Background Arteriovenous fistula (AVF) maturation failure is a main limitation of vascular access. Maturation is determined by the intricate balance between outward remodeling and intimal hyperplasia, whereby endothelial cell dysfunction, platelet aggregation, and vascular smooth muscle cell (VSMC) proliferation play a crucial role. von Willebrand Factor (vWF) is an endothelial cell–derived protein involved in platelet aggregation and VSMC proliferation. We investigated AVF vascular remodeling in vWF‐deficient mice and vWF expression in failed and matured human AVFs. Methods and Results Jugular‐carotid AVFs were created in wild‐type and vWF −/− mice. AVF flow was determined longitudinally using ultrasonography, whereupon AVFs were harvested 14 days after surgery. VSMCs were isolated from vena cavae to study the effect of vWF on VSMC proliferation. Patient‐matched samples of the basilic vein were obtained before brachio‐basilic AVF construction and during superficialization or salvage procedure 6 weeks after AVF creation. vWF deficiency reduced VSMC proliferation and macrophage infiltration in the intimal hyperplasia. vWF −/− mice showed reduced outward remodeling (1.5‐fold, P =0.002) and intimal hyperplasia (10.2‐fold, P <0.0001). AVF flow in wild‐type mice was incremental over 2 weeks, whereas flow in vWF −/− mice did not increase, resulting in a two‐fold lower flow at 14 days compared with wild‐type mice ( P =0.016). Outward remodeling in matured patient AVFs coincided with increased local vWF expression in the media of the venous outflow tract. Absence of vWF in the intimal layer correlated with an increase in the intima‐media ratio. Conclusions vWF enhances AVF maturation because its positive effect on outward remodeling outweighs its stimulating effect on intimal hyperplasia.

Vascular access is the lifeline for patients receiving haemodialysis as kidney replacement therapy. As a surgically created arteriovenous fistula (AVF) provides a high-flow conduit suitable for cannulation, it remains the vascular access of choice. In order to use an AVF successfully, the luminal diameter and the vessel wall of the venous outflow tract have to increase. This process is referred to as AVF maturation. AVF non-maturation is an important limitation of AVFs that contributes to their poor primary patency rates. To date, there is no clear overview of the overall role of the extracellular matrix (ECM) in AVF maturation. The ECM is essential for vascular functioning, as it provides structural and mechanical strength and communicates with vascular cells to regulate their differentiation and proliferation. Thus, the ECM is involved in multiple processes that regulate AVF maturation, and it is essential to study its anatomy and vascular response to AVF surgery to define therapeutic targets to improve AVF maturation. In this review, we discuss the composition of both the arterial and venous ECM and its incorporation in the three vessel layers: the tunica intima, media, and adventitia. Furthermore, we examine the effect of chronic kidney failure on the vasculature, the timing of ECM remodelling post-AVF surgery, and current ECM interventions to improve AVF maturation. Lastly, the suitability of ECM interventions as a therapeutic target for AVF maturation will be discussed.

BACKGROUND AND AIMS Nonmaturation of arteriovenous fistulas (AVFs) remains a bottleneck in creating a long-lasting lifeline to hemodialysis. AVF maturation is determined by the intricate balance between outward remodeling (OR) and formation of intimal hyperplasia (IH), both processes in which vascular smooth muscle cell (VSMC) proliferation is crucial. Recently, we observed that von Willebrand Factor (vWF) is an essential protein in AVF maturation in mice, through its induction of VSMC proliferation and thereby positive effect on OR. In this study we investigate the role of vWF in human AVFs. METHOD Humane plasma and patient-matched samples were obtained during two stage brachio-basalic AVF surgery: native veins were collected at time of AVF creation and venous AVF samples at time of transposition or salvage procedure. Random selection was performed using propensity score matching while adjusting for age, sex, ethnicity, diabetes, and dialysis status. AVF maturation was defined as a luminal AVF diameter of &gt;6 mm using intravascular probes. Sixteen mature AVFs and 15 failed AVFs were available for matched-pair analysis. Histological samples were stained for vWF, CD31 and aSMA. Positively stained tissue in the medial layer was quantified using Histoquant and analyzed with the Wilcoxon signed-rank test. Intima/media (I/M) area ratio was calculated by dividing the IH by the medial layer. vWF antigen levels in patient plasma were determined by ELISA for 8 patients with AVF failure and 10 with AVF maturation. RESULTS vWF is expressed in the IH, tunica media and vasa vasorum of ESRD (end-stage renal disease) patient AVFs. Furthermore, aSMA+/vWF+ co-localization was observed in tunica media of matured AVF. Although no difference in IH was observed across the pair-matched samples of patients with matured versus failed AVFs, the I/M ratio increased from the native vein to the venous AVF sample with 2.6-fold in mature AVFs (P = 0.02) and 4.0-fold in failed AVFs (P = 0.0001). Concurrently, OR, wall thickening and luminal area were significantly increased 2.1-fold (P = 0.02), 6.6-fold (P = 0.009) and 4.7-fold (P = 0.02), respectively, in patients with matured AVFs compared to failed AVFs. While there was no difference in systemic vWF antigen levels between groups or before versus after AVF creation, wall thickening and OR coincided with 167% (P = 0.03) increase in vWF expression in the medial layer of preaccess veins to matured venous AVF samples, but not in patient samples of failed AVFs (Figure 1). Moreover, we observed a reduction of vWF+ endothelial cells lining the intima, from the native vein to the venous AVF outflow tract. Matured AVFs presented with 3.6-fold more vWF+ intima than failed AVF samples (P = 0.04). The I/M ratio and vWF+ intima of AVFs were negatively correlated using linear regression: when the vWF+ intimal layer is disrupted, the I/M ratio increases (Figure 2; slope P = 0.0017 for matured AVFs and P = 0.0264 for failed AVFs). CONCLUSION Our results suggest that vWF is involved in AVF maturation in ESRD patients through its local action. Despite comparable systemic vWF antigen levels between between patient groups, matured AVFs show increased local vWF expression in the intimal layer and tunica medica, coinciding with increased venous wall thickening as well as outward remodeling.

Background and Aims Low arteriovenous fistula (AVF) maturation rate remains a bottleneck in creating a long lasting lifeline to hemodialysis. AVF maturation is determined by the intricate balance between outward remodeling (OR) and formation of intimal hyperplasia (IH), during which vascular smooth muscle cell (VSMC) proliferation is essential. Recently, the von Willebrand Factor (VWF) has been shown to induce VSMC proliferation in vitro and in vivo. In this study we investigate the role of VWF on VSMC proliferation and AVF functionality to gain a better understanding of the relationship between the two latter processes. Method AVFs were created in wild-type (WT) and VWF (-/-) deficient (B6.129S2-Vwf tm1Wgr) mice using the external jugular vein and the common carotid artery (CCA). Mean velocity and diameter of the CCA was measured at baseline before surgery, t=0 immediately post-surgery, t=7 and t=14 using ultrasonography to determine flow volume (mL/s). The mice were sacrificed at t=14, AVFs were collected and OR and IH were analyzed using Weigert’s elastin and aSMA staining, using the Mann-Whitney test. To study VSMC proliferation in vitro, VSMCs were isolated from vena cava explants. VSMCs from VWF -/- mice were cultured with 10% VWF -/- plasma with or without addition of VWF (Wilfactin). Proliferation was measured after 40 hours. Humane patient-matched samples were obtained during two stage brachio-basalic AVF surgery: the native veins at creation and venous AVF samples at transposition. Maturation was defined as a luminal AVF diameter of &gt; 6 mm. Samples were stained for VWF and aSMA. Positively stained tissue in the medial layer was quantified using Histoquant and analyzed with the Wilcoxon signed-rank test. Results WT mice showed better outcomes in AVF functionality. Whereas blood flow did not increase in VWF deficient mice over two weeks after AVF-creation, WT mice showed a steady increase in flow rate from t=0 to t=7 (1.97 fold) and t=14 (3.0 fold) . At 14 days, VWF -/- mice showed significantly reduced OR compared to WT mice, with a 1.46 fold smaller venous perimeter (p=0.008). Formation of IH was also significantly reduced in VWF -/- mice (9.58 fold change, p = 0,0001), with a non-significant 1.42 fold difference in luminal area. Lastly, an 8.05 fold decrease was observed in aSMA+ cells in the IH of VWF -/-mice (p = 0.0002). In vitro, addition of VWF caused an 1.5 fold increase in proliferation compared to VWF deficient VSMCs cultured without VWF (p=0.016). As for chronic kidney disease (CKD) patients, our study showed significant wall thickening and increase in aSMA+ tissue only in matured AVFs. There was significant upregulation of VWF+ tissue in the media from native veins to venous AVF samples, with 183% in failed AVFs (p= 0.026) and 425% in matured AVFs (p= 0.013). Matured AVFs had a 4.6 fold increase in VWF+ area in the medial layer compared to failed AVFs (p= 0.033). Conclusion Our study demonstrates VWF as an inducer of VSMC proliferation and thereby both OR and IH. Secondly, we show that VWF is essential for AVF maturation and that enhanced expression in the media coincides with AVF maturation in CKD patients.