Early Immunomodulatory Effects of Implanted Human Perivascular Stromal Cells during Bone Formation

Human perivascular stem/stromal cells (PSC) are a multipotent mesodermal progenitor cell population defined by their perivascular residence. PSC are most commonly derived from subcutaneous adipose tissue, and recent studies have demonstrated the high potential for clinical translation of this fluorescence-activated cell sorting-derived cell population for bone tissue engineering. Specifically, purified PSC induce greater bone formation than unpurified stroma taken from the same patient sample. In this study, we examined the differences in early innate immune response to human PSC or unpurified stroma (stromal vascular fraction [SVF]) during the in vivo process of bone formation. Briefly, SVF or PSC from the same patient sample were implanted intramuscularly in the hindlimb of severe combined immunodeficient (SCID) mice using an osteoinductive demineralized bone matrix carrier. Histological examination of early inflammatory infiltrates was examined by hematoxylin and eosin and immunohistochemical staining (Ly-6G, F4/80). Results showed significantly greater neutrophilic and macrophage infiltrates within and around SVF in comparison to PSC-laden implants. Differences in early postoperative inflammation among SVF-laden implants were associated with reduced osteogenic differentiation and bone formation. Similar findings were recapitulated with PSC implantation in immunocompetent mice. Exaggerated postoperative inflammation was associated with increased IL-1α, IL-1β, IFN-γ, and TNF-α gene expression among SVF samples, and conversely increased IL-6 and IL-10 expression among PSC samples. These data document a robust immunomodulatory effect of implanted PSC, and an inverse correlation between host inflammatory cell infiltration and stromal progenitor cell-mediated ossification.