56 5. BIOMATERIALS AS VEHICLES FOR STEM CELL DELIVERY

environment after acute MI. With proper selection or design, the biomaterials can serve as an artificial extracellular matrix, to replace the degraded ECM after MI and provide the required temporal support to allow cell engraftment and retention. Using biomaterial scaffolds, thicker cell constructs could be created in vitro, to achieve high density of cells in future grafts. In addition, biomaterials could provide various biochemical and biophysical cues to enhance cell survival, induce angiogenesis at infarct,and direct and control stem/progenitor cell differentiation,by implementation of controlled delivery mechanisms for growth factors, integration of specific cell-matrix interactions, and more. The successful implementation of biomaterials as cell delivery vehicle promises to yield an improved cell engraftment and long-term functionality.

5.2STEM CELL DELIVERY BY BIOMATERIALS

Table 5.1 summarizes the results of several studies using biomaterials as vehicles for stem cell delivery in the infarct.

5.2.1HUMAN EMBRYONIC STEM CELL-DERIVED CELLS

A complimentary approach to the use of human ESC-derived cardiomyocytes for preservation of myocardial tissue after MI is the rescue of the vascular network compromised during ischemia insult. Myocardial injection of hESC-derived vascular cells in an in situ formed bioactive (thymosin β4-containing) PEG hydrogel into infarcted hearts in rats has shown that the delivered cells formed capillaries in the infarct zone. In addition, magnetic resonance imaging (MRI) revealed that the microvascular grafts effectively preserved contractile performance, attenuated left ventricular dilation, and decreased infarct size [4]. In another study, implantation of porous fibrin scaffold co-seeded with hESC-derived endothelial and smooth muscle cells, in a porcine model of ischemia/reperfusion resulted in significant LV functional improvement as judged by cardiac MRI. The authors point to neovascularization as an underlying mechanism behind function restoration [5].

Although the latter study confirmed improvements in cell engraftment due the delivery in biomaterial scaffolds (shown by bioluminescent imaging of luciferase-labeled cells after four weeks), both studies did not provide a direct measure of the effect of scaffolds on the extent of cell retention, as the control group of direct cell injection was missing. Nevertheless, in addition to some beneficial effect of sole material, the authors point to neovascularization as a major paracrine mechanism responsible for functional repair by the delivered cells.

5.2.2ADULT BONE MARROW-DERIVED STEM CELLS

To date, a handful of studies have attempted at regenerating the ischemic myocardium via implantation of a tissue patch pre-seeded either with bone marrow cells or BM-derived mesenchymal stem cells.

Piao et al used bone marrow-derived mononuclear cell (BMMNC)-seeded biodegradable poly-glycolide-co-caprolactone (PGCL) scaffolds in a rat MI model. Patch implantation resulted

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Table 5.1: Selected biomaterial-based strategies for stem cell delivery

58 5. BIOMATERIALS AS VEHICLES FOR STEM CELL DELIVERY

in improved neovascularization and the presence of α-myosin heavy chain (MHC) and troponin I markers in some BMMNCs. Interestingly, when either BMMNCand acellular-PGCL patches were used, attenuation of LV remodeling and LV dysfunction was observed, suggesting the potential of empty scaffolding as an effective treatment alternative for MI [11]. Lin et al used self-assembling peptide nanofibers (NFs) for BMMNC delivery in a pig MI model. NF injection significantly improved diastolic function and reduced ventricular remodeling 28 days after treatment. Injection of BMMNCs alone ameliorated systolic function only, whereas the injection of BMMNCs with NFs significantly improved both systolic and diastolic functions, increased transplanted cell retention ( 30 cells/mm2 in BMMNC-only group and 230 cells/mm2 in BMMNCs/NFs-treated animals) and increased capillary density in the peri-infarct area. The authors indicated an improved cell retention, survival and function, together with a synergistic effect of using biomaterials, as the major mechanisms behind the observed beneficial results [10].

Kim and co-workers showed that epicardial implantation of a poly(lactide-co-ε-caprolactone) (PLCL) patch seeded with MSC induced an enhanced expression of cardiac markers such as MHC, α-actin and troponin I as well as the cardiac transcription factor GATA-4 when compared to MSC suspension injections. Though MSC injections significantly improved heart function, the MSCPLC patch had a greater positive effect on LV EF and infarct size. However, the likely possibility that these beneficial effects, at least partially, stem from greater cell retention in PLC scaffolds, has not been elaborated, as the retention parameter was not compared between the cell-treated groups [13]. Chen et al developed bioengineered tissue graft, by using a porous acellular bovine pericardium sandwiched with multilayered sheets of mesenchymal stromal cells. This tissue graft (sandwiched patch) was used to replace the resected infarct scar area in a syngeneic Lewis rat model with an experimentally chronic MI. Patch application one month after MI improved cardiac function, compared to empty patch-treated animals. The effect of the biomaterial on cell retention was not evaluated [16].

Yu et al examined the effect of encapsulation of human MSCs (hMSCs) in RGD-modified alginate hydrogel microspheres on cell retention, differentiation, and myocardial repair in rats.The encapsulation significantly improved cell survival, compared to simple cell injection. Specifically, hMSC presence (presented as percentage of human cells to host (rat) cells, quantified by real-time PCR), at seven days post-injection in immunodeficient rats, was evident only in the hMSCs-encapsulated group (0.58%). After an additional week, the cells in the microbead group still indicated good retention (0.53%). There were no significant differences in angiogenesis degree between the treatment groups; all showed higher vessel density compared to untreated controls. Alginate microspheres, with or without encapsulated MSCs, successfully maintained LV shape and prevented negative LV remodeling after MI, emphasizing again the importance of the biomaterial in preserving mechanical and passive properties of the myocardium [14].

In a recent study, Norol and co-workers directly compared the engraftment rates of rat MSC when delivered within porous pullulan/dextran scaffold or endocardial injection of their cell suspension in a rat model of MI. Cellular engraftment was measured by quantitative RT-PCR using