Molecular mechanisms of interrod spacing-mediated osseointegration via modulating inflammatory response and osteogenic differentiation

Osseointegration is initiated by immune cell-derived inflammatory responses to the surfaces of implanted materials. However, the bio-molecular mechanisms by which nanotopographic features of implant surfaces immunomodulate osseointegration remain to be clarified. To this end, herein, three kinds of TiO₂ nanorod-like arrays on Ti with the interrod spacing of ∼0, 45 and 85 nm, termed as S0, S45 and S85, respectively, were hydrothermally grown, and their actions on the polarization of macrophages (MΦs) and MΦs-mediated contact osteogenesis of bone marrow-derived mesenchymal stromal cells (BMSCs) were explored, especially the involved signaling pathways. Compared to pure Ti (namely P-Ti), the nanorods-arrayed Ti activated FG-integrin α_Mβ₂-IKK-NF-κB signaling pathway in the adhered MΦs at initial 6 h while FN-integrin α₅β₁-PI3K-Akt1 pathway since 24 h, accelerating the transition of MΦs from pro-inflammatory (M1) to anti-inflammatory (M2) phenotype. With the cytokines secreted by M2 cells, the coatings activated the BMP2-TGFβ1-SMAD signaling pathway to enhance osteogenic differentiation of the recruited BMSCs in vitro. Also, the nanorods-coated Ti induced a proper immune microenvironment and accelerated bone apposition in rabbit tibia marrow cavities in vivo. Furthermore, owing to the favorable capacity of osteoimmunomodulation, the ∼85 nm spaced nanorods-arrayed coating exhibited satisfactorily in orchestrating osteogenic differentiation in vitro and in vivo. This work provides insights into nanotopographic features of implant surfaces immunomodulating contact osteogenesis.