A Microfluidics-Based Screening Tool to Assess the Impact of Blood Plasma Factors on Microvascular Integrity

This study provides a method to assess the impact of circulating plasma factors on microvascular integrity by using a recently developed microvessel-on-a-chip platform featuring the human endothelium that is partly surrounded by the extracellular matrix. The system is high-throughput, which allows parallel analysis of organ-level microvessel pathophysiology, including vascular leakage. Ethylenediaminetetraacetic acid plasma samples are mixed with inhibitors for recalcification of the plasma samples to avoid activation of the coagulation- or complement system. Moreover, the assay is validated by spiking vascular endothelial growth factor, histamine, or tumor necrosis factor alpha to recalcified plasma and confirms their modulation of microvessel barrier function at physiologically relevant concentrations. Finally, this study shows that perfusing the microvessels with recalcified plasma samples of coronavirus disease-2019 patients, with a confirmed proinflammatory profile, results in markedly increased leakage of the microvessels. The assay provides opportunities for diagnostic screening of inflammatory or endothelial disrupting plasma factors associated with endothelial dysfunction.

Abstract 147: Coordinate Regulation of Chemo-attractive And -repulsive Guidance Cues Instructs Leukocyte-endothelial Interactions in Atherosclerosis

Leukocyte influx into the arterial wall plays a critical role in the development and progression of atherosclerosis. Emerging evidence suggests that neuronal guidance cues, typically expressed during development, are involved in both physiological and pathological immune responses. We hypothesized that endothelial expression of such guidance cues plays a role in regulating leukocyte trafficking into the vascular wall during atherogenesis. Gene expression profiling revealed that members of the Netrin, Semaphorin and Ephrin family of guidance molecules are differentially regulated in athero-prone and athero-protected regions of the aortae of Ldlr-/- mice. Netrin-1 and Semaphorin 3A are highly expressed in the outer curvature of the aorta, and potently inhibit chemokine-directed migration of monocytes in vitro. Endothelial expression of these negative guidance cues is down-regulated by pro-atherogenic factors associated with monocyte entry into the vessel wall, including oscillatory shear stress and pro-inflammatory cytokines. Furthermore, using intravital microscopy we show that in vivo delivery of Netrin-1- or Semaphorin3A-blocking peptides increases leukocyte adhesion to the endothelium ~2-fold compared to the control peptides. Conversely, we show that a guidance cue that is highly expressed in atheroprone regions of the aorta, EphrinB2, is up-regulated in endothelium cells under pro-atherosclerotic flow conditions (eg. oscillatory shear stress, pro-inflammatory cytokines) and functions as a monocyte chemoattractant. These data suggest a new paradigm for leukocyte-endothelial interactions in which the coordinate regulation of chemo-attractive and -repulsive guidance cues regulates leukocyte trafficking in health and disease.

Abstract 45: Antiatherosclerotic Effects of miR-33 Inhibition: Increased Reverse Cholesterol Transport and Alternative-Activation (M2) of Macrophages

Plasma HDL levels have a protective role in atherosclerosis, yet clinical therapies to raise HDL and exploit its atheroprotective effects have remained elusive. Recent studies identified miR-33 as an intronic microRNA, located within the SREBF2 gene, that suppresses expression of the cholesterol transporter ABC transporter A1 (ABCA1) and lowers HDL levels. Conversely, mechanisms that inhibit miR-33 increase ABCA1 and plasma HDL, suggesting that antagonism of miR-33 may be atheroprotective. We hypothesized that systemic delivery of an oligonucleotide inhibitor of miR-33 would increase plasma HDL and promote reverse cholesterol transport (RCT), and therefore have a beneficial impact on atherosclerosis. To test this, we treated Ldlr -/- mice with established atherosclerotic plaques with anti-miR33 or a control anti-miR for 4 weeks. Treatment with anti-miR-33 increased circulating HDL levels by 37% and enhanced RCT to the plasma, liver, and feces by up to 80%. Consistent with this, anti-miR33-treated mice showed a marked reduction in plaque size and lipid content, as well as an increase in indicators of plaque stability. Laser capture microdissection of lesional CD68+ cells demonstrated that anti-miR33 oligonucleotides directly targeted the plaque macrophages, where they enhanced ABCA1 expression and cholesterol removal. Moreover, macrophages from anti-miR33-treated mice showed an enrichment in anti-inflammatory M2 markers (Arg1, Il10) and reduced expression of proinflammatory M1 markers (iNos and Tnfa). Notably, overexpression of miR-33 in pMφ in vitro decreases markers of the M2 phenotype, Arg1 and Il-4, and increases the expression of inflammatory cytokines such as Tnfa and Il-1b. In contrast, anti-miR-33 polarizes pMφ to an M2 phenotype (Arg1, Fizz1, Il-10 and Il-4), with an associated downregulation of inflammatory genes (Tnfa, Il-1b). Overall, these results indicate that anti-miR33 has multiple beneficial effects on atherosclerosis, including increasing HDL and RCT, and reducing lesional inflammation by promoting macrophage polarization to the reparative M2 state, highlighting the promise for anti-miR33 therapy for the treatment of cardiovascular disease.

Epac1-Rap1 signaling regulates monocyte adhesion and chemotaxis

Extravasation of leukocytes is a crucial process in the immunological defense. In response to a local concentration of chemokines, circulating leukocytes adhere to and migrate across the vascular endothelium toward the inflamed tissue. The small guanosinetriphosphatase Rap1 plays an important role in the regulation of leukocyte adhesion, polarization, and chemotaxis. We investigated the role of a guanine nucleotide exchange protein for Rap1 directly activated by cAMP (Epac1) in adhesion and chemotaxis in a promonocytic cell line and in primary monocytes. We found that Epac1 is expressed in primary leukocytes, platelets, CD34-positive hematopoietic cells, and the leukemic cell lines U937 and HL60. Epac activation with an Epac-specific cAMP analog induced Rap1 activation, beta1-integrin-dependent cell adhesion, and cell polarization. In addition, activated Epac1 enhanced chemotaxis of U937 cells and primary monocytes. Similar to activation of Epac1, stimulation of cells with serotonin to induce cAMP production resulted in Rap1 activation, increased cell adhesion and polarization, and enhanced chemotaxis. The effects of serotonin on U937 cell adhesion were dependent on cAMP production but could not be blocked by a protein kinase A inhibitor, implicating Epac in the regulation of serotonin-induced adhesion. In summary, our work reveals the existence of previously unrecognized cAMP-dependent signaling in leukocytes regulating cell adhesion and chemotaxis through the activation of Epac1.