In the present study, the feasibility of augmenting the angiogenic effect of BM-MSCs using fibrin was demonstrated. We showed that BM-MSCs had robust cell attachment, migration and viability when cultured in fibrin in vitro
, a finding in concurrence with published literature [18
]. When delivered together as an injectable therapy to the infarct scar, the BM-MSCs in fibrin treatment could enhance capillary formation. A significant improvement in capillary density was associated with increased levels of VEGF expressed in the infarct scar. Although the present method used for VEGF assessment can only be semiquantified, this technique provided spatial information regarding VEGF localization within the infarct tissue.
The enhancement of VEGF expression after treatment of BM-MSCs in fibrin can provide cardioprotective effects to cardiomyocytes. This finding is supported by in vivo
studies in which treatment of BM-MSCs overexpressing VEGF improved cardiac function [29
]. In related studies, Markel et al
. demonstrated that knockdown of VEGF in BM-MSCs by siRNA impaired VEGF production and diminished stem cell-mediated postischemic myocardial recovery [30
Although the BM-MSCs demonstrated low cell survival and no evidence of cardiovascular differentiation at 5 weeks after delivery - a finding supported by other reports [31
] - the transient retention of BM-MSCs may be sufficient to promote the paracrine release of angiogenic cytokines such as VEGF. Moreover, our findings suggest that fibrin could modulate the angiogenic effect of BM-MSCs by promoting significantly increased neovascularization, when compared with the treatment by each individual component. However, the inability of BM-MSCs and fibrin to reduce the infarct size in the present study suggests that other additional treatment factors may be required, such as prosurvival cues or cell-cell interaction [33
The ability of fibrin to regulate BM-MSC behavior has been demonstrated in previous studies [18
], in which fibrin stimulated BM-MSC proliferation and migration and could modulate BM-MSC phenotype towards cardiovascular lineages. The effect of fibrin on BM-MSCs could be due to factors such as matrix elasticity [35
] or specific integrin binding [36
]. These potential factors are interesting and warrant further investigation.
An advantage of this approach of delivering BM-MSCs in fibrin is the ability to enhance VEGF levels in the infarct without relying on viral approaches to induce VEGF overexpression [30
]. Furthermore, the treatment of BM-MSCs in fibrin can potentially become an off-the-shelf therapy. BM-MSCs can be easily isolated from numerous sources and expanded in vitro
one million-fold while sustaining multilineage potential [38
]. Also, BM-MSCs lack MHC II antigens that are responsible for immune rejection, making them a favorable candidate for allogeneic cell transplantation [41
]. In addition, fibrin is already an FDA-approved material. Therefore, the delivery of BM-MSCs and fibrin could provide a treatment strategy for patients nearing congestive heart failure who require immediate therapeutic intervention.
In contrast to previous studies that demonstrated angiogenic effects of fibrin treatment alone [14
], the current study showed no enhancement in neovascularization in the fibrin group. This finding is supported by others and by our previous work showing that fibrin alone does not affect capillary formation or microvascular density [26
]. The difference in results may also be related to the type of animal model used. Earlier reports suggest that fibrin’s ability to modulate angiogenesis may, in part, be related to its ability to recruit inflammatory cells that secrete angiogenic factors [43
]. However, since the animals used in this study were immunocompromised, they may have had a limited ability to respond to the treatment of fibrin. Instead, our results suggest that it may be necessary to deliver both BM-MSCs and fibrin in order to augment capillary formation and VEGF expression for repair of chronic MI in the immunocompromised setting.
Earlier reports on the combined delivery of cells with fibrin for myocardial repair remain limited. Previous studies of delivering myoblasts in fibrin in an acute MI model also demonstrated significant increases in neovascularization and cardiac functional improvement [12
], but myoblasts lack the ability to form electrical connections with cardiomyocytes and therefore pose the risk of arrhythmia [44
]. Transplantation of endothelial cells with fibrin into the ischemic myocardium has been shown to improve ejection fraction and neovascularization [47
], but endothelial cells also have limited ability to transdifferentiate into cardiomyocytes. The therapeutic effect of delivering bone marrow mononuclear cells alone or in conjunction with fibrin resulted in significantly higher microvascular density and improvement in cardiac function [42
]. In addition, Zhang et al
. demonstrated that controlled release of stromal cell-derived factor-1α conjugated to a poly(ethylene glycol) fibrin patch could improve recruitment of c-kit-expressing cells and improve cardiac function [50
]. With respect to BM-MSCs and fibrin delivery for myocardial repair, Liu et al
. transplanted patches containing BM-MSCs in fibrin onto acutely infarcted porcine hearts and reported enhanced angiogenesis when compared with acellular fibrin constructs [20
]. However, the patch approach relied on migration of the BM-MSCs from the patch into the myocardium, a process reported by the authors as only 10% efficient. By contrast, the current study utilizes an injectable delivery system that efficiently delivers the cells locally into the myocardium. To date, this is the only study that has investigated the injectable delivery of BM-MSCs in fibrin using a chronic model of MI.