The Role of Myeloid Cell-Derived PDGF-B in Neotissue Formation in a Tissue-Engineered Vascular Graft

Abstract

Aim: Inflammatory myeloid lineage cells mediate neotissue formation in tissueengineered vascular grafts, but the molecular mechanism is not completely understood. We examined the role of vasculogenic PDGF-B in tissue-engineered vascular graft neotissue development. Materials & methods: Myeloid cell-specific PDGF-B knockout mice (PDGF-KO) were generated using bone marrow transplantation, and scaffolds were implanted as inferior vena cava interposition grafts in either PDGF-KO or wildtype mice. Results: After 2 weeks, grafts from PDGF-KO mice had more remaining scaffold polymer and less intimal neotissue development. Increased macrophage apoptosis, decreased smooth muscle cell proliferation and decreased collagen content was also observed. Conclusion: Myeloid cell-derived PDGF contributes to vascular neotissue formation by regulating macrophage apoptosis, smooth muscle cell proliferation and extracellular matrix deposition. Progress in tissue engineering continues to promise a solution to the lack of tissue available for reconstructive and replacement surgery. Our lab has worked over the past several years developing a tissue-engineered vascular graft (TEVG) for use in congenital heart and vascular surgery. Traditionally used prosthetic materials in the form of Dacron ® or Gore-Tex ® pose significant risk for thromboembolic and infectious complications and also lack growth capacity, making them less ideal for use in the pediatric population. Our ability to create a TEVG with growth capacity [1,2] has therefore been an important advancement in the field, but graft stenosis continues to be a major limitation to widespread clinical application. We have therefore concentrated our efforts on understanding the underlying processes of TEVG neotissue formation and stenosis development.We developed a murine model of TEVG stenosis [3] and found that seeding the grafts with bone marrow-derived mononuclear cells prior to implantation increases graft patency [4] in a dose responsive manner [5], but the seeded cells disappear shortly after implantation [6,7]. We thus hypothesized that a paracrine mechanism, rather than proliferation of seeded cells, was responsible for the remodeling of the TEVG and then demonstrated that host endothelial and smooth muscle cells from the adjacent vessel migrate to form the TEVG neotissue.Host monocytes and macrophages were identified as key mediators of graft remodeling. They infiltrate the graft in large numbers shortly after implantation, and excessive infiltration leads to graft stenosis [8].

Implantation of VEGF‐functionalized cell‐free vascular grafts: regenerative and immunological response

“…They also establish new ECM, predominantly fibronectin, that serves as substrate for EC attachment and migration (27,69). MCs and Mfs are indispensable for vascular graft patency and remodeling, providing key signals for circulating endothelial progenitor cells and ingrowth of anastomotic ECs, as shown in previous studies (15,(46)(47)(48)(70)(71)(72). In a key study, Hibino et al (70) found that Mf infiltration was crucial for neotissue formation.…”

“…In addition, the thicker cellular component of SHV grafts indicates a more robust cellular infiltration, likely due to the presence of VEGF, a well‐known mitogen for ECs and strong chemoattractant for MCs and Mϕs (44, 45). Recruited MCs and Mϕs might have induced robust proliferation of medial‐layer cells (especially smooth muscle cells and fibroblasts) in a well‐orchestrated series of events typical of the wound healing process (15, 46–48). Increased cell infiltration was accompanied by reduced thickness of the SIS matrix remaining in the grafts at 1 mo postimplantation, suggesting that SHV grafts remodeled more efficiently than SH grafts.…”

“…It is important to note that these vessels, although successful, still require intensive culture times-a key issue when considering the ability of a graft to be truly off-the-shelf ready. In addition to devitalized and decellularized grafts, nonbiologic grafts composed of various polymeric biomaterials have also been used to engineer cell-free vascular grafts (15)(16)(17)(18)(19)(20)(21)(22)(23). Besides biocompatibility, lack of immunogenicity, and mechanical properties matching those of native vessels, these materials must promote endothelialization of the lumen to achieve patency and promote development of the vascular wall through extensive, long-term remodeling.…”

Endothelialization of arterial vascular grafts by circulating monocytes

“…Decellularized tissue engineered constructs have been utilized with increasing frequency and demonstrated improved patency and regeneration potential in pre-clinical studies [1][2][3][4][5][6][7][8][9][10][11][12][13] and clinical trials 14,15 . In addition to decellularized grafts, nonbiological grafts composed of various polymeric biomaterials have also been used to engineer cell-free vascular grafts [16][17][18][19][20][21][22][23][24] . All of these acellular materials must promote endothelialization of the lumen to achieve patency and promote development of the vascular wall through extensive, long-term remodeling.…”

Seeding and Implantation of a Biosynthetic Tissue-engineered Tracheal Graft in a Mouse Model

“…Wash with 0.1% BSA-PBS for 5 min. Incubate in primary antibody with concentrations for keratin 5 (1:1000) 6 , keratin 14 (1:250) 6 and F4/80 (1:100) 7 for 18 h overnight at 4 °C.…”