ROLE OF CYTOKINES IN BONE TISSUE REMODELING IN NORM AND PATHOLOGY

Significant successes in the study of physiological and pathophysiological patterns of bone remodeling in recent years have highlighted immune factors important role in bone tissue pathology and significantly revised our ideas about postmenopausal osteoporosis development mechanisms. Advanced osteoimmunology and evidence of immune mechanisms key role in bone remodeling disorders gave us possibility for identification of osteoporosis as chronic immune-mediated disease. Moreover, instead of the term “Osteoporosis”, the term “Immunoporosis” was reasonably used. Bone tissue is constantly in state of continuous renewal (remodeling), which is balanced by formation and resorption processes and is achieved through the coordinated functioning of the three main bone cells types. Constant and active interaction between osteocytes, osteoblasts and osteoclasts is ensured by cytokines (RANKL, osteoprotegerin, macrophage colony-stimulating factor, vascular endothelial growth factor, etc) secretion. Moreover, predominantly, bone remodeling regulation is limited by Osteocyte-Osteoblast-Osteoclast system. With pathological changes in immune reactivity, which may be caused by deficiency of estrogen, vitamin D, calcium, inflammatory diseases, etc., various types of immunocompetent cells are activated. This is accompanied with increased RANKL production by leukocyte cells, which potentiates processes of maturation, differentiation of osteoclasts, and increase in their activity. In addition to RANKL secretion, activated leukocytes, including T lymphocytes, enhance other osteoclastogenic cytokines production. IL-1, IL-6, IL-17, TNF and TGF-β are main mediators of accelerated bone loss in postmenopausal women.