Coronary artery atherosclerosis is associated with an inflammatory process (2) and contributes to significant morbidity and mortality (3). Surgeons often implant bypass grafts to deliver oxygenated blood around a stenosis to distal coronary beds. Current gold standard treatments use vessels harvested from other parts of the body since this autologous approach outperforms synthetic grafts. Unfortunately, these vessels require surgical harvesting and are prone to restenosis due to intimal hyperplasia. A current clinical focus is the development of a long-lasting tissue-engineered vascular graft (TEVG) (4). Despite significant efforts, ideal TEVGs have remained elusive. Many groups are working on developing TEVGs with appropriate mechanical properties, bioactivity, and biocompatibility (4, 5). Protein-coated polytetrafluoroethylene (ePTFE) grafts lined with autologous endothelial cells have shown long-term patency in almost 500 patients. The complexity of the cell sourcing and seeding procedures, however, does not make this technique amenable to routine use in vascular surgery. Yet, the clinical successes indicate the potential at this early stage of tissue engineering efforts. Ongoing efforts in vitro and in vivo seek to optimize long-term patency, mechanical properties, and reendothelialization (6). Recent advancements suggest continued improvements are possible and continued development could eliminate many of the issues associated with current synthetic grafts.

Atherosclerosis can also be present in peripheral arteries, and peripheral artery disease (PAD) can lead to intermittent claudication and critical limb ischemia in later stages of disease progression (7). The risk of developing lower-limb PAD increases with obesity, a history of atherosclerosis, high triglycerides, low high-density lipoprotein, and aging (8). Ongoing efforts seek to develop non-invasive interventions to treat atherosclerosis and prevent deleterious remodeling of the vascular wall. A recent study showed an association between serum levels of sortilin, a glycoprotein involved in glucose and lipid metabolism, with aortic calcification and general cardiovascular disease risk (9). Carotid artery atherosclerosis revealed PCSK6 as a novel protease, possibly making these lesions prone to rupture (10). The development of a novel platelet lysate hydrogel has shown promise to promote angiogenic activity of mesenchymal stem cells (MSC) that can also be delivered concomitantly (11). While each of these individual findings may lead to a therapeutic breakthrough, the combination of multiple studies over the next 50 years has the potential to improve our mechanistic insight into atherosclerosis and PAD, providing unique treatment solutions.

Accumulating evidence from basic science and clinical medicine suggests that inflammation plays critical roles in the pathogenesis of atherosclerotic vascular diseases and their clinical complications (12). Emerging evidence indicates that macrophages are a heterogeneous population (13). Similarly, we know that monocytes, generally considered as macrophage precursors, are also heterogeneous (14). Changes in macrophage behavior and attributes in response to systemic or local environmental cues may help execute specific functions during the disease process. Traditionally macrophages were thought to adopt a pro-inflammatory or anti/non-inflammatory phenotype in response to stimuli (M1 vs. M2 polarization), but new evidence suggest that macrophage heterogeneity is more multi-dimentional (15–17). Studies using single cell analyses have demonstrated the dynamic and complex nature of human primary monocytes and macrophages heterogeneity (18–20). Understanding the underlying mechanisms of monocyte/macrophage heterogeneity and related therapeutic implications may require innovative approaches such as machine learning from large clinical studies.

Abdominal aortic aneurysm (AAA) is an inflammatory disease of the aorta resulting in pathologic dilation of the vessel wall. Clinically, an aortic diameter 50% larger than normal is considered aneurysmal, and only surgical treatment options currently exist (21). Between 5 and 10% of people in the industrialized world over the age of 65 suffer from AAAs (22, 23), accounting for roughly 16,000 deaths and 150,000 inpatient hospitalizations per year in the U.S (24, 25). Although recent studies have provided insight into the pathogenesis of AAA, a detailed understanding of the underlying mechanisms that lead to AAA expansion remains incomplete.

Development of novel therapies that will interrupt development of an AAA or halt aneurysm progression remains a challenge (26). Efforts are focusing on investigating the association between genetic variants and aneurysm formation (27) and the role of enzyme activity in extracellular matrix (ECM) changes within the aortic wall (28). Further work has focused on the role of the inflammasome, including both innate immunity and inflammation, in aneurysm formation and progression (29). Recent studies have shown that serum amyloid A, a protein that associates with high-density lipoprotein when in circulation, exacerbates acute vascular events by activating the inflammasome (30, 31). Others have investigated the correlation between circulating biomarkers and aortic disease, showing that elevated circulating levels of the soluble receptor for advanced glycation end products is associated with a variety of aortopathies, independent of aortic diameter (32). Identifying patients at increased risk for aneurysm development and then increasing aortic wall strength through pharmacologic means could slow growth of AAAs to large diameters where rupture is more likely.

Beyond aortic wall research, blood flow hemodynamics have been shown to be critical to the formation and growth of aneurysms, dissections, and thrombus (33). This provides strong motivation to develop sophisticated data-driven models of blood flow, pressure, and wall elasticity associated with AAAs. Recent work focused on implementing a multi-modality imaging approach that combined high frequency ultrasound (US) and optical coherence tomography (OCT) as inputs for a murine computational modeling study (34). The results showed that differences in final lesion size and compositions correlated with vortical structures obtained through mouse-specific fluid dynamic simulations, suggesting that differences in morphology and hemodynamics play crucial roles in AAA formation. These data agree with a large amount of previous work where imaging-based computational findings have suggested a link between hemodynamic perturbations and aneurysmal disease heterogeneity (35). The combination of imaging, hemodynamic simulations, and biomechanical analysis is proving to be useful for exploring potential translational strategies that could soon be useful to predict possible aneurysm expansion and rupture (36, 37). Taken together, these recent advancements suggest a bright future for multi-disciplinary cardiovascular research in clinical medicine, genetics, biology, and engineering to address unmet clinical need associated with AAA.

Unidirectional blood flow from the left ventricle to the aorta for systemic distribution normally occurs through coordinated action of three leaflets. Leaflet action is controlled by a layered and highly organized ECM microarchitecture (40–43). The ECM structure is maintained by two cell populations: valvular endothelial cells (VECs) and valvular interstitial cells (VICs). VECs appear phenotypically distinct from other endothelial cell populations in vascular tissues and exhibit regional heterogeneity with side-specific differences in gene expression (44–46). VICs are a poorly defined population of cells with subpopulations of fibroblasts, myofibroblasts, smooth muscle cells, and neuron-like cells previously identified within the leaflets (47, 48). Phenotypic changes in VECs and VICs have been associated with aortic valve remodeling (49), but the relative contributions of these cells and associated subpopulations remain unknown. The role of inflammation in valve remodeling (50, 51) is especially relevant when considering approaches to valve disease in developing countries, where rheumatic heart disease is a major contributor. VECs and VICs also display mechanosensitivity and readily respond to changes in the mechanical environment (52–55). The complex cellular and biomechanical environment is difficult to recapitulate in vitro and animal models of aortic valve disease are lacking (56), making mechanistic studies on the biomechanical and biochemical initiators of disease difficult to perform.

Recent studies have sought to overcome this limitation by using large, unbiased proteomic and transcriptomic approaches to characterize molecular changes in aortic valve leaflets obtained from patients undergoing replacement surgeries (57). Combining pathological characterization of the leaflets following resection with network-based analysis of the proteomic and transcriptomic data has yielded new insight into the potential molecular drivers of aortic valve disease. Coupled with new genome wide association studies that have revealed new lipid associations with aortic valve disease, these big data approaches may provide new clues about points of non-invasive therapeutic intervention and the development of drug-based therapies (58, 59). However, challenges remain in identification of patients during the early stages of disease before gross remodeling of the aortic valve leaflets necessitate replacement.

Given the lack of non-invasive treatment or suitable options for CAVD, the traditional clinical approach has been surgical valve replacement. First introduced in 1960, early iterations of devices for aortic valve replacement utilized mechanical valves consisting of caged-ball or tilting disk designs surgically implanted into the aortic orifice following removal of the diseased valve (60). These devices provided the first viable clinical solution for patients with aortic valve abnormalities and offered extraordinary reduction in mortality associated with CAVD. Of note, inoperable patients with CAVD have a 2–3 year mortality of < 50% (61, 62). Though these devices helped correct valve dysfunction, nearly all patients who received mechanical valves suffered valve-related complications within 10 years, and many died of these complications (63).

To enhance biocompatibility and create a more normal geometry, bioprosthetic valves were introduced in the clinic in the late 1960s as an alternative to mechanical valves (64). Bioprosthetic valves are fabricated from glutaraldehyde treated (and hence non-viable) porcine aortic valve or bovine pericardial tissue formed into a tri-leaflet structure. Bovine pericardium is used most frequently today. These valves do not require lifelong anticoagulation therapy, and bioprosthetic valves more adequately recapitulate the biomechanics and hemodynamics of native aortic valves. Nevertheless, bioprosthetic valves frequently undergo calcification, leading to stenosis or tearing with regurgitation. After ~15–20 years, bioprosthetic valves often must be replaced, requiring the patient to undergo an additional invasive surgical procedure. The mineral forms due to phosphorus in devitalized cell remnants and possibly residual aldehyde affinity for mineral. Newer versions of bioprosthetic valves overcome this limitation through detergent-based treatments that reduce cell-based material and inhibit mineral deposition (65). To avoid multiple surgeries, modern mechanical valves have been deemed more suitable for younger patients who need aortic valve replacement. Clinicians must weigh the relative risks of reoperation to replace bioprosthetic valves vs. the risks associated with anticoagulant therapy in patients with mechanical valves (66).

The advent of transcatheter aortic valve implantation (TAVI) has begun to revolutionize aortic valve replacement. Synthetic or bovine pericardial-based aortic valves are placed into the aortic annulus using an endovascular catheter. The catheter is most often introduced through the femoral artery and guided to the annulus whereupon the replacement aortic valve is deployed, displacing the diseased aortic valve (67). First introduced for elderly patients and those deemed unfit for surgical-based replacements, TAVI is becoming standard care for many patients with CAVD (68). Patients undergoing TAVI procedures have similar outcomes as those who receive surgical aortic valves (69). Early analyses indicated that TAVI may induce stroke, paravalvular leak, and vascular wall damage during catheterization; however, subsequent studies have shown that other complications may be less of a concern than those arising from surgery (70, 71). TAVI “valve-in-valve” approaches also obviate the need for open surgical procedures for patients with degeneration of a previously implanted bioprosthetic valve. After the initial bioprosthetic valve deteriorates, a TAVI procedure can introduce a new valve that is likely to exceed the expected lifespan of the patient.

The leading cause of aortic valve disease in developing countries is rheumatic heart disease, but the local infrastructure is not generally well-suited for open heart procedures. TAVI may provide a more appropriate option for patients in these regions (72); however, two specific limitations must be overcome. Typically, aortic valve disease and bioprosthetic degeneration are associated with the deposition of calcific mineral on the leaflets. This mineral provides a structure to anchor TAVI valves, but rheumatic-induced aortic valve remodeling does not usually involve heavy calcification. Positioning the catheter during TAVI also requires imaging modalities not commonly available in developing countries. Recently developed TAVI strategies designed specifically for low resource settings may help overcome these limitations (73). The new design employs a supra-annular anchoring technique that latches to the non-calcified valve structure and provides tactile feedback that allows the clinician to locate the correct annular position without the need for fluoroscopic imaging. This technique could address a major unmet clinical need in developing countries.

Since the first replacement aortic valves were introduced, advancements in both valve design and replacement techniques have provided lifesaving options for many patients. However, issues remain, particularly for pediatric patients who require aortic valve replacement due to congenital valvular abnormalities. These children often require multiple procedures to replace valves that do not adapt to somatic growth, and calcification of valves and conduits is accelerated in young recipients. These patients would benefit from engineered aortic valve constructs that fully integrate with native host tissues, do not degenerate, and adapt to size and pressure changes in the cardiovascular system. Efforts to develop tissue engineered aortic valves should integrate knowledge of the complex biological environment, dynamic biomechanics, material durability, and delivery/implantation methods discussed in the previous sections. Early attempts to engineer living aortic valve tissues employed biodegradable scaffolds seeded with mixed populations of arterial-derived endothelial cells and fibroblasts (74). These constructs yielded ECM deposition consistent with native valve structure after 2 months in an ovine model, demonstrating the potential utility of a living engineered tissue that can actively remodel appropriately after implantation (75).

Translation of these proof-of-concept techniques to clinical practice for human patients remains elusive, however. Questions persist on the appropriate cell source, the most appropriate material for the scaffolds, the minimum biomechanical functionality required for implantation, and the methods to assess remodeling in situ following implantation (76). In the early 2000s, the first tissue engineered aortic valve replacement surgeries were performed in neonates with severe congenital malformations (77). These valves exhibited beneficial early remodeling in an ovine model, and gross long-term ECM remodeling was attributed to a problem with the animal model. The outcomes from initial clinical trials, however, were largely poor. Many of the valves exhibited remodeling concomitant with inflammation, including fibrosis and deterioration, comparable to the observations made in the ovine endpoints (77).

These early outcomes reduced enthusiasm for aortic valve tissue engineering, but the clinical need for pediatric patients with aortic valve dysfunction remains. Early clinical successes have been noted in pediatric mitral valve repair using constructs of porcine small intestinal submucosa handmade in the clinic to resemble valve leaflets (78). The ad hoc use of this material in patients with few other clinical options has yielded promising results in short-term clinical follow-ups and work by recruiting endogenous cells that stimulate leaflet remodeling and growth (79, 80). Similar strategies are being developed for aortic valve replacement. Many approaches currently in pre-clinical development seek to recruit host cells after implantation of a polymer matrix without cells or other biological adjuncts. In such an approach, proper ECM development and leaflet maturation takes cues from and depends on processes that occur in native valve development (so-called “in situ tissue engineering”) (81). This strategy enables off-the-shelf availability of constructs without the need for maintenance of cellular viability, and could provide a clinically feasible solution that avoids cell sourcing complications (82).

Whether these tissue engineered constructs can adequately recapitulate the function of native aortic valves remains to be seen. Perhaps the complete recapitulation of the complex biological structure and biomechanical properties of native valve are not required to produce adequate and lasting function. Imperfect strategies that offer new life to a patient with no other options provide clinical value, and knowledge gained through iterations of incremental improvement will help fill gaps in our current understanding of aortic valve biology and function. Ultimately, non-invasive therapeutics may prevent or reverse adult-onset aortic valve remodeling, and minimally invasive implantation of tissue engineered valves may fix congenital abnormalities in pediatric patients. Achieving these goals will require concerted interdisciplinary efforts of basic scientists, engineers, and clinicians. All are well-represented within ISACB.

When considering scaffolds for cardiac tissue engineering, many factors must be considered for the vast applicability of these materials (83). Cost, sustainability, and labor skill requirements must all be considered for widespread use. A potential starting point for engineered cardiac tissue is an acellular scaffold (84). Investigators have used a master bank of human cells to produce the ECM for these scaffolds. This allows for a controlled initial material source, which helps bring costs down and maintain quality control over the product. Decellularization leaves an ECM that is attractive for native cells to adhere and proliferate (85–87). Lyophilization and sterilization yields an “off the shelf” product, which also helps bring down costs. These scaffolds can be produced in a Good Manufacturing Practices (GMP) facility with appropriate quality controls, allowing for consistent production of scaffolds with the same properties. Most pre-clinical work to date with this scaffold has been in a congenital model, showing that the scaffold grows with the animal. The Emmert/Hoerstrup group is currently working to develop the scaffold as a cardiac patch.

Difficulties remain in vascularizing scaffolds to maintain cell viability. Instead of using mammalian cells to produce a scaffold, investigators are looking toward the plant kingdom, specifically spinach leaves (88). After the decellularizing process, the vascular network inherent to the plant remains and can be used to perfuse fluid. Microspheres, of similar size to red blood cells, were also able to pass through the plant vasculature, and the scaffold is able to serve as a basement membrane for contracting cardiac myocytes. Further work, however, is required for clinical realization of this technique.

Clinical trials on cell therapy for heart disease have demonstrated only limited success (89). This may be due in part to the variability in the cells used in cardiac cell therapy. Most cardiac clinical trials have utilized MSC. While this cell type has not demonstrated deleterious effects, improvement in cardiac function appears to be limited.

Embryonic stem cells (ESCs) and induced pluripotent stem cells (iPS cells) have demonstrated the potential to form contractile myocytes. ESCs can proliferate to provide a plentiful source of cells. They are also able to differentiate into contracting cells with many properties similar to adult cardiac myocytes. ESCs, however, remain a topic of controversy. An exciting discovery in 2006 introduced a new cell type—iPS cells that can be produced from adult differentiated cells (e.g., fibroblast) through genetic engineering (90). By inserting specific genes regulating transcription factors, the adult cells can be induced to becoming an embryonic-like stem cell. These cells can then be differentiated into contractile cells with properties similar to cardiac myocytes. Thus, a patient's own cells can potentially yield cardiac myocytes that restore cardiac function, eliminating any immune rejection response from the recipient. However, significant concerns still remain for both ESCs and iPS cells prior to their use in the clinic. Cell sorting and validation is essential to moving the field forward. Incorporation of the wrong cell type in the heart can lead to fatal arrhythmias or worse, and proliferation of these cells must be regulated.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.