STRO-1 Immunoselection Enhances the Biological Properties and Cardiovascular Paracrine Effects of Bone Marrow Mesenchymal Cells

Abstract

Purpose: In contrast to traditional plastic adherence isolation, immunoselection has been advocated as a strategy to enrich the purity of mesenchymal stromal cells (MSC) from bone marrow (BM) preparations. This study set out to determine whether immunoselection using the STRO-1 monoclonal antibody confers biological differences to BM MSC compared to conventional plastic adherence isolation. It also investigated whether the paracrine effects of MSC on cardiac and endothelial cells are enhanced by higher levels of STRO-1 expression. Methods: Plastic adherent (PlAd) and STRO-1 positive (SP) MSC were isolated from healthy human BM. Assays were performed to study the clonogenicity, replicative potential and plasticity of the two populations. Conditioned media (CM) from 4th passage cells was used to compare for differences in paracrine effects on human umbilical vein endothelial cells and rat cardiac muscle cells (CMC). Paracrine assays were also repeated using CM from fractionated MSC populations with highly discrepant STRO-1 expression (STRO-1Bright (SB) and STRO- 1Dim (SD)). Finally, SB and SD MSC were assessed for their in vivo capacity to induce mitogenic and angiogenic effects in a rat model of myocardial infarction. Results: Cells isolated by STRO-1 immunoselection yielded more colony forming units-fibroblastic and more population doublings than those prepared by plastic adherence. During ex vivo culture, SP MSC maintained higher levels of (1) STRO-1 surface expression; (2) gene expression for early stem cell markers (TWIST-1, DERMO-1); and (3) gene and protein expression of various cytokines (SDF-1/CXCL12, HGF, IGF-1 and VEGF). SP CM augmented the proliferation and migration of CMC and endothelial tube formation to a greater extent than PlAd CM. Paracrine responses were most enhanced by the presence of SB CM, while SD CM had the converse effect. These observations were supported in vivo, as injection of SB MSC into rat myocardium was associated with higher numbers of proliferating (Ki67-positive) cardiomyocytes and greater vascular density than SD MSC. Conclusions: STRO-1-based immunoselection gives rise to a population of mesenchymal cells with favourable biological characteristics, compared to plastic adherence isolation. In addition, higher STRO-1 expression is associated with enhanced cardiovascular-relevant paracrine effects. Immunoselection may be a useful strategy for optimising the cardiac reparative properties of MSC.