The cytokines tumour necrosis factor-alpha (TNFalpha) and interleukin-1beta (IL-1B) induce endothelial cells to recruit leukocytes. However, the exact adhesion and activation mechanisms induced by each cytokine, and their relative sensitivities to modulation by endothelial exposure to shear stress remain unclear. We cultured human umbilical vein endothelial cells (HUVEC) in glass capillaries at various shear stresses, with TNFalpha or IL-1B added for the last 4 h. Subsequently, human neutrophils were perfused over the HUVEC, and adhesion and migration were recorded. Both cytokines induced dose-dependent capture of neutrophils. However, while conditioning of HUVEC by increasing shear stress for 24 h diminished their response to TNFalpha, the response of HUVEC to IL-1B was similar at all shear stresses. The differing sensitivities were evident at levels of adhesive function and mRNA for adhesion molecules and chemokines. Analysis of nuclear factor kappaB (NF-kappaB)/Rel family of transcription factors showed that their expression and activation were modified by exposure to shear stress, but did not obviously explain differential responses to TNFalpha and IL-1B. Antibodies against selectins were effective against capture of neutrophils on TNFalpha-treated but not IL-1B-treated HUVEC. Stable adhesion was supported by beta2-integrins in each case. Activation of neutrophils occurred dominantly through CXC-chemokine receptor 2 (CXCR2) for TNFalpha-treated HUVEC, while blockade of CXCR1, CXCR2 and of platelet-activating factor receptors caused additive inhibition of migration on IL-1B-treated HUVEC. The mechanisms which underlie neutrophil recruitment, and their modulation by the haemodynamic environment, differ between cytokines. Interventions aimed against leukocyte recruitment may not operate equally in different inflammatory milieu.

We investigated the adhesive behavior of mesenchymal stem cells (MSC) in blood, which might influence their fate when infused as therapy. Isolated human bone marrow MSC (BMMSC) or umbilical cord MSC (UCMSC) adhered efficiently from flow to the matrix proteins, collagen, or fibronectin, but did not adhere to endothelial selectins. However, when suspended in blood, BMMSC no longer adhered to collagen, while UCMSC adhered along with many aggregated platelets. Neither MSC adhered to fibronectin from flowing blood, although the fibronectin surface did become coated with a platelet monolayer. UCMSC induced platelet aggregation in platelet rich plasma, and caused a marked drop in platelet count when mixed with whole human or mouse blood in vitro, or when infused into mice. In contrast, BMMSC did not activate platelets or induce changes in platelet count. Interestingly, isolated UCMSC and BMMSC both adhered to predeposited platelets. The differences in behavior in blood were attributable to expression of podoplanin (an activating ligand for the platelet receptor CLEC-2), which was detected on UCMSC, but not BMMSC. Thus, platelets were activated when bound to UCMSC, but not BMMSC. Platelet aggregation by UCMSC was inhibited by recombinant soluble CLEC-2, and UCMSC did not cause a reduction in platelet count when mixed with blood from mice deficient in CLEC-2. We predict that both MSC would carry platelets in the blood, but their interaction with vascular endothelium would depend on podoplanin-induced activation of the bound platelets. Such interactions with platelets might target MSC to damaged tissue, but could also be thrombotic. Stem Cells 2018;36:1062-1074.

Bone morphogenetic proteins 9 and 10 (BMP9/BMP10) are circulating cytokines with important roles in endothelial homeostasis. The aim of this study was to investigate the roles of BMP9 and BMP10 in mediating monocyte-endothelial interactions using an in vitro flow adhesion assay. Herein, we report that whereas BMP9/BMP10 alone had no effect on monocyte recruitment, at higher concentrations both cytokines synergized with tumor necrosis factor-α (TNFα) to increase recruitment to the vascular endothelium. The BMP9/BMP10-mediated increase in monocyte recruitment in the presence of TNFα was associated with up-regulated expression levels of E-selectin, vascular cell adhesion molecule (VCAM-1), and intercellular adhesion molecule 1 (ICAM-1) on endothelial cells. Using siRNAs to type I and II BMP receptors and the signaling intermediaries (Smads), we demonstrated a key role for ALK2 in the BMP9/BMP10-induced surface expression of E-selectin, and both ALK1 and ALK2 in the up-regulation of VCAM-1 and ICAM-1. The type II receptors, BMPR-II and ACTR-IIA were both required for this response, as was Smad1/5. The up-regulation of cell surface adhesion molecules by BMP9/10 in the presence of TNFα was inhibited by LDN193189, which inhibits ALK2 but not ALK1. Furthermore, LDN193189 inhibited monocyte recruitment induced by TNFα and BMP9/10. BMP9/10 increased basal IκBα protein expression, but did not alter p65/RelA levels. Our findings suggest that higher concentrations of BMP9/BMP10 synergize with TNFα to induce the up-regulation of endothelial selectins and adhesion molecules, ultimately resulting in increased monocyte recruitment to the vascular endothelium. This process is mediated mainly via the ALK2 type I receptor, BMPR-II/ACTR-IIA type II receptors, and downstream Smad1/5 signaling.