Editorial: Innate Immunity in the Context of Osteoimmunology

The term “osteoimmunology” identifies the research field aimed at studying the crosstalk between cells of the skeletal and immune systems (1). The close relationship between these two systems is apparent based on the sharing of the same microenvironment (2), but it also extends beyond this through a network of signaling pathways and molecules acting in the pathophysiology of bone and immune cells (3). A large proportion of research in osteoimmunology has long focused on the effects elicited by adaptive immunity on bone, with rheumatoid arthritis as a prototypical disease condition (4). Only recently has the innate arm of the immune system received increasing attention in this framework (5). Indeed, osteonal macrophages, mast cells, and dendritic cells, in particular, have emerged as active players in skeletal remodeling and repair and in inflammation-induced bone loss (6–8). In parallel, novel concepts have been proposed regarding the capacity of bone cells to regulate immunity, suggesting, for example, the classification of osteoclasts as professional antigenpresenting cells and inflammatory osteoclasts as a different population compared to homeostatic osteoclasts (9, 10). Hereby, the overall picture on the bone-immune system interplay gained additional complexity. This collection of articles reflects this topic and focusses on osteoimmunology with regard to innate immune cells/bone cells crosstalk.