Adaptive immune response inhibits ectopic mature bone formation induced by BMSCs/BCP/plasma composite in immune-competent mice.

A combination of autologous bone marrow stromal cells (BMSCs) and biomaterials is a strategy largely developed in bone tissue engineering, and subcutaneous implantation in rodents or large animals is often a first step to evaluate the potential of new biomaterials. This study aimed at investigating the influence of the immune status of the recipient animal on BMSCs-induced bone formation. BMSCs prepared from C57BL/6 mice, composed of a mixture of mesenchymal stromal and monocytic cells, were combined with a biomaterial that consisted of biphasic calcium phosphate (BCP) particles and plasma clot. This composite was implanted subcutaneously either in syngenic C57BL/6 immune-competent mice or in T-lymphocyte-deficient Nude (Nude) mice. Using histology, immunohistochemistry, and histomorphometry, we show here that this BMSC/BCP/plasma clot composite implanted in Nude mice induces the formation of mature lamellar bone associated to hematopoietic areas and numerous vessels. Comparatively, implantation in C57BL/6 results in the formation of woven bone without hematopoietic tissue, a lower number of new vessels, and numerous multinucleated giant cells (MNGCs). In situ hybridization, which enabled to follow the fate of the BMSCs, revealed that BMSCs implanted in Nude mice survived longer than BMSCs implanted in C57BL/6 mice. Quantitative expression analysis of 280 genes in the implants indicated that the differences between C57BL/6 and Nude implants corresponded almost exclusively to genes related to the immune response. Gene expression profile in C57BL/6 implants was consistent with a mild chronic inflammation reaction characterized by Th1, Th2, and cytotoxic T-lymphocyte activation. In the implants retrieved from T-deficient Nude mice, Mmp14, Il6st, and Tgfbr3 genes were over-expressed, suggesting their putative role in bone regeneration and hematopoiesis. In conclusion, we show here that the T-mediated inflammatory microenvironment is detrimental to BMSCs-induced bone formation and shortens the survival of implanted cells. Conversely, the lack of T-lymphocyte reaction in T-deficient animals is beneficial to BMSCs-induced mature bone formation. This should be taken into account when evaluating cell/biomaterial composites in these models.