Academic literature on the topic 'Patrolling monocyte'

Key Points SCD patients with a recent VOC episode have lower frequencies and numbers of HO-1hi patrolling monocytes. Heme-driven SCD vaso-occlusion is exacerbated in mice lacking patrolling monocytes and reversed following patrolling monocyte transfer.

Nonclassical monocytes undergo intravascular patrolling in blood vessels, positioning them ideally to coordinate responses to inflammatory stimuli. Under some circumstances, the actions of monocytes have been shown to involve promotion of neutrophil recruitment. However, the mechanisms whereby patrolling monocytes control the actions of neutrophils in the circulation are unclear. Here, we examined the contributions of monocytes to antibody- and neutrophil-dependent inflammation in a model of in situ immune complex-mediated glomerulonephritis. Multiphoton and spinning disk confocal intravital microscopy revealed that monocytes patrol both uninflamed and inflamed glomeruli using β2 and α4 integrins and CX3CR1. Monocyte depletion reduced glomerular injury, demonstrating that these cells promote inappropriate inflammation in this setting. Monocyte depletion also resulted in reductions in neutrophil recruitment and dwell time in glomerular capillaries and in reactive oxygen species (ROS) generation by neutrophils, suggesting a role for cross-talk between monocytes and neutrophils in induction of glomerulonephritis. Consistent with this hypothesis, patrolling monocytes and neutrophils underwent prolonged interactions in glomerular capillaries, with the duration of these interactions increasing during inflammation. Moreover, neutrophils that interacted with monocytes showed increased retention and a greater propensity for ROS generation in the glomerulus. Also, renal patrolling monocytes, but not neutrophils, produced TNF during inflammation, and TNF inhibition reduced neutrophil dwell time and ROS production, as well as renal injury. These findings show that monocytes and neutrophils undergo interactions within the glomerular microvasculature. Moreover, evidence indicates that, in response to an inflammatory stimulus, these interactions allow monocytes to promote neutrophil recruitment and activation within the glomerular microvasculature, leading to neutrophil-dependent tissue injury.

Abstract Causing leukocyte activation and upregulation of adhesion molecules on endothelial cells. CD16+ monocytes, also known as endothelial patrolling monocytes, normally scavenge the damaged cells and debris from the vasculature. As compared to other monocyte subsets or immune cell types, the CD16+monocyte subset expresses higher levels of the anti-inflammatory heme oxygenase 1 (HO-1), a heme degrading enzyme. Given the role of CD16+ monocytes as scavengers of debris on endothelial cells, we tested the hypothesis that this subset may protect SCD vasculature from the ongoing hemolytic insult through expression of high levels of HO-1. We found roughly 35% of circulating CD16+ monocytes from SCD patients expressed very high levels of HO-1 as compared to 5% in healthy controls. The HO-1hi SCD monocytes expressed significantly (30%) less TNF-a compared to HO-1lo monocytes following stimulation, consistent with anti-inflammatory effects of HO-1. We hypothesized that uptake of free hemoglobin/heme was responsible for high HO-1 expression levels in SCD CD16+ monocytes. To test this, healthy donors (HDs) or SCD patient monocytes were treated with different doses of free heme or hemolysed RBCs. We found dose-dependent HO-1 induction (five-fold at 20mM heme) in purified CD16- monocytes, but surprisingly none in CD16+ subset. However, upon co-culture with human umbilical vein endothelial cells (HUVEC), continuous or prior exposure to heme induced HO-1hi expression exclusively in CD16+ monocytes (5 fold in HD and further two fold in SCD compared to non-heme treated cocultures, p<0.001). Using imagining flow cytometric analysis, we found marked increase in uptake of heme-exposed endothelial cell-derived material by CD16+ monocytes (HD: 2% to 13% ± 3%; in SCD: 20% ± 3% to 30% ± 4%, p< 0.001) but none by CD16- monocytes. Our transwell studies demonstrated that cell-cell contact between CD16+ monocytes and heme-exposed HUVEC was required for HO-1hi expression. We found roughly 4-fold increase in expression of phosphatidylserine (PS, annexin V+), ICAM-1 and vCAM-1 on heme-treated HUVEC cells. Antibody blocking studies identified PS moieties as well as ICAM-1 as key molecules involved in monocyte-HUVEC interactions that mediated HO-1hi induction, suggesting that high levels of HO-1 expression in SCD CD16+ monocytes is in part the result of attachment to and engulfment of apoptotic, activated endothelial cells damaged by heme. SCD patients suffer from vaso-occlusive crisis (VOC), resulting from increased attachment of SCD RBCs to damaged and activated endothelium. We hypothesized that inadequate numbers or lower HO-1hi levels in CD16+ monocyte will predispose SCD patients to episodes of VOC due to decreased removal by CD16+ monocytes of damaged endothelial and sickle RBCs. Amongst SCD patients receiving chronic transfusions, we found a two-fold lower frequency of circulating CD16+ monocytes and half the numbers of CD16+HO-1hi monocytes in patients with a recent history of VOC episode as compared to those without VOC (p< 0.01); the former group also expressed higher levels of circulating sVCAM-1 (997 ± 210 vs 765 ± 236 ng/m, p=0.02), a marker of endothelial activation. To formally test the role of patrolling monocytes in endothelial damage induced by SCD RBCs and heme, we injected RBCs from Townes SCD mice alone or after 24hrs with heme into Nr4a1-knockout mice which have a selective loss of patrolling monocytes. Immunofluorescence analysis of liver vasculature showed a 3 fold increase in the activated endothelial marker, ICAM-1 within 24hrs following injection of sickle RBCs and two-fold increase in circulating sVCAM-1 levels in mice treated with sickle RBC plus heme (p <0.001). Transfer of HO-1+ patrolling monocytes (LY6Clo), but not a classical monocyte subset (LY6C+) reversed activated endothelial phenotype, indicating that patrolling monocytes can inhibit SCD-induced endothelial activation. Altogether, these data suggest that SCD patrolling monocytes remove hemolysis-damaged endothelial cells, resulting in HO-1 upregulation and dampening of vascular inflammation. Perturbations in CD16+ monocyte numbers resulting in lower local HO-1 levels can predispose SCD patients to VOC, thus identifying HO-1+ patrolling monocytes as key players in VOC pathophysiology and as therapeutic targets. Disclosures No relevant conflicts of interest to declare.

Objective— Nonclassical monocytes (NCM) function to maintain vascular homeostasis by crawling or patrolling along the vessel wall. This subset of monocytes responds to viruses, tumor cells, and other pathogens to aid in protection of the host. In this study, we wished to determine how early atherogenesis impacts NCM patrolling in the vasculature. Approach and Results— To study the role of NCM in early atherogenesis, we quantified the patrolling behaviors of NCM in ApoE −/− (apolipoprotein E) and C57BL/6J mice fed a Western diet. Using intravital imaging, we found that NCM from Western diet–fed mice display a 4-fold increase in patrolling activity within large peripheral blood vessels. Both human and mouse NCM preferentially engulfed OxLDL (oxidized low-density lipoprotein) in the vasculature, and we observed that OxLDL selectively induced NCM patrolling in vivo. Induction of patrolling during early atherogenesis required scavenger receptor CD36, as CD36 −/− mice revealed a significant reduction in patrolling activity along the femoral vasculature. Mechanistically, we found that CD36-regulated patrolling was mediated by a SFK (src family kinase) through DAP12 (DNAX activating protein of 12KDa) adaptor protein. Conclusions— Our studies show a novel pathway for induction of NCM patrolling along the vascular wall during early atherogenesis. Mice fed a Western diet showed increased NCM patrolling activity with a concurrent increase in SFK phosphorylation. This patrolling activity was lost in the absence of either CD36 or DAP12. These data suggest that NCM function in an atheroprotective manner through sensing and responding to oxidized lipoprotein moieties via scavenger receptor engagement during early atherogenesis.

Abstract Monocytes are a heterogeneous group of mononuclear innate immune cells that have diverse inflammatory responses. In the context of cancer, monocytes and monocyte-derived cells have been evaluated for their pro- and anti-tumoral effects in the tumor microenvironment. These functions range from induction of tumor cell death to suppression of T cells, promotion of angiogenesis and remodeling of the extracellular matrix. Outside of the primary tumor, monocytes in circulation maintain their dichotomous role in cancer immunosurveillance. Specifically, patrolling non-classical CD14−CD16+ monocytes have been found to play a role in the prevention of metastasis. In contrast, CD14+ monocyte-derived cells from patients with solid tumors have been shown to promote tumor progression. Therefore, understanding this monocytic heterogeneity as well as other unexplored roles (i.e. monocyte-mediated phagocytosis of tumor cells) is crucially important for malignancies with high rates of metastasis. Yet, the phenotypic and functional diversity of circulating monocytes in patients with metastatic solid malignancies is still largely unknown. In this study, we sought to characterize and compare peripheral blood monocytes obtained from healthy donors and patients with advanced stage solid tumors. Through flow cytometric analysis and functional assays, we determined the subpopulation distributions as well as the phagocytic and suppressive activities of the monocytic compartment in patients with advanced cancer and healthy controls. Providing new insights into their cancer-related functions, we highlight the need for consideration of circulating monocytes into immune-targeting approaches in metastatic solid malignancies.

Abstract Monocytes are innate immune cells recognized for their ability to play both tumor permissive and surveillant roles in cancer. Circulating classical monocytes (CD14+CD16−) can home to the tumor and suppress other immune cells through various mechanisms, including the production of arginase and the release of reactive oxygen species (ROS). Conversely, patrolling nonclassical monocytes (CD14−CD16+) have been shown to employ processes such as phagocytosis and presentation of tumor antigens to prevent metastasis. This heterogeneous monocyte function is influenced by tumor-derived factors that are released during cancer development and progression. Phenotypic and transcriptional alterations in circulating monocytes and other myeloid cells in patients with solid tumors have been reported and associated with poor clinical outcomes. However, perturbations of specific monocyte functions in the setting of solid tumors have not been well explored. Here we present a characterization of monocytes by coupling flow cytometry-based functional assays with sequencing (Func-seq). Healthy donor primary monocytes and monocytic cell lines were used to examine the production of ROS and arginase in response to osteosarcoma-conditioned media and monocyte-mediated phagocytosis of osteosarcoma cells. Bulk RNA-seq and ATAC-seq were performed on FACS-sorted populations to compare differentially expressed genes and establish transcriptomic and epigenetic signatures associated with monocyte-mediated immunosuppression and tumor-cell phagocytosis. The incorporation of functional selection into -omic characterization provides insights into monocyte behavior and potential therapeutic targets to alter their activity in solid tumors. Funding: US National Institutes of Health grants ZIA BC 011332 and ZIA BC 011855 and NCI cancer moonshot.

Abstract Nonclassical (patrolling) monocytes are a population of monocytes which monitor the vasculature and participate in tissue repair. Though nonclassical monocytes are thought to be developmentally derived from classical monocytes, the mechanisms that control the differentiation of classical monocytes into nonclassical monocytes are still not completely understood. Signaling through the Notch2 receptor appears to trigger development of nonclassical monocytes. Also, the transcription factors C/EBPβ and Nur77 (Nr4a1) are required for nonclassical monocyte development in vivo. We have now identified additional transcription factor requirements for the differentiation and/or survival of nonclassical monocytes in vivo. We find that mice with conditional deletion of Bcl6 in myeloid cells or germline IRF2 deficiency show a severe loss of nonclassical monocytes. Both Bcl6 and IRF2 are induced during the transition from classical to nonclassical monocyte. In vitro culture of myeloid progenitors on stromal cells expressing DLL1, a Notch2 ligand, recapitulates aspects of nonclassical monocyte development seen in vivo, confirming Notch signaling as a key initiating event in the development of nonclassical monocytes. This system allows for the identification of the downstream targets of Notch2 signaling that drive nonclassical monocyte development, as well as the analysis of the impact of other transcription factors on the differentiation and survival of these cells in vivo. We present a model of the steps required for development of murine nonclassical monocytes. Supported by NIH (RO1 AI150297, RO1 CA248919)

Monocytes circulate in the blood and migrate to inflammatory tissues, but their functions can be either detrimental or beneficial, depending on their phenotypes. In humans, classical monocytes are inflammatory cluster of differentiation (CD)14++CD16−CCR2++ cells originated from the bone marrow or spleen reservoirs and comprise ≥92% of monocytes. Intermediate monocytes (CD14++CD16+CCR2+) are involved in the production of anti-inflammatory cytokines [such as interleukin (IL)-10], reactive oxygen species (ROS), and proinflammatory mediators [such as tumor necrosis factor-α (TNF-α) and IL-1β). Nonclassical monocytes (CD14+CD16++CCR2−) are patrolling cells involved in tissue repair and debris removal from the vasculature. Many studies in both humans and animals have shown the importance of monocyte chemoattractant protein-1 (MCP-1) and its receptor [chemokine receptor of MCP-1 (CCR2)] in pathologies, such as atherosclerosis and myocardial infarction (MI). This review presents the importance of these monocyte subsets in cardiovascular diseases (CVDs), and sheds light on new strategies for the blocking of the MCP-1/CCR2 axis as a therapeutic goal for treating vascular disorders.

Human monocytes have been classified into three distinct groups, classical (anti-inflammatory; CD14+/CD16−), nonclassical (patrolling; CD14+/CD16++), and intermediate (proinflammatory; CD14++/CD16+). Adhesion of nonclassical/intermediate monocytes with the endothelium is important for innate immunity, and also vascular inflammatory disease. However, there is an incomplete understanding of the mechanisms that regulate CD16+ versus CD16− monocyte adhesion to the inflamed endothelium. Here, we tested the hypothesis that a high-mannose (HM) N-glycoform of intercellular adhesion molecule-1 (ICAM-1) on the endothelium mediates the selective recruitment of CD16+ monocytes. Using TNF-α treatment of human umbilical vein endothelial cells (HUVECs), and using proximity ligation assay for detecting proximity of specific N-glycans and ICAM-1, we show that TNF-α induces HM-ICAM-1 formation on the endothelial surface in a time-dependent manner. We next measured CD16− or CD16+ monocyte rolling and adhesion to TNF-α-treated HUVECs in which HM- or hybrid ICAM-1 N-glycoforms were generated using the α-mannosidase class I and II inhibitors, kifunensine and swainsonine, respectively. Expression of HM-ICAM-1 selectively enhanced CD16+ monocyte adhesion under flow with no effect on CD16− monocytes noted. CD16+ monocyte adhesion was abrogated by blocking either HM epitopes or ICAM-1. A critical role for HM-ICAM-1 in mediating CD16+ monocyte rolling and adhesion was confirmed using COS-1 cells engineered to express HM or complex ICAM-1 N-glycoforms. These data suggest that HM-ICAM-1 selectively recruits nonclassical/intermediate CD16+ monocytes to the activated endothelium. NEW & NOTEWORTHY Monocyte subsets have been associated with cardiovascular disease, yet it is unknown how different subsets are recruited to the endothelium. This study demonstrates the formation of distinct ICAM-1 N-glycoforms in the activated endothelium and reveals a key role for high mannose ICAM-1 in mediating proinflammatory CD16+ monocyte adhesion. Presented data identify roles for endothelial N-glycans in recruiting specific monocyte subsets during inflammation.

Mononuclear phagocytes are crucial arms of the innate immune system, performing various fonctions, from clearance of pathogenic microorganisms, to maintenance of tissue homeostasis. Two cell types have been studied; first, monocytes, bone marrow-derived cells, of which two main subsets have been described. Ly6C+ monocytes are short-lived circulating cells that have the ability to quickly migrate into tissues, and produce a range of molecules, or differentiate into inflammatory cells. Ly6Clow monocytes are a patrolling cell type, which crawls on the luminal side of vessels where they perform a monitoring of the vasculature. In a collaborative effort, using nucleoside analogs and experimentally parameterised, computer-assisted modeling, the dynamics of these two subsets has been re-examined, in the light of genetic data obtained previously in the laboratory. The results suggest that the transition between a Ly6C+ to a Ly6Clow phenotype occurs more prominently in the bone marrow, rather than the blood as suggested by the literature. Second, macrophages in the kidney were investigated in the context of circulating small immune complexes. Macrophages are large, resident cells which develop during embryonic life, maintaining their numbers locally and independently from bone marrow-derived cells in most tissues. Kidney resident macrophages perform a constant monitoring of the circulation, in collaboration with the endothelial cells they underlie. The experimental model for the study of their response to immune complexes was studied, and the results provide insight into the unique character of this function for kidney resident macrophages, as well as clarifying the means of access of the immune complexes to the macrophages. Together, the work presented in this thesis sheds light on particular aspects of the biology of myeloid populations, in steady state and inflammatory settings.