Modulation of stem cell delivery strategy by platelet lysate utilization and cell aggregation for enhanced bone regeneration
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Challenges to control delivered cell behavior, including viability and differentiation, remain a significant barrier to the translation of cell-based bone tissue engineering strategies. Our research objectives were to evaluate a bioluminescent imaging (BLI) technique for the longitudinal monitoring of delivered mesenchymal stem cell (MSC) number and subsequently evaluate the effect of two MSC delivery strategies on cell survival and facilitated bone regeneration. A BLI protocol for tracking MSCs implanted subcutaneously within an alginate/mesh delivery platform was successfully established, during which the potential role of confounding factors, including fibrotic and vascular tissue development, on BLI signal correlation strength was highlighted. The utility of human platelet lysate (hPL) as a pre-treatment or co-delivery strategy was subsequently evaluated. While hPL utilization did not improve delivered hMSC survival nor promote construct vasculature within an immunocompromised rat model, limitations in the applicability of hPL pre-clinical testing via rodent testbeds were identified. Finally, the effect of MSC aggregation was investigated. Within an immunocompromised rat model, delivery of spheroids had no impact on cell survival, construct vascularization, nor critically-sized bone defect repair. When examined within a syngeneic rodent model, rMSC aggregates elicited a surviving cell fraction and construct vasculature comparable to that of single cell delivery. Despite in vitro observation that the osteoinductive potential of alginate/mesh constructs was increased with rMSC seeding, delivery of rMSC-containing treatments to the femoral defect space attenuated bone repair. Overall, this research implemented a novel imaging platform to relate key cell-based tissue regeneration metrics, namely delivered cell survival, construct vasculature, and functional outcomes, in an effort to elucidate fundamental principles for development of an effective MSC-based large bone defect therapeutic strategy. Importantly, this body of work also drew attention to several aspects of rodent model selection including xenogenicity, cross-reactivity, and biological variability.