken cDCs. Further analysis revealed that chicken cDCs expressed CSF1R but lacked the expression of CSF2R. Collectively, the cell surface phenotypes of chicken cDCs were partially conserved with mammalian XCR1⁺ cDC1, with distinct differences in CSF1R and CSF2R expression compared with mammalian orthologues. These original reagents allow the efficient identification of chicken cDCs to investigate their important roles in the chicken immunity and diseases.

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.

Analysis of lung stromal expression of the atypical chemokine receptor ACKR2 reveals unanticipated expression in murine blood endothelial cells

Analysis of chemokine receptor, and atypical chemokine receptor, expression is frequently hampered by the lack of availability of high‐quality antibodies and the species specificity of those that are available. We have previously described methodology utilizing Alexa‐Fluor‐labeled chemokine ligands as versatile reagents to detect receptor expression. Previously this has been limited to hematopoietic cells and methodology for assessing expression of receptors on stromal cells has been lacking. Among chemokine receptors, the ones most frequently expressed on stromal cells belong to the atypical chemokine receptor subfamily. These receptors do not signal in the classic sense in response to ligand but scavenge their ligands and degrade them and thus sculpt in vivo chemokine gradients. Here, we demonstrate the ability to use either intratracheal or intravenous, Alexa‐Fluor‐labeled chemokine administration to detect stromal cell populations expressing the atypical chemokine receptor ACKR2. Using this methodology, we demonstrate, for the first time, expression of ACKR2 on blood endothelial cells. This observation sets the lung aside from other tissues in which ACKR2 is exclusively expressed on lymphatic endothelial cells and suggest unique roles for ACKR2 in the pulmonary environment.

The Atypical Chemokine Receptor Ackr2 Constrains NK Cell Migratory Activity and Promotes Metastasis

Chemokines have been shown to be essential players in a range of cancer contexts. In this study, we demonstrate that mice deficient in the atypical chemokine receptor Ackr2 display impaired development of metastasis in vivo in both cell line and spontaneous models. Further analysis reveals that this relates to increased expression of the chemokine receptor CCR2, specifically by KLRG1⁺ NK cells from the Ackr2^−/− mice. This leads to increased recruitment of KLRG1⁺ NK cells to CCL2-expressing tumors and enhanced tumor killing. Together, these data indicate that Ackr2 limits the expression of CCR2 on NK cells and restricts their tumoricidal activity. Our data have important implications for our understanding of the roles for chemokines in the metastatic process and highlight Ackr2 and CCR2 as potentially manipulable therapeutic targets in metastasis.

Molecular vaccine prepared by fusion of XCL1 to the multi-epitope protein of foot-and-mouth disease virus enhances the specific humoural immune response in cattle

Targeting antigen to dendritic cells (DCs) is a promising way to manipulate the immune response and to design prophylactic molecular vaccines. In this study, the cattle XCL1, ligand of XCR1, was fused to the type O foot-and-mouth disease virus (FMDV) multi-epitope protein (XCL-OB7) to create a molecular vaccine antigen, and an ^△XCL-OB7 protein with a mutation in XCL1 was used as the control. XCL-OB7 protein specifically bound to the XCR1 receptor, as detected by flow cytometry. Cattle vaccinated with XCL-OB7 showed a significantly higher antibody response than that to the ^△XCL-OB7 control (P < 0.05). In contrast, when XCL-OB7 was incorporated with poly (I:C) to prepare the vaccine, the antibody response of the immunized cattle was significantly decreased in this group and was lower than that in the ^△XCL-OB7 plus poly (I:C) group. The FMDV challenge indicated that cattle immunized with the XCL-OB7 alone or the ^△XCL-OB7 plus poly (I:C) obtained an 80% (4/5) clinical protective rate. However, cattle vaccinated with ^△XCL-OB7 plus poly (I:C) showed more effective inhibition of virus replication than that in the XCL-OB7 group after viral challenge, according to the presence of antibodies against FMDV non-structural protein 3B. This is the first test of DC-targeted vaccines in veterinary medicine to use XCL1 fused to FMDV antigens. This primary result showed that an XCL1-based molecular vaccine enhanced the antibody response in cattle. This knowledge should be valuable for the development of antibody-dependent vaccines for some infectious diseases in cattle.

Chemokine Detection Using Receptors Immobilized on an SPR Sensor Surface

Chemokines and their receptors take part in many physiological and pathological processes, and their dysregulated expression is linked to chronic inflammatory and autoimmune diseases, immunodeficiencies, and cancer. The chemokine receptors, members of the G protein-coupled receptor family, are integral membrane proteins, with seven-transmembrane domains that bind the chemokines and transmit signals through GTP-binding proteins. Many assays used to study the structure, conformation, or activation mechanism of these receptors are based on ligand-binding measurement, as are techniques to detect new agonists and antagonists that modulate chemokine function. Such methods require labeling of the chemokine and/or its receptor, which can alter their binding characteristics. Surface plasmon resonance (SPR) is a powerful technique for analysis of the interaction between immobilized receptors and ligands in solution, in real time, and without labeling. SPR measurements nonetheless require expression and purification steps that can alter the conformation, stability, and function of the chemokine and/or the chemokine receptor. In this review, we focus on distinct methods to immobilize chemokine receptors on the surface of an optical biosensor. We expose the advantages and disadvantages of different protocols used and describe in detail the method to retain viral particles as receptor carriers that can be used for SPR determinations.