Deoxygenated hydroxyapatite inhibits macrophage inflammation through fibronectin restricted adsorption

Macrophages can determine the ultimate outcome of the foreign body reaction (FBR). Although researchers confirmed that differences in the elemental composition of the implant interface can lead to varying levels of biological function, the mechanism underlying the polarization directions of macrophages induced by varying oxygen proportions remains unclear. This research presented the fabrication of a deoxygenated hydroxyapatite (dHAP) surface to investigate the impact of oxygen content on macrophage activation. The dHAP surface exhibited a pronounced inhibitory effect on the inflammatory activation of macrophages when compared to the HAP surface. Results from total internal reflection microscopy (TIRFM) and molecular dynamic (MD) simulation have revealed that the significant extracellular matrix adhesion protein, Fibronectin (Fn), showed a lower level of adsorption on dHAP surfaces. The Arg-Gly-Asp (RGD) structural domain showed a reduction in the exposure. The diminished adhesion capacity and impaired active site recognition ability of Fn resulted in lower activation of the integrin-focal adhesion kinase (FAK) pathway of macrophages on the dHAP surface, thereby suppressing the inflammation. In summary, this work explains the mechanism of the FBR impacted by the proportion of oxygen at the protein level. It also introduces a new approach to enhance the compatibility of biomaterials. STATEMENT OF SIGNIFICANCE: Macrophages are key in the foreign body response (FBR). Researches indicate that implant material's elemental interface content can regulate the functionality of biomaterials, but the mechanism of this regulation is unclear. To study the relationship between the elemental content at the interface and macrophages in the FBR, we prepared a deoxygenated hydroxyapatite (dHAP). Our results showed that the dHAP surface inhibited the adsorption behavior and changed the orientation of an ECM protein-fibronectin (Fn)-as well as the exposure of fewer active sites of the Arg-Gly-Asp (RGD) sequence, leading to less integrin activation. And then, the activation of the integrin- focal adhesion kinase (FAK) signaling pathway was reduced, leading to a greater activation of macrophages towards a pro-regenerative direction.