Anti-inflammatory/tissue repair macrophages enhance the cartilage-forming capacity of human bone marrow-derived mesenchymal stromal cells

Macrophages are key players in healing processes. However, little is known on their capacity to modulate the differentiation potential of mesenchymal stem/stromal cells (MSC). Here we investigated whether macrophages (Mf) with, respectively, pro-inflammatory and tissue-remodeling traits differentially modulate chondrogenesis of bone marrow derived-MSC (BM-MSC). We demonstrated that coculture in collagen scaffolds of BM-MSC with Mf derived from monocytes polarized with M-CSF (M-Mf), but not with GM-CSF (GM-Mf) resulted in significantly higher glycosaminoglycan (GAG) content than what would be expected from an equal number of BM-MSC alone (defined as chondro-induction). Moreover, type II collagen was expressed at significantly higher levels in BM-MSC/M-Mf as compared to BM-MSC/GM-Mf constructs, while type X collagen expression was unaffected. In order to understand the possible cellular mechanism accounting for chondro-induction, developing monoculture and coculture tissues were digested and the properties of the isolated BM-MSC analysed. We observed that as compared to monocultures, in coculture with M-Mf, BM-MSC decreased less markedly in number and exhibited higher clonogenic and chondrogenic capacity. Despite their chondro-inductive effect in vitro, M-Mf did not modulate the cartilage tissue maturation in subcutaneous pockets of nude mice, as evidenced by similar accumulation of type X collagen and calcified tissue. Our results demonstrate that coculture of BM-MSC with M-Mf results in synergistic cartilage tissue formation in vitro. Such effect seems to result from the survival of BM-MSC with high chondrogenic capacity. Studies in an orthotopic in vivo model are necessary to assess the clinical relevance of our findings in the context of cartilage repair.