The aim of the paper is to summarize the current understanding of the molecular biology of arteriovenous fistula (AVF). It intends to encourage vascular access teams, care providers, and scientists, to explore new molecular tools for assessing the suitability of patients for AVF as vascular access for maintenance hemodialysis (HD). This review also highlights most recent discoveries and may serve as a guide to explore biomarkers and technologies for the assessment of kidney disease patients choosing to start kidney replacement therapy. Objective criteria for AVF eligibility are lacking partly because the underlying physiology of AVF maturation is poorly understood. Several molecular processes during a life cycle of an AVF, even before creation, can be characterized by measuring molecular fingerprints using newest “omics” technologies. In addition to hypothesis-driven strategies, untargeted approaches have the potential to reveal the interplay of hundreds of metabolites, transcripts, proteins, and genes underlying cardiovascular adaptation and vascular access-related adjustments at any given timepoint of a patient with kidney disease. As a result, regular monitoring of modifiable, molecular risk factors together with clinical assessment could help to reduce AVF failure rates, increase patency, and improve long-term outcomes. For the future, identification of vulnerable patients based on the assessment of biological markers of AVF maturation at different stages of the life cycle may aid in individualizing vascular access recommendations.

© 2022 S. Karger AG, Basel

Introduction

Arteriovenous fistula (AVF) was first introduced by Brescia et al. [1] and, if it matures, remains the most preferred option for vascular access in patients on maintenance hemodialysis (HD) due to its superior patency and lower infection rates compared to other vascular access choices. Traditionally, creation of an AVF involves a surgical procedure that connects an artery to a vein, usually in the forearm (radial-cephalic) or the upper arm (brachial-cephalic) of the upper limb. The aim of this procedure is to enlarge and strengthen the vein to sustain increased blood flow rates during HD and to allow for easy and reliable access to blood vessels. More recently, noninvasive endovascular and percutaneous techniques have been used effectively for forearm AVF creation. These procedures showed comparable safety, patency rates with increased patient satisfaction [2].

Once an AVF is created, it takes several weeks to months until the first cannulation with large international differences. Of those AVFs that mature, the median time to first use is 10 days in Japan, 46 days in Europe, Australia, and New Zealand, and 82 days in the USA [3]. Successful AVF use is 87% in Japan, 67% in Europe, Australia, and New Zealand, and 64% in the USA. Use of Doppler ultrasound plays a crucial role in the creation and adequate maturation of AVFs [4]. Primary AVF failure, as defined by insufficient maturation for use in dialysis 3 months after creation, is up to 40% compared to 18% in the group of patients who undergo ultrasound examination [5]. Lower rates of maturation are related to age, gender, ethnicity, comorbidities, and previous vascular access, such as arterial venous grafts, central venous catheter, or no prior AVF [6, 7].

Routinely, AVF maturation is determined by the vascular access team based on the physical examination and ultrasound findings, nonetheless, the criteria to define a mature AVF differ widely among clinics, which causes delays or unreliable AVF cannulation. A practical approach used by many clinicians to determine the likelihood to have a mature fistula is the “Rule of 6’s,” which includes an AVF flow >600 mL/min, a diameter of at least 6 mm, and vein depth from the surface of the skin <6 mm [8]. These AVF clinical maturation criteria are opinion-based and not validated. For example, the updated KDOQI guidelines include new results from the Hemodialysis Fistula Maturation (HFM) study showing that fistulas with greater depths are more likely to have poorer maturation outcomes [9]. Therefore, more objective screening programs based on the detection of circulating biomarkers would be helpful to identify patients at risk of AVF maturation failure.

AVF maturation requires cardiovascular adaptation, processes involved in venous wall thickening and remodeling, as well as vascular access-related adjustments [10]. The molecular biology of AVF maturation is complex and remains poorly understood. Previous studies have revealed molecular pathogenic factors, such as inflammation, hypoxic injury, wall shear stress, uremia, and oxidative stress, which result in AVF maturation failure. Most of these mechanistic studies were conducted in animal models and whether the revealed mechanisms apply to humans needs to be further investigated. Moreover, there is a lack of modifiable risk factors that prevent AVF maturation failure. Reliable clinical or nonclinical biomarkers to predict AVF maturation prior to access creation are also missing. In a recent systematic review and meta-analysis comprising thirteen studies with a combined population of 1,512 subjects, 48 biomarkers were assessed as a part of routine clinical care [11]. No significant association between any of the assessed routine circulating biomarkers and AVF failure was observed.

This article will review the current understanding of AVF molecular biology and underline the need for new therapeutic targets and predictive biomarkers, which would help to individualize dialysis care and increase patency and successful use of an AVF. Herein, we highlight recent discoveries of molecular processes and signaling pathways involved in AVF maturation to spark interest in further exploring a targeted, hypothesis-driven approach. We also discuss how new molecular targets and yet unknown pathways can be discovered through an untargeted, unbiased approach using omics technologies. Both approaches would be needed to develop therapeutic and diagnostic tools for improving AVF success rates and tailor vascular access strategies to individual conditions. We used two review articles on molecular targets published in the last few years [12, 13] and browsed recent literature for newest updates.

From Inflow Artery and Outflow Vein to Functional AVF: An Overview of the Remodeling Process and Its Molecular Implications

The venous wall conducts blood flow in a low-pressure environment where the pressure immediately increases after AVF creation surgery due to the abnormal connection to a high-pressure artery. The walls of veins and arteries consist of a unique cellular composition and tissue architecture that surrounds the lumen: the outer layer called adventitia, the middle layer called media, which is thicker in arteries, and the inner layer called intima. In the pre-access vein, the predominant resident cells in the intima are endothelial cells (ECs), smooth muscle cells (SMCs), and myofibroblast/fibroblast with diverse phenotypes. Formerly, intimal hyperplasia was considered uncommon in the pre-access vein, but several studies, including the HFM study, proved otherwise, finding a 35% prevalence of severe intimal hyperplasia (>41% lumen occlusion) in the pre-access veins of the participants [14]. The contribution of different cells and mediators in the development of intimal hyperplasia of variable degrees is not completely understood, but there is evidence that both ECs and paracrine factors may play an important role [15].

If matured well, the AVF wall thickens and expands outwardly to increase lumen diameter for successful vascular access required for maintenance HD (Fig. 1a). After the AVF creation, cells from the media and intima layers act as mechanosensors of hemodynamic changes and vascular trauma; the intima shows eccentric expansion and thickens its diameter approximately 4-fold, while the media frequently shows atrophy. At this point, the predominant cells in the intima are synthetic SMCs, which are associated with high proliferation and migration rates and high secretion of extracellular matrix (ECM) along with myofibroblasts [15]. Neointimal hyperplasia forms when SMCs, myofibroblasts, and ECM build up in the intima impairing outward remodeling. It could progress to become stenotic and eventually thrombotic leading to AVF failure. On the other hand, studies have suggested that intimal hyperplasia in AVF, whether preexisting or postoperative, is not associated with AVF failure [16, 17].

Fig. 1.

Remodeling phases and risk factors affecting molecular signatures of a functional AVF after creation surgery. a Abundance of biomarkers characterizes each phase during an AVF life cycle starting at creation surgery until a functional fistula is formed. Shown are the outer, middle, and inner layer of the baseline blood vessel and functional AVF. b Risk factors affecting AVF maturation and downstream molecular signatures. Created with BioRender.com.

After AVF creation, diverse cell types in the blood vessel wall adapt to many different stimuli, such as increased shear stress, oxygen tension, inflammation, and uremia (Fig. 1b). These factors affect numerous signaling pathways upon AVF creation and during its maturation. Understanding their regulation pre- and post-surgery is essential for guiding patients with individualized vascular access options, improving vascular access success rates, and developing effective therapeutic tools for non-maturing AVF. Recent reviews highlighted the need for unraveling the missing, yet vital, pieces of information to understand the complex molecular pathways involved in the process of blood vessels transitioning into functional AVFs [12, 13, 18].

Biological Processes and Molecular Pathways in AVF Maturation – Recent, Seminal FindingsBiological ProcessesInflammation

Inflammation is an integral part of the immune response to stress caused by initial vascular injury and subsequent altered hemodynamics. At the cellular level, the elicited inflammatory response is a well-balanced process characterized by coordinated infiltration of immune cells, including macrophages, several types of T cells, natural killer T cells, dendritic cells, neutrophils, and mast cells into the vascular wall. The mechanism by which the immune cells modulate AVF adaptation is poorly understood and most information comes from animal studies [19-22], which have shown that inhibition of macrophages and T cells accumulation prevent AVF from maturing. Local inflammation is essential for AVF maturation as different subsets of inflammatory cells participate in venous remodeling [22]. However, an excessive, systemic inflammatory response has been associated with AVF failure [23, 24]; increased macrophage and T cell infiltration has been documented in stenotic and thrombosed AVF segments [25, 26]. So far, unequivocal demonstration of improving AVF success or patency by manipulating inflammation remains elusive likely due to the complex nature of inflammatory responses in AVF.

Accumulation of T cells precedes that of macrophages during AVF maturation in a mouse model [27]. CD4+ T cells promote AVF maturation by reducing excessive wall thickening and improving blood flow [19]. Adoptive transfer of CD4+ T cells was shown to improve AVF maturation in an athymic rat. It has been suggested that CD4+ T cells, i.e., Th1, Th2, and Treg cells, regulate macrophage accumulation and polarization by secreting interferon-γ (IFN-γ), interleukin-2 (IL-2), and transforming growth factor β1 (TGF-β1) cytokines [27]. Moreover, inhibition of T cell differentiation reduces venous wall thickening and AVF patency [28]. These findings suggest that balanced inflammation during AVF maturation requires coordinated regulation of different subsets of T cells and macrophages.

Macrophages are classified into two major subtypes: M1 and M2. M1 subtype secretes proinflammatory cytokines such as tumor necrosis factor α (TNF-α), IL-1β, IL-6, IL-12, and IL-23 [29]; M2 macrophages are anti-inflammatory by releasing IL-10 and TGF-β and help resolve inflammation and promote wound healing [29]. Both types of macrophages infiltrate into the venous wall after AVF creation: M1 in the early phase of maturation and M2 in the subsequent later phases. How these two types of macrophages function in AVF maturation in a coordinated manner is not clear. The Akt1-mTORC1 pathway has been shown to promote accumulation of both types of macrophages in AVF maturation [21]. Rapamycin, an mTor inhibitor of Akt1-mTORC1 signaling, promotes venous modeling while reducing accumulation of both M1 and M2 macrophages [21]. Intraoperative implantation of collagen membranes eluting rapamycin improved AVF maturation rate to 87% by KDOQI criteria without causing systemic immunosuppression or any other significant side effect in a nonrandomized study of 30 patients undergoing AVF creation surgery [30]. A Phase 3 randomized, single-blind, multicenter study (ACCESS Study) evaluating the efficacy and safety of collagen implants eluting rapamycin was completed in 2021 (clinicaltrials.gov, #NCT02513303). The results are not yet available as of May 2022.

Another prominent regulator of macrophage infiltration is the well-established fractalkine receptor 1 (CX3­CR1). It mediates macrophage infiltration into the vasculature through its sole ligand fractalkine, which is expressed in inflamed endothelium. Binding of CX3CR1 to fractalkine initiates the innate immune system, which recruits monocytes to the lumen of blood vessels. After maturation and differentiation, macrophages play a major role in either pro- or anti-inflammatory response in different environmental conditions. Interestingly, a recent study showed that blockade of CX3CR1 signaling leads to reduced venous stenosis and improved AVF patency using a humanized mouse model [31]. In another study using a mouse stenosis model [32], macrophages were found to infiltrate into the outer layer at a fast rate and slowly decline over time and only minimal macrophage infiltration happened in the neointimal later in the AVF stenosis. The findings from these two animal studies on AVF stenosis suggest that after AVF forms, early intervention by inhibiting CX3CR1 signaling may prevent stenosis and improve patency.

Given that excessive inflammation is associated with AVF failure, anti-inflammatory agents could be beneficial to AVF maturation. Drugs with pleiotropic effects, such as statins, have been investigated for their anti-inflammatory effects on AVF, but the results are mixed [33]. More recently, two animal studies have again suggested a beneficial role of statins in AVF patency. Cui et al. [34] found that oral intake of atorvastatin reduced macrophage accumulation and fibrinogenesis and improved AVF patency in an AVF mouse model. Zhao et al. [35] also showed an AVF improvement by locally delivering simvastatin-loaded microparticles to mouse AVF. Therefore, randomized controlled clinical trials studying statin benefits on patient AVF outcome would be needed. Systemic or local statin delivery via novel surgical or endovascular techniques should also be considered.

Anti-inflammatory steroids such as glucocorticoids are also good drug candidates for improving AVF success. Nanoparticle therapeutics, such as liposomes, have been developed to deliver the drug selectively to inflamed tissues where liposomes are phagocytized by macrophages [36]. Preclinical studies successfully demonstrated that prednisolone delivered in polyethylene glycol-coated liposomes enhanced outward remodeling of AVF veins [37]. However, a promising, investigator-initiated, multicenter, double-blinded, placebo-controlled randomized controlled trial “Liposomal Prednisolone to Improve Hemodialysis Fistula Maturation” (LIPMAT study) was terminated prematurely in 2018 because of slow enrollment [38]. Although no significant difference was observed between the control and prednisolone liposome-treated cases, the pilot study showed that the approach was safe and well-tolerated by patients with end-stage renal disease [39]. Manipulating macrophages using pegylated liposomes emerges as an attractive tool for selectively delivering novel discovered regulators or drugs to inflamed tissues; however, the extent and timing of treatment need to be determined.

Hypoxia and Oxidative Stress

Cells respond to oxygen levels by regulating the key transcription factors, hypoxia induced factor (HIF) proteins, at posttranslational level. Under normal oxygen conditions, the cellular oxygen sensor PHD (prolyl hydroxylase domain) proteins become activated via oxygen binding and catalyze the hydroxylation of proline residues on HIF1α. The hydroxylated HIF1α interacts with the von Hippel–Lindau E3 ubiquitin ligase and then is directed to the proteasome for degradation. Under hypoxic conditions, HIF1α is not hydroxylated and therefore stabilized. It enters the nucleus and forms a complex with its partner HIF1β, eliciting the transcription of a large number of genes as a response to hypoxia [40].

It has been shown that HIF1 mRNA and proteins are induced after AVF creation [41, 42]. The induction of HIF genes could be a response to both hypoxia and oxidative stress caused by AVF surgery [42]. A few direct HIF target genes have been identified to play important roles in AVF maturation, such as heme oxygenase-1 (HO-1) and vascular endothelial growth factor A (VEGF-A). Targeting the HIF pathway could be a clinical strategy to improve AVF maturation. Further supporting this idea, local delivery of HIF inhibitors such as siHIF1/2α, everolimus (Eve), and topoisomerase (TOPO), inhibits neointimal hyperplasia in a mouse model of AVF [43]. It is also plausible to hypothesize that the adaptive process following reperfusion after anastomosis exposes the vein to relatively high oxygen tension potentially affecting the outcome of AVF maturation.

HO-1 encodes an inducible form of heme oxygenase, which can degrade harmful free heme molecules [44]. One of the degradation products, biliverdin, is converted to bilirubin, a cellular antioxidant that can remove free radicals. Another product, carbon monoxide, can increase blood flow of AVF [45]. Knockout of HO-1 in mouse causes increased neointimal hyperplasia and venous wall thickening, likely by promoting the expression of monocyte chemoattractant protein-1 (MCP1), matrix metalloproteinase-2 (MMP-2), and matrix metalloproteinase-9 (MMP-9) [46]. MCP1 is one of the key cytokines that regulate migration of infiltrating macrophages and SMCs [46, 47] and its elevated levels were found to be associated with restenosis of AVF in human patients [48]. Matrix metalloproteinases can degrade ECM to facilitate cell migration during venous wall remodeling [49]. Using a chronic kidney disease (CKD) mouse model, Kang et al. [45] demonstrated beneficial effects of upregulating HO-1 on AVF maturation. Thus, delivering HO-1 gene via AAV vector in HO-1 knockout mouse model reduced excessive wall thickening and increased blood flow of AVF. Interestingly, longer GT repeats at the HO-1 promoter are associated with decreased AVF patency in human patients likely due to inhibitory effects of this long GT repeat on HO-1 gene transcription [50, 51]. These findings suggest that upregulating HO-1 expression therapeutically via small molecule drugs or gene therapy using CRISPR technology can be a strategy for improving AVF maturation.

The angiogenic cytokine VEGF-A is another key HIF target that is directly involved in vascular remodeling [41, 52]. VEGF-A binds to its receptors VEGFR-1/2 and promotes endothelial proliferation and migration. Sadaghianloo et al. [42] showed that expression of HIF and VEGF genes increases during early AVF maturation in a mouse AVF model. Moreover, increased expression of VEGF-A and its receptors were found to be associated with venous stenosis in a mouse model with renal insufficiency [41]. Increased expression of VEGF-A has been confirmed in human stenotic AVF specimen [46]. Interestingly, a single-nucleotide polymorphism (SNP) of VEGF-A, the VEGF-936C/C genotype, was shown to confer an increased risk of late AVF thrombosis in patients undergoing HD [53]. Therefore, elevated VEGF-A signaling is one of the major culprits for AVF stenosis. Attenuation of VEGF activity by locally delivered shRNA or Bevacizumab, a monoclonal antibody to VEGF-A, has been shown to reduce venous stenosis in mouse AVF models [54, 55]. This suggests that anti-VEGF-A therapy could be beneficial to patients by preventing stenosis. Indeed, a study of 14 HD patients receiving an arteriovenous access and intravitreal Bevacizumab showed a significant improvement in patency. A follow-up clinical study (clinicaltrials.gov, #NCT02695641) was initiated to evaluate the pharmacokinetics of low-dose Bevacizumab and its efficacy on reducing plasma free VEGF-A levels in HD patients. Unfortunately, the study was withdrawn because of difficulty in patient enrollment.

Molecular PathwaysTGF-β Signaling Pathway

TGF-β1 plays a key role in cell differentiation, survival, and proliferation of diverse cell types and is critical for regulating physiological and pathological tissue repair responses [56]. TGF-β1 is upregulated in early and late phases of AVF maturation to promote SMC proliferation and migration [13]. TGF-β1 binds to its receptor TBRI/II and induces expression of ECM genes, such as collagen genes, via activating transcription factor Smad2/3 [57]. However, excessive ECM deposition causes fibrosis leading to intimal hyperplasia and AVF nonmaturation. Using a two-stage surgery, it was found that fibrosis is the leading cause of AVF failure [17]. Xie et al. [58] showed that response of SMCs following vascular injury was limited by LMO7, one of TGF-β1 target genes, via a negative feedback mechanism. LMO7 interacts with c-Fos and c-Jun and promotes their degradation leading to a decrease in TGF-β1 autoinduction [58]. Non-canonical TGF-β1 signaling mediated by TGF-β-activated kinase 1 is also involved in cellular proliferation and ECM deposition during maturation, but the mechanism is not clear [59].

Interestingly, TGF-β signaling has been suggested to contribute to observed gender differences in AVF outcomes, i.e., poor AVF outcome in female patients [60, 61]. Using an AVF mouse model with CKD, Cai et al. [61] suggested that the poor AVF outcome of female mice is due to increased fibrosis possibly mediated through an increase in TGF-β1 and a decrease in bone morphogenetic protein 7 pathway compared to male mice. Moreover, higher expression levels of TGF-β1 in humans, caused by polymorphisms in its genes, lead to decreased AVF patency [62]. These findings suggest manipulation of the TGF-β1 pathway could be a potentially new therapeutic strategy. However, TGF-β1 is a master regulator of the healing process and systemic blockade could tip the tight control to excessive pathological repair. One strategy to control AVF maturation could be manipulation of miR-133a expression, a recently discovered, putative TGF-β1-induced regulator of ɑ-smooth muscle actin, connective tissue growth factor, and collagens [63].

Ephrin-B2/Eph-4B Signaling

From a developmental perspective, AVF maturation can be thought of as a process where the vein adopts some molecular features of the artery. ECs of the vascular system have different genetic expression profiles, indicating that different hemodynamic environments of arterial and venous ECs regulate their phenotypes.

Eph receptors are a family of receptor tyrosine kinases and play key roles in determining vascular identities. One member of Eph family, Eph-B4, together with its ligand ephrin-B2, has been shown to elicit signaling cascades determining vascular identities (vein vs. artery) via their binding during murine embryonic vascular development [64, 65]. The elicited signaling in embryos occurs in both the EC bearing Eph4 receptor and its adjacent EC bearing transmembrane ligand ephrin-B2. In adults, Eph-B4 expresses exclusively in veins as venous marker; ephrin-B2 predominantly in arteries as arterial marker. During AVF maturation, surprisingly ephrin-B2 was found to be expressed in ECs in the venous limb, where Eph-B4 was also increased compared to normal veins in both human and mouse AVFs [66]. Increased Eph-B4 signaling is required for venous remodeling in mouse AVF via an Akt-mediated mechanism and regained ephrin-B2 signaling also regulates venous remodeling by activating several pathways such as eNOS, Akt1, ERK1/2, and p38 [67]. These findings suggest that a matured AVF adopts a balanced dual identity of both vein and artery and targeting Eph-B4/Ephrin-B2 signaling may be another strategy for improving AVF outcomes.

Non-Pathway Specific Approaches: Identification of New Biomarkers and Molecular Mechanisms

The majority of studies described above suggest pathways to be therapeutically targeted and lay the basis for hypothesis-driven identifications of circulating biomarkers. In contrast, a non-pathway specific approach offers a unique opportunity to identify targets or biomarkers that have previously not been considered. A specific strength of such an unbiased approach is the ability to identify novel biomarkers that may be causal and/or therapeutically modifiable mediators of a particular pathophysiology. Another advantage is that the application of a personalized, unbiased approach may assist patients in their decision of choosing the right renal replacement therapy. This strategy is recommended by the newest patient-centered KDOQI guidelines “Right Access, for Right Patient, at Right Time, for the Right Reason” [8]. With respect to AVF, it will reduce the number and costs of required procedures to successfully use vascular accesses and increase their patency. Indeed, Medicare costs for dialysis vascular access services required by elderly US Medicare patients (age ≥66 years) initiating HD therapy during 2010–2013 was 2.8 billion US dollars each year [68].

To date, there is no documented association between biomarkers assessed in routine clinical evaluation and AVF failure [11]. There is a need for an unbiased approach to characterize molecular signatures of baseline blood vessels before and during their use for vascular access. A comprehensive analysis of all measurable, circulating molecules will allow us to outline the relationship between them and may lay the basis to individualize future clinical evaluation of kidney failure patients. Thus far, there are only a few published studies using “omics”- and system biology-based technologies in biomarker discovery for vascular access (Fig. 2). For a larger impact, a significant investment will be needed to establish and validate these new technologies. An excellent example is the recently completed multicenter observational HFM study, which collected a comprehensive biobank of various sample types from a total of 602 participants [69]. This biorepository is available for future studies, using omics and other tools, tailored to identify new biomarkers and ultimately change of focus on metabolic processes other than the markers assessed in routine clinical care. In this regard, proteomic and metabolomic analyses of blood samples from this study would be helpful to identify predictive biomarkers for AVF outcomes.

Fig. 2.

Characterization of molecular signatures from one sample of blood at any timepoint during a kidney disease patient’s care. Shown are different molecules available in the circulation for systematic analysis using different sample processing tools and state-of-the-art “omics” technologies. Such a system biology approach may serve as a powerful strategy to generate phenotype-associated molecular signatures and new mechanisms for further clinical development. Created with BioRender.com.

Only a few studies applied genomics tools to the field of vascular biology, especially AVF [70-74]. Using microarray technology, previous studies found that during the early phase of AVFs, hundreds of genes are significantly upregulated or downregulated [70-72]. These genes are enriched in metabolic and cell proliferation pathways, consistent with venous wall expanding [72]. Studies to identify miRNAs that are involved in AVF maturation using microRNA microarrays and RT-PCRs yielded inconsistent results [73, 75]. In a recent elegant study of a small number of human patients who underwent two-stage upper-arm AVF creation surgeries, Martinez et al. [74] obtained paired venous samples from both stages, i.e., pre-access veins (native veins) and AVFs, and used RNA-Seq to analyze their gene expression profiles in two patient groups with different AVF outcomes (failed vs. matured). By correlating transcriptional profiles of pre-access veins to their AVF outcomes, a unique inflammatory fingerprint (CSF3R, FPR1, S100A8, S100A9, and VNN2) in SMCs of native veins was identified to be associated with AVF nonmaturation, suggesting that genetic background or previous trauma to vessels could predict AVF outcome. This study lays the groundwork for future opportunities, where the dynamic molecular transformation of blood vessels can be studied longitudinally in humans using omics technologies to create “molecular signatures.” Molecular signatures identified in a cluster of individuals or universally shared among all kidney disease patients will help achieve a reliable, functional and long-lasting vascular access for patients.

Despite these progresses, the molecular mechanism of AVF maturation at a systems level is still poorly understood and predictive biomarkers are also lacking. Data analysis of a completed clinical trial with a transcriptomics component of studying vascular biology during AVF maturation might help answer these questions [76]. A genome-wide association study of AVF outcomes will also be beneficial to identifying key genetic risk factors for AVF failure [77]. With much advancement of RNA-seq technology, especially single cell RNA-seq, coupled with spatial transcriptomics, studies would be feasible to investigate roles of different cell types or distinct subpopulations of cells in AVF maturation at both cellular and molecular levels using animal models and human tissues [78].

Conclusion

Fistula care has evolved from the goal of “Fistula first” to a concept underpinned by “Right Access, Right Patient, Right Time, and for the Right Reason.” The pathophysiology of AVF maturation is poorly understood with the lack of modifiable targets to improve outcomes along the entire life cycle of AVF. There are several factors which show promise as potential targets, but conclusive evidence is lacking. The role of inflammatory cells like T cells and macrophages provides potential new modifiable targets such as AKT1-mTOC1 inhibition with rapamycin [21, 30], or through anti-inflammatory agents such as statins [33-35] and glucocorticoids [36, 37]. The vascular adaptation to the hypoxia and oxidative stress could also influence AVF outcomes, so factors that affect HIF, like HIF1 mRNA, VEGF-A, and HIF inhibitors, could modify the adaptive process thereby improving AVF maturation. Systemic effects for these approaches will need careful assessment [62, 66].

Emerging basic science studies provide many avenues to explore. A combination of approaches incorporating proteomic, transcriptomic, and metabolomic studies could accelerate the identification of intervention targets and predictive biomarkers to improve AVF function and longevity, which still remains an unmet need in improving dialysis care.

Conflict of Interest Statement

The authors have no conflicts of interest to declare.

Funding Sources

Dr S. Mitra is supported by funding from National Institute of Health Research MedTech & In Vitro Diagnostics Co-operatives (D4D).

Author Contributions

Xin Wang, Leticia M. Tapia Silva, Milind Nikam, Sandip Mitra, Syed Shaukat Abbas Zaidi, and Nadja Grobe contributed to this work according to the ICMJE Criteria for Authorship.

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