Interactions between bone and immune systems: A focus on the role of inflammation in bone resorption and fracture healing

Functional interactions between the immune system and bone tissues are reflected in a number of cytokines, chemokines, hormones and other mediators regulating the functions of both bone and immune cells. Investigations of the mechanisms of those interactions have become important for the understanding of the pathogeneses of diseases like inflammatory arthritis, inflammatory bowel disease, periodontal disease and osteoporosis. This review first addresses the roles of the inflammatory mediators and mechanisms by which they cause inflammation-induced bone loss. In the second part of the review we stress the importance of proinflammatory mediators for normal fracture healing. Defective bone remodeling underlying different pathological processes may be caused by disturbed differentiation and function of either osteoclast or osteoblast lineage cells. Understanding of the mechanisms governing enhanced differentiation and activation of osteoclast progenitors in the inflammatory conditions on the one hand, and the role of inflammation in the recruitment and differentiation of multipotent progenitors into the skeletal lineage during the fracture healing on the other hand is a critical first step in developing interventions that modulate bone regeneration processes and in designing novel pharmacological strategies.