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Leukocyte extravasation into inflamed tissue is a complex process that is difficult to capture as a whole in vitro. We employed a blood-vessel-on-a-chip model in which endothelial cells were cultured in a tube-like lumen in a collagen-1 matrix. The vessels are leak-tight, creating a barrier for molecules and leukocytes. Addition of inflammatory cytokine TNF-α caused vasoconstriction, actin remodelling and upregulation of ICAM-1. Introducing leukocytes into the vessels allowed real-time visualisation of all different steps of the leukocyte transmigration cascade including migration into the extracellular matrix. Individual cell tracking over time distinguished striking differences in migratory behaviour between T-cells and neutrophils. Neutrophils cross the endothelial layer more efficiently than T-cells, but upon entering the matrix, neutrophils display high speed but low persistence, whereas T-cells migrate with low speed and rather linear migration. In conclusion, 3D imaging in real-time of leukocyte extravasation in a vessel-on-a-chip enables detailed qualitative and quantitative analysis of different stages of the full leukocyte extravasation process in a single assay.SummaryA functional hydrogel-based blood-vessel-on-a-chip model is used to study the complete leukocyte transendothelial migration process in real time. T-lymphocytes and neutrophils exhibit distinct migration dynamics in the extravascular matrix after transendothelial migration, which can be altered using a chemotactic gradient.

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK) is a member of the TNF ligand family involved in various diseases including brain inflammatory pathologies such as multiple sclerosis. It has been demonstrated that TWEAK can induce cerebrovascular permeability in an in vitro model of the blood–brain barrier. The molecular mechanisms playing a role in TWEAK versus TNFα signaling on cerebral microvascular endothelial cells are not well defined. Therefore, we aimed to identify gene expression changes in cultures of human brain microvascular endothelial cells (hCMEC/D3) to address changes initiated by TWEAK exposure. Taken together, our studies highlighted that gene involved in leukocyte extravasation, notably claudin-5, were differentially modulated by TWEAK and TNFα. We identified differential gene expression of hCMEC/D3 cells at three timepoints following TWEAK versus TNFα stimulation and also found distinct modulations of several canonical pathways including the actin cytoskeleton, vascular endothelial growth factor (VEGF), Rho family GTPases, and phosphatase and tensin homolog (PTEN) pathways. To our knowledge, this is the first study to interrogate and compare the effects of TWEAK versus TNFα on gene expression in brain microvascular endothelial cells.

For leukocytes crossing the vessel wall to solve inflammation, the endothelium acts as customs control. In the past years, progress has been made on the molecular mechanisms that are used by both the leukocytes and endothelium to allow efficient crossing, although not all the exact rules these vascular customs play by are completely understood. In this review, we focus on the contribution of the endothelium to the process of leukocyte extravasation and summarize the different molecular mechanisms involved in efficient leukocyte passage and prevention of local leakage at the same time. We will highlight the dynamic regulation of the endothelial actin cytoskeleton, which under the influence of different stimuli is a key player in leukocyte transendothelial migration.

Background: Identification of clinically useful biomarkers for Nasal Polyposis in chronic rhinosinusitis (CRSwNP) has proven dif-ficult. We analyzed gene expression profiling data to find explanations for this. Methods: We analyzed mRNA expression profiling data, GSE36830, of six uncinate tissues from healthy controls and six NP from CRSwNP patients. We performed Ingenuity Pathway Analysis (IPA) of differentially expressed genes to identify pathways and predicted upstream regulators. Results: We identified 1,608 differentially expressed genes and 177 significant pathways, of which Th1 and Th2 activation pathway and leukocyte extravasation signaling were most significant. We identified 75 upstream regulators whose activity was predicted to be upregulated. These included regulators of known pathogenic and therapeutic relevance, like IL-4. However, only seven of the 75 regulators were actually differentially expressed in NP, namely CSF1, TYROBP, CCL2, CCL11, SELP, ADORA3, ICAM1. Interes-tingly, these did not include IL-4, and four of the seven were receptors. This suggested a potential explanation for the discrepancy between the predicted and observed expression levels of the regulators, namely that the receptors, and not their ligands, were upregulated. Indeed, we found that 10 receptors of key predicted upstream regulators were upregulated, including IL4R. Conclusion: Our findings indicate that the difficulties in finding specific biomarkers for CRSwNP depend on the complex underly-ing mechanisms, which include multiple pathways and regulators, each of which may be subdivided into multiple components such as ligands, soluble and membrane-bound receptors. This suggests that combinations of biomarkers may be needed for CRSwNP diagnostics.

During inflammation, endothelial cells are bombarded with cytokines and other stimuli from surrounding cells. Leukocyte extravasation and vascular leakage are both prominent but believed to be uncoupled as they occur in separate spatiotemporal patterns. In this study, we investigated a “double-hit” approach on primary human endothelial cells primed with LPS followed by histamine. Using neutrophil transendothelial migration (TEM) under physiological flow assays, we found that an LPS-primed endothelium synergistically enhanced neutrophil TEM when additionally treated with histamine, whereas the effects on neutrophil TEM of the individual stimuli were moderate to undetectable. Interestingly, the double-hit–induced TEM increase was not due to decreased endothelial barrier, increased adhesion molecule expression, or Weibel–Palade body release. Instead, we found that it was directly correlated with junctional remodeling. Compounds that increased junctional “linearity” (i.e., stability) counteracted the double-hit effect on neutrophil TEM. We conclude that a compound, in this case histamine (which has a short primary effect on vascular permeability), can have severe secondary effects on neutrophil TEM in combination with an inflammatory stimulus. This effect is due to synergic modifications of the endothelial cytoskeleton and junctional remodeling. Therefore, we hypothesize that junctional linearity is a better and more predictive readout than endothelial resistance for compounds aiming to attenuate inflammation.

Rationale: Patients with elevated levels of lipoprotein(a) [Lp(a)] are hallmarked by increased metabolic activity in the arterial wall on positron emission tomography/computed tomography, indicative of a proinflammatory state. Objective: We hypothesized that Lp(a) induces endothelial cell inflammation by rewiring endothelial metabolism. Methods and Results: We evaluated the impact of Lp(a) on the endothelium and describe that Lp(a), through its oxidized phospholipid content, activates arterial endothelial cells, facilitating increased transendothelial migration of monocytes. Transcriptome analysis of Lp(a)-stimulated human arterial endothelial cells revealed upregulation of inflammatory pathways comprising monocyte adhesion and migration, coinciding with increased 6-phophofructo-2-kinase/fructose-2,6-biphosphatase (PFKFB)-3–mediated glycolysis. ICAM (intercellular adhesion molecule)-1 and PFKFB3 were also found to be upregulated in carotid plaques of patients with elevated levels of Lp(a). Inhibition of PFKFB3 abolished the inflammatory signature with concomitant attenuation of transendothelial migration. Conclusions: Collectively, our findings show that Lp(a) activates the endothelium by enhancing PFKFB3-mediated glycolysis, leading to a proadhesive state, which can be reversed by inhibition of glycolysis. These findings pave the way for therapeutic agents targeting metabolism aimed at reducing inflammation in patients with cardiovascular disease.

Arteriogenesis strongly depends on leukocyte and platelet recruitment to the perivascular space of growing collateral vessels. The standard approach for analyzing collateral arteries and leukocytes in arteriogenesis is ex vivo (immuno-) histological methodology. However, this technique does not allow the measurement of dynamic processes such as blood flow, shear stress, cell-cell interactions, and particle velocity. This paper presents a protocol to monitor in vivo processes in growing collateral arteries during arteriogenesis utilizing intravital imaging. The method described here is a reliable tool for dynamics measurement and offers a high-contrast analysis with minimal photo-cytotoxicity, provided by multiphoton excitation microscopy. Prior to analyzing growing collateral arteries, arteriogenesis was induced in the adductor muscle of mice by unilateral ligation of the femoral artery. After the ligation, the preexisting collateral arteries started to grow due to increased shear stress. Twenty-four hours after surgery, the skin and subcutaneous fat above the collateral arteries were removed, constructing a pocket for further analyses. To visualize blood flow and immune cells during in vivo imaging, CD41-fluorescein isothiocyanate (FITC) (platelets) and CD45-phycoerythrin (PE) (leukocytes) antibodies were injected intravenously (i.v.) via a catheter placed in the tail vein of a mouse. This article introduces intravital multiphoton imaging as an alternative or in vivo complementation to the commonly used static ex vivo (immuno-) histological analyses to study processes relevant for arteriogenesis. In summary, this paper describes a novel and dynamic in vivo method to investigate immune cell trafficking, blood flow, and shear stress in a hindlimb model of arteriogenesis, which enhances evaluation possibilities notably.

Immune checkpoint inhibitors (ICIs) have become the standard of care for several cancers. However, ICI therapy has also been associated with various immune-related adverse events (irAEs). Clinical manifestations of immune-related colitis resemble those of inflammatory bowel diseases such as ulcerative colitis (UC). The composition of the bowel microflora is thought to influence the development of inflammatory bowel disease and irAE colitis. We profiled the gene expressions and microbe compositions of colonic mucosa from patients with solid cancers receiving anti-PD-L1 antibody treatment; we then compared the expression profiles associated with irAE colitis with those associated with UC. The pathway enrichment analysis revealed functional similarities between inflamed regions of irAE colitis and UC. The common enriched pathways included leukocyte extravasation and immune responses, whereas non-inflamed mucosa from patients with irAE colitis was distinct from patients with UC and was characterized by the recruitment of immune cells. A similarity between the microbiota profiles was also identified. A decreased abundance of Bacteroides species was observed in inflamed regions from both irAE colitis and UC based on a microbiota composition analysis of 16S rDNA sequencing. Pathways associated with molecule transport systems, including fatty acids, were enriched in inflamed and non-inflamed irAE colitis and inflamed UC, similar to Piphillin-inferred KEGG pathways. While UC is characterized by local regions of inflammation, ICI treatment extends to non-inflammatory regions of the colonial mucosa where immune cells are reconstituted. This analysis of the similarity and heterogeneity of irAE colitis and UC provides important information for the management of irAE colitis.

The endothelium is a crucial regulator of vascular homeostasis by controlling barrier integrity as well acting as an important signal transducer, thereby illustrating that endothelial cells are not inert cells. In the context of atherosclerosis, this barrier function is impaired and endothelial cells become activated, resulting in the upregulation of adhesion molecules, secretion of cytokines and chemokines and internalization of integrins. Finally, this leads to increased vessel permeability, thereby facilitating leukocyte extravasation as well as fostering a pro-inflammatory environment. Additionally, activated endothelial cells can form migrating tip cells and proliferative stalk cells, resulting in the formation of new blood vessels. Emerging evidence has accumulated indicating that cellular metabolism is crucial in fueling these pro-atherosclerotic processes, including neovascularization and inflammation, thereby contributing to plaque progression and altering plaque stability. Therefore, further research is necessary to unravel the complex mechanisms underlying endothelial cell metabolic changes, and exploit this knowledge for finding and developing potential future therapeutic strategies. In this review we discuss the metabolic alterations endothelial cells undergo in the context of inflammation and atherosclerosis and how this relates to changes in endothelial functioning. Finally, we will describe several metabolic targets that are currently being used for therapeutic interventions.

A progressive loss of functional nephrons defines chronic kidney disease (CKD). Complications related to cardiovascular disease (CVD) are the principal causes of mortality in CKD; however, the acceleration of CVD in CKD remains unresolved. Our study used a complementary proteomic approach to assess mild and advanced CKD patients with different atherosclerosis stages and two groups of patients with different classical CVD progression but without renal dysfunction. We utilized a label-free approach based on LC-MS/MS and functional bioinformatic analyses to profile CKD and CVD leukocyte proteins. We revealed dysregulation of proteins involved in different phases of leukocytes' diapedesis process that is very pronounced in CKD's advanced stage. We also showed an upregulation of apoptosis-related proteins in CKD as compared to CVD. The differential abundance of selected proteins was validated by multiple reaction monitoring, ELISA, Western blotting, and at the mRNA level by ddPCR. An increased rate of apoptosis was then functionally confirmed on the cellular level. Hence, we suggest that the disturbances in leukocyte extravasation proteins may alter cell integrity and trigger cell death, as demonstrated by flow cytometry and microscopy analyses. Our proteomics data set has been deposited to the ProteomeXchange Consortium via the PRIDE repository with the data set identifier PXD018596.

Chromosome Instability (CIN) in tumors affects carcinogenesis, drug resistance, and recurrence/prognosis. Thus, it has a high impact on outcomes in clinic. However, how CIN occurs in human tumors remains elusive. Although cells with CIN (i.e., pre/early cancer cells) are proposed to be removed by apoptosis and/or a surveillance mechanism, this surveillance mechanism is poorly understood. Here we employed a novel data-mining strategy (Gene Expression to Copy Number Alterations [CNA], “GE-CNA”) to comprehensively identify 1578 genes that associate with CIN, indicated by genomic CNA as its surrogate marker, in human lung adenocarcinoma. We found that (a) amplification/insertion CNA is facilitated by over-expressions of DNA replication stressor and suppressed by a broad range of immune cells (T-, B-, NK-cells, leukocytes), and (b) deletion CNA is facilitated by over-expressions of mitotic regulator genes and suppressed predominantly by leukocytes guided by leukocyte extravasation signaling. Among the 39 CNA- and survival-associated genes, the purine metabolism (PPAT, PAICS), immune-regulating CD4-LCK-MEC2C and CCL14-CCR1 axes, and ALOX5 emerged as survival-critical pathways. These findings revealed a broad role of the immune system in suppressing CIN/CNA and cancer development in lung, and identified components representing potential targets for future chemotherapy, chemoprevention, and immunomodulation approaches for lung adenocarcinoma.

Introduction Blood platelets are increasingly recognized as modulators of leukocyte effector functions in various pathologies including acute lung injury (ALI). ALI is a life-threatening disease, caused by damage to the alveolar epi- and endothelium. Excessive accumulation of leukocytes leads to severe lung inflammation, resulting in impaired lung function and hypoxemia. Objective Since leukocyte migration is modulated by activated platelets and phosphatidylinositol 3-kinase (PI3K) signaling is involved in platelet function, we aimed to elucidate the effect of PI3K on platelet-mediated immune responses. Materials and Methods We generated a mouse model with a platelet-specific deletion of p85α, the most important regulatory subunit of the class IA PI3K, and evaluated platelet function and platelet–leukocyte interactions. Moreover, we analyzed the impact of platelet-specific p85α gene deficiency during sterile peritonitis and acid-induced ALI. Results In vitro analyses of platelets revealed that lack of p85α led to decreased downstream signaling and diminished expression of surface activation markers, for example, CD62P and CD63, as well as reduced platelet aggregation. Moreover, platelet PI3K essentially mediated direct interactions of platelets with monocytes and neutrophils. In mice, platelet-specific p85α deficiency prevented leukocyte infiltration into the peritoneum and the bronchoalveolar compartment during sterile peritonitis and ALI, respectively. Additionally, the release of the inflammatory cytokine interleukin-12/23 was diminished in platelet p85α-deficient mice during ALI. In contrast to PI3K, neither overexpression nor depletion of platelet phosphatase and tensin homolog, the endogenous antagonist of PI3K, significantly modulated platelet function. Conclusion Our data indicate a crucial role of platelet PI3K signaling for leukocyte extravasation upon inflammatory stimuli in various diseases models.

Precise spatiotemporal regulation of leukocyte extravasation is key for generating an efficient immune response to injury or infection. The integrins LFA-1(CD11a/CD18) and Mac-1(CD11b/CD18) play overlapping roles in neutrophil migration because they bind the same as well as different ligands in response to extracellular signaling. Using two-photon intravital imaging and transmission electron microscopy, we observed the existence of preferred sites for neutrophil entrance into the endothelial cell monolayer and exit from the basement membrane and pericyte sheath during neutrophil extravasation, namely, hotspots I and II, by elucidating distinctive roles of LFA-1 and Mac-1. To penetrate the vascular endothelium, neutrophils must first penetrate the endothelial cell layer through hotspot I (i.e., the point of entry into the endothelium). Neutrophils frequently remain in the space between the endothelial cell layer and the basement membrane for a prolonged period (>20 min). Subsequently, neutrophils penetrate the basement membrane and pericyte sheath at hotspot II, which is the final stage of exiting the vascular endothelium. To further investigate the roles of LFA-1 and Mac-1, we newly generated LFA-1 FRET (CD11a-YFP/CD18-CFP) mice and Mac-1 FRET (CD11b-YFP/CD18-CFP) mice. Using both FRET mice, we were able to determine that LFA-1 and Mac-1 distinctly regulate the neutrophil extravasation cascade. Our data suggest that the vascular endothelium functions as a double-layered barrier in the steps of neutrophil extravasation. We propose that the harmonized regulation of neutrophil penetration through the endothelium via hotspots I and II may be critical for vascular homeostasis during inflammation., Immune response: White blood cells follow the leader In response to infection or tissue damage, white blood cells known as leukocytes exit blood vessels at specific sites, “hotspots.” Many leukocytes follow each other out through the same hotspot. As part of the body’s initial response to infection or tissue damage, leukocytes are carried to infected or damaged tissues by blood vessels. To fight infection, they must exit the vessels, but how they exited was poorly understood. Young-Min Hyun at the Yonsei University College of Medicine in Seoul, South Korea and co-workers used advanced imaging techniques to visualize leukocyte delivery. They found that many leukocytes use a single hotspot to enter the blood vessel wall, travel through the wall’s interior, and then exit the wall at another hotspot. Using hotspots is thought to minimize the number of perforations in the vessel wall, maintaining vessel integrity. These results illuminate a key aspect of how efficiently the body fights infection from the viewpoint of leukocyte migration.

Leukocyte transmigration through the blood vessel wall is a fundamental step of the inflammatory response and requires expression of adhesion molecule PECAM-1. Accumulating evidence implicates that semaphorin (Sema) 3F and its receptor neuropilin (NRP) 2 are central regulators in vascular biology. Herein, we assess the role of Sema3F in leukocyte migration in vitro and in vivo. To determine the impact of Sema3F on leukocyte recruitment in vivo, we used the thioglycollate-induced peritonitis model. After the induction of peritonitis, C57BL/6 mice were intraperitoneally (i.p.) injected daily with recombinant Sema3F or solvent for 3 days. Compared with solvent-treated controls, leukocyte count was increased in the peritoneal lavage of Sema3F-treated mice indicating that Sema3F promotes leukocyte extravasation into the peritoneal cavity. In line with this observation, stimulation of human endothelial cells with Sema3F enhanced the passage of peripheral blood mononuclear cells (PBMCs) through the endothelial monolayer in the transwell migration assays. Conversely, silencing of endothelial Sema3F by siRNA transfection dampened diapedesis of PBMCs through the endothelium in vitro. xMechanistically, Sema3F induced upregulation of adhesion molecule PECAM-1 in endothelial cells and in murine heart tissue shown by immunofluorescence and western blotting. The inhibition of PECAM-1 by blocking antibody HEC7 blunted Sema3F-induced leukocyte migration in transwell assays. SiRNA-based NRP2 knockdown reduced PECAM-1 expression and migration of PBMCs in Sema3F-treated endothelial cells, indicating that PECAM-1 expression and leukocyte migration in response to Sema3F depend on endothelial NRP2. To assess the regulation of Sema3F in human inflammatory disease, we collected serum samples of patients from day 0 to day 7 after survived out-of-hospital cardiac arrest (OHCA, n = 41). First, we demonstrated enhanced migration of PBMCs through endothelial cells exposed to the serum of patients after OHCA in comparison to the serum of patients with stable coronary artery disease or healthy volunteers. Remarkably, serum samples of OHCA patients contained significantly higher Sema3F protein levels compared with CAD patients (CAD, n = 37) and healthy volunteers (n = 11), suggesting a role of Sema3F in the pathophysiology of the inflammatory response after OHCA. Subgroup analysis revealed that elevated serum Sema3F levels after ROSC are associated with decreased survival, myocardial dysfunction, and prolonged vasopressor therapy, clinical findings that determine the outcome of post-resuscitation period after OHCA. The present study provides novel evidence that endothelial Sema3F controls leukocyte recruitment through a NRP2/PECAM-1-dependent mechanism. Sema3F serum concentrations are elevated following successful resuscitation suggesting that Sema3F might be involved in the inflammatory response after survived OHCA. Targeting the Sema3F/NRP2/PECAM-1 pathway could provide a novel approach to abolish overwhelming inflammation after resuscitation.

We investigated whether resveratrol (RSV) can attenuate obesity and diabetes progression and improve diabetes-induced vascular dysfunction, and we attempted to delineate its underlying mechanisms. Male C57Bl/6 mice were administered a high-fat diet (HFD) for 17 weeks. Mice developed type 2 diabetes with increased body weight, hyperglycemia, hyperinsulinemia, and hyperlipidemia. Oral gavage with RSV significantly reversed the symptoms induced by the HFD. Insulin sensitivity likewise improved after the RSV intervention in these mice. Phenylephrine-induced cremaster arteriolar constriction was impaired, whereas RSV treatment significantly mitigated the vessel responsiveness to phenylephrine. The obese diabetic mice exhibited increased leukocyte rolling, adhesion, and transmigration in the postcapillary venules of the cremaster muscle. By contrast, RSV treatment significantly attenuated HFD-induced extravasation. RSV significantly recovered phosphorylated Akt and eNOS expression in the thoracic aorta. In addition, activated adenosine monophosphate-activated protein kinase in the thoracic aorta was involved in the improvement of epithelial function after RSV intervention. RSV considerably upregulated the plasma NO level in HFD mice. Moreover, RSV-enhanced human umbilical vein endothelial cells healing through Sirt1/ER pathway may be involved in the prevention of leukocyte extravasation. Collectively, RSV attenuates diabetes-induced vascular dysfunction by activating Akt/eNOS/NO and Sirt1/ER pathway. Our mechanistic study provides a potential RSV-based therapeutic strategy against cardiovascular disease.

Dengue virus, the causative agent of dengue disease which may have hemorrhagic complications, poses a global health threat. Among the numerous target cells for dengue virus in humans are monocytes, macrophages and mast cells which are important regulators of vascular integrity and which undergo dramatic cellular responses after infection by dengue virus. The strategic locations of these three cell types, inside blood vessels (monocytes) or outside blood vessels (macrophages and mast cells) allow them to respond to dengue virus infection with the production of both intracellular and secretory factors which affect virus replication, vascular permeability and/or leukocyte extravasation. Moreover, the expression of Fc receptors on the surface of monocytes, macrophages and mast cells makes them important target cells for antibody-enhanced dengue virus infection which is a major risk factor for severe dengue disease, involving hemorrhage. Collectively, these features of monocytes, macrophages and mast cells contribute to both beneficial and harmful responses of importance to understanding and controlling dengue infection and disease.

The vascular endothelium is a highly specialized barrier that controls passage of fluids and migration of cells from the lumen into the vessel wall. Endothelial cells assist leukocytes to extravasate and despite the variety in the specific mechanisms utilized by different leukocytes to cross different vascular beds, there is a general principle of capture, rolling, slow rolling, arrest, crawling, and ultimately diapedesis via a paracellular or transcellular route. In atherosclerosis, the barrier function of the endothelium is impaired leading to uncontrolled leukocyte extravasation and vascular leakage. This is also observed in the neovessels that grow into the atherosclerotic plaque leading to intraplaque hemorrhage and plaque destabilization. This review focuses on the vascular endothelial barrier function and the interaction between endothelial cells and leukocytes during transmigration. We will discuss the role of endothelial dysfunction, transendothelial migration of leukocytes and plaque angiogenesis in atherosclerosis.

Previously, we discovered antigen-presenting cells to express DC-HIL receptor and to secrete its soluble form (soluble DC-HIL [sDC-HIL]), both of which bind to syndecan-4 on T cells and endothelial cells (ECs), with the former binding attenuating T-cell function and the latter binding promoting angiogenesis. In this study, we examined the effects of sDC-HIL binding to EC on T-cell extravasation using an allergic contact dermatitis model in mice. The hapten oxazolone applied to ear skin in sensitized mice upregulated cutaneous expression of sDC-HIL, which downregulated the allergic reaction by reducing transendothelial migration of T cells but not other immune cells (neutrophils and mast cells). Moreover, intravenously infused sDC-HIL bound to EC in blood vessels of oxazolone-challenged skin in a scattered and patchy pattern, and intravital microscopic analysis revealed that blood-circulating T cells firmly adhere to DC-HIL-treated endothelia. This regulatory property of sDC-HIL requires syndecan-4 expression by both EC and T cells. Our findings indicate that the DC-HIL/syndecan-4 pathway mediates a cross-talk between T cells and ECs, regulating the cutaneous immune response by preventing extravasation of activated T cells into inflamed skin.

Vascular adhesion protein-1 (VAP-1) is an inflammation-inducible adhesion molecule and a primary amine oxidase involved in immune cell trafficking. Leukocyte extravasation into tissues is mediated by adhesion molecules expressed on endothelial cells and pericytes. Pericytes play a major role in the angiogenesis and vascularization of cycling endometrium. However, the functional properties of pericytes in the human endometrium are not known. Here we show that pericytes surrounding the spiral arterioles in midluteal human endometrium constitutively express VAP-1. We first characterize these pericytes and demonstrate that knockdown of VAP-1 perturbed their biophysical properties and compromised their contractile, migratory, adhesive and clonogenic capacities. Furthermore, we show that loss of VAP-1 disrupts pericyte-uterine natural killer cell interactions in vitro. Taken together, the data not only reveal that endometrial pericytes represent a cell population with distinct biophysical and functional properties but also suggest a pivotal role for VAP-1 in regulating the recruitment of innate immune cells in human endometrium. We posit that VAP-1 could serve as a potential biomarker for pregnancy pathologies caused by a compromised perivascular environment prior to conception.