**1. Introduction**

The gold standard for treatment of advanced heart valve disease is surgical heart valve re‐ placement. None of the currently available mechanical and bioprosthetic heart valve substi‐ tutes resembles normal heart valve function. While mechanical heart valves offer excellent durability, they require life-long anticoagulation to control thromboembolism, which inher‐ ently leads to an increased risk of hemorrhage complications. Bioprosthetic valves on the other hand, retain a more physiological blood flow pattern, but these valves are prone to calcifica‐ tion and structural deterioration, limiting their lifespan. For both types of replacement valves, the main limitation is that they are non-living prostheses, incapable of adapting to changes in the hemodynamic environment. It was shown that a living autograft implanted in the aortic position (the Ross procedure) improves long-term clinical outcome compared to a non-living homograft [1]. This illustrates the importance of the regulatory and adaptive properties of a living valve substitute. Tissue engineered aortic valves can provide such an autologous, viable valve with the potential to grow, adapt, and regenerate within the hemodynamic environ‐ ment. Evidently, the pediatric and young adult population would benefit most from such a tis‐ sue engineered aortic valve. The valve's ability to grow as the recipient grows and matures, eliminates the need for repetitive surgeries. [2-7].

Foundational principle of regenerative medicine is restoring the native tissue structure and function by providing a microenvironment necessary to promote tissue regeneration. Tissue engineering scaffolds are biomaterials designed to create this microenvironment and to pro‐ mote tissue regeneration [8]. The traditional tissue engineering paradigm for creating trileaflet heart valves consists of harvesting autologous cells from the patient, expanding the cells *in vi‐ tro,* and subsequently seeding the cells into a biodegradable scaffold. The cell-scaffold con‐ structs are conditioned in a bioreactor to promote extracellular matrix formation (ECM), while

© 2013 van Loon et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the scaffold is degraded [9]. Although this approach leads to an autologous, living heart valve, the *in vitro* process is a very costly and time consuming procedure. Therefore, a novel approach emerged from this, in which the *in vitro* phase is completely omitted, the so-called *in situ* tissue engineering, or 'guided tissue regeneration' (figure 1). This approach relies on the body's nat‐ ural regenerative potential and uses the human body as a bioreactor [4]. Key to this process is the use of a functional scaffold, capable of host cell repopulation and subsequent *in situ* tissue remodeling. After implantation, cells will colonize the scaffold to form tissue, while the scaf‐ fold withstands physiological stresses and strains from its hemodynamic environment and may gradually degrade [2,4]. Clearly, these characteristics put more stringent demands on the biomaterial used, as it should provide both mechanical strength and the cellular niche, balanc‐ ing between material degradation and tissue formation. The scaffold can be biological or syn‐ thetic or a combination of both (a hybrid scaffold), loaded with bioactive components and/or cells to provide stimuli for favorable host cell repopulation, differentiation and tissue forma‐ tion. The *in situ* approach allows for the minimization of risks and costs associated with cell and tissue culture, while providing off-the-shelf availability.

The *in situ* tissue engineering approach is heavily reliable on the wound healing response. Both the injury incurred during the implantation process and the host inflammatory response to the implanted biomaterial and its degradation products, influence the local microenvironment created by the scaffold. The biomaterial intensifies the inflammatory response by inducing a foreign body response (FBR), propagated by infiltrating immune cells [10,11]. This response is characterized by the presence of macrophages and their fusion into multinucleated giant cells associated with chronic inflammation arising from the persistent presence of a foreign body. The FBR, especially inflammation, may drive valve calcification. However, inflamma‐ tion is not merely a detrimental response to biomaterial scaffolds. Rather, it can be considered as a natural agent of tissue remodeling, orchestrated by various cell types and potent signaling molecules. By unraveling the inflammatory response towards the foreign biomaterial and the triggers for pathological outcome, targets may be identified to control the inflammatory response through modifications of the biomaterial. The goal is to develop strategies that harness the beneficial aspects of the inflammatory response, while limiting its potential

The Immune Response in *In Situ* Tissue Engineering of Aortic Heart Valves

http://dx.doi.org/10.5772/54354

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deleterious effects by modulating the inflammatory response towards regeneration.

technology.

**2. Biomaterial scaffolds**

mimicked in the applied scaffold materials [3].

**2.1. Biomaterial scaffold use in** *in situ* **heart valve tissue engineering**

This chapter deals with the use of biomaterials for *in situ* heart valve tissue engineering and the immune response to the implanted biomaterial. The FBR to biomaterials is discussed, leading to biomaterial design approaches directed to immunomodulation towards tissue regeneration, identifying pitfalls as well as current research challenges for this innovative

Biomaterials are materials that interact with the body and its cells. As such, they are central to many strategies for regenerative medicine. They are employed as vehicles for transplanting (progenitor) cells, timed and localized delivery of bioactive moieties, and/or as 3D scaffolds for tissue engineering. Scaffolds are biomaterials designed to create a microenvironment that promotes regeneration. Besides creating and maintaining a defined space for tissue growth, biomaterial scaffolds also provide mechanical stability, and support cell adhesion and migration. Ideally, a scaffold for tissue engineering should be bioresorbable, biocompatible and have a highly porous macrostructure necessary for cell growth, nutrient supply, and waste removal [2,3,6,8,12]. By engineering the proper cellular niche, such scaffolds can provide an environment suitable to modify host responses and direct cell survival, migration, proliferation, differentiation, as well as matrix formation and remodeling. The premise is that in order to unlock the full potential of the cells, at least some aspects of the native 3D tissue environment associated with their renewal, differentiation and organization needs to be

Trileaflet heart valves are sophisticated tissues with an anisotropic three-layered structure, optimized to withstand the repetitive hemodynamic loads it is subjected to. A human heart

**Figure 1.** (A) The *in situ* tissue engineering paradigm in which the scaffold is directly implanted, omitting lengthy *in vitro* conditioning phases. (B) The scaffold consists of the bare biomaterial (*i*), which may harbor incorporated bioac‐ tive moieties (*ii*) and/or cells (e.g. bone marrow stromal cells) seeded directly prior to implantation (*iii*). (C) Hypothe‐ sized mechanism of *in situ* heart valve tissue engineering: after implantation, the scaffold will trigger the host immune response, leading to recruitment of various immune cells and macrophage infiltration. The infiltrated cells secrete cy‐ tokines and growth factors to attract additional immune cells, as well as tissue cells, originating from surrounding tis‐ sue and/or circulating (progenitor) cells. Endothelial cells cover the blood-scaffold interface and activated myofibroblasts migrate into the scaffold to produce extracellular matrix. Scaffold degradation correlates to a decrease in pro-inflammatory stimuli, eventually leading to resolution of inflammation. Ideally, the valve remodels into the physiological three-layered structure with endothelial cells covering the blood-contact area and quiescent myofibro‐ blasts as valve interstitial-like cells populating the spongiosa layer. Illustration by Anthal Smits.

The *in situ* tissue engineering approach is heavily reliable on the wound healing response. Both the injury incurred during the implantation process and the host inflammatory response to the implanted biomaterial and its degradation products, influence the local microenvironment created by the scaffold. The biomaterial intensifies the inflammatory response by inducing a foreign body response (FBR), propagated by infiltrating immune cells [10,11]. This response is characterized by the presence of macrophages and their fusion into multinucleated giant cells associated with chronic inflammation arising from the persistent presence of a foreign body. The FBR, especially inflammation, may drive valve calcification. However, inflamma‐ tion is not merely a detrimental response to biomaterial scaffolds. Rather, it can be considered as a natural agent of tissue remodeling, orchestrated by various cell types and potent signaling molecules. By unraveling the inflammatory response towards the foreign biomaterial and the triggers for pathological outcome, targets may be identified to control the inflammatory response through modifications of the biomaterial. The goal is to develop strategies that harness the beneficial aspects of the inflammatory response, while limiting its potential deleterious effects by modulating the inflammatory response towards regeneration.

This chapter deals with the use of biomaterials for *in situ* heart valve tissue engineering and the immune response to the implanted biomaterial. The FBR to biomaterials is discussed, leading to biomaterial design approaches directed to immunomodulation towards tissue regeneration, identifying pitfalls as well as current research challenges for this innovative technology.
