Most regenerative strategies have not yet proven to be safe or reasonably efficient in the clinic. In addition to stem cells and growth factors, the immune system plays a crucial role in the tissue healing process. Here, we propose that controlling the immune-mediated mechanisms of tissue repair and regeneration may support existing regenerative strategies or could be an alternative to using stem cells and growth factors. The first part of this review we highlight key immune mechanisms involved in the tissue healing process and marks them as potential target for designing regenerative strategies. In the second part, we discuss various approaches using biomaterials and drug delivery systems that aim at modulating the components of the immune system to promote tissue regeneration.

Growth factors (GFs) are critical in tissue repair, but their translation to clinical use has been modest. Physiologically, GF interactions with extracellular matrix (ECM) components facilitate localized and spatially regulated signaling; therefore, we reasoned that the lack of ECM binding in their clinically used forms could underlie the limited translation. We discovered that a domain in placenta growth factor-2 (PlGF-2(123-144)) binds exceptionally strongly and promiscuously to ECM proteins. By fusing this domain to the GFs vascular endothelial growth factor-A, platelet-derived growth factor-BB, and bone morphogenetic protein-2, we generated engineered GF variants with super-affinity to the ECM. These ECM super-affinity GFs induced repair in rodent models of chronic wounds and bone defects that was greatly enhanced as compared to treatment with the wild-type GFs, demonstrating that this approach may be useful in several regenerative medicine applications.

By binding growth factors (GFs), the ECM tightly regulates their activity. We recently reported that the heparin-binding domain II of fibronectin acts as a promiscuous high-affinity GF-binding domain. Here we hypothesized that fibrin, the provisional ECM during tissue repair, also could be highly promiscuous in its GF-binding capacity. Using multiple affinity-based assays, we found that fibrin(ogen) and its heparin-binding domain bind several GFs from the PDGF/VEGF and FGF families and some GFs from the TGF-β and neurotrophin families. Overall, we identified 15 unique binding interactions. The GF binding ability of fibrinogen caused prolonged retention of many of the identified GFs within fibrin. Thus, based on the promiscuous and high-affinity interactions in fibrin, GF binding may be one of fibrin’s main physiological functions, and these interactions may potentially play an important and ubiquitous role during tissue repair. To prove this role in a gain-of-function model, we incorporated the heparin-binding domain of fibrin into a synthetic fibrin-mimetic matrix. In vivo, the multifunctional synthetic matrix could fully mimic the effect of fibrin in a diabetic mouse model of impaired wound healing, demonstrating the benefits of generating a hybrid biomaterial consisting of a synthetic polymeric scaffold and recombinant bioactive ECM domains. The reproduction of GF–ECM interactions with a fibrin-mimetic matrix could be clinically useful, and has the significant benefit of a more straightforward regulatory path associated with chemical synthesis rather than human sourcing.

Modern synthetic biomaterials are being designed to integrate bioactive ligands within hydrogel scaffolds for cells to respond and assimilate within the matrix. These advanced biomaterials are only beginning to be used to simulate the complex spatio-temporal control of the natural healing microenvironment. With increasing understanding of the role of growth factors and cytokines and their interactions with components of the extracellular matrix, novel biomaterials are being developed that more closely mimic the natural healing environments of tissues, resulting in increased efficacy in applications of tissue repair and regeneration. Herein, the important aspects of the healing microenvironment, and how these features can be incorporated within innovative hydrogel scaffolds, are presented.

Growth factors are important morphogenetic proteins that instruct cell behavior and guide tissue repair and renewal. Although their therapeutic potential holds great promise in regenerative medicine applications, translation of growth factors into clinical treatments has been hindered by limitations including poor protein stability, low recombinant expression yield, and suboptimal efficacy. This review highlights current tools, technologies, and approaches to design integrated and effective growth factor-based therapies for regenerative medicine applications. The first section describes rational and combinatorial protein engineering approaches that have been utilized to improve growth factor stability, expression yield, biodistribution, and serum half-life, or alter their cell trafficking behavior or receptor binding affinity. The second section highlights elegant biomaterial-based systems, inspired by the natural extracellular matrix milieu, that have been developed for effective spatial and temporal delivery of growth factors to cell surface receptors. Although appearing distinct, these two approaches are highly complementary and involve principles of molecular design and engineering to be considered in parallel when developing optimal materials for clinical applications.

Significance: Growth factors are very promising molecules for the treatment of skin wounds. However, their translation to clinical use has been seriously limited, facing issues related to safety and cost-effectiveness. These problems may derive from the fact that growth factors are used at vastly supraphysiological levels without optimized delivery systems. Recent Advances: The extracellular matrix (ECM) plays a fundamental role in coordinating growth factor signaling. Therefore, understanding the mechanisms by which the ECM modulates growth factor activity is key for designing efficient growth factor-based therapies. Recently, several growth factorbinding domains have been discovered within various ECM proteins, and growth factor delivery systems integrating these ECM growth factor-binding domains showed promising results in animal models of skin wound healing. Moreover, a novel strategy consisting of engineering growth factors to target endogenous ECM could substantially enhance their efficacy, even when used at low doses. Critical Issues: Optimal delivery of growth factors often requires complex engineered biomaterial matrices, which can face regulatory issues for clinical translation. To simplify delivery systems and render strategies more applicable, growth factors can be engineered to optimally function with clinically approved biomaterials or with endogenous ECM present at the delivery site. Future Directions: Further development and clinical trials will reveal whether growth factor-based therapies can be used as main therapeutic approaches for skin wound healing. The future impact of these therapies will depend on our capacity to deliver growth factors more precisely, to improve efficacy, safety, and cost-effectiveness.

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Blood vessel growth plays a key role in regenerative medicine, both to restore blood supply to ischemic tissues and to ensure rapid vascularization of clinical-size tissue-engineered grafts. For example, vascular endothelial growth factor (VEGF) is the master regulator of physiological blood vessel growth and is one of the main molecular targets of therapeutic angiogenesis approaches. However, angiogenesis is a complex process and there is a need to develop rational therapeutic strategies based on a firm understanding of basic vascular biology principles, as evidenced by the disappointing results of initial clinical trials of angiogenic factor delivery. In particular, the spatial localization of angiogenic signals in the extracellular matrix (ECM) is crucial to ensure the proper assembly and maturation of new vascular structures. Here, we discuss the therapeutic implications of matrix interactions of angiogenic factors, with a special emphasis on VEGF, as well as provide an overview of current approaches, based on protein and biomaterial engineering that mimic the regulatory functions of ECM to optimize the signaling microenvironment of vascular growth factors.