Interaction between bone and immune cells: Implications for postmenopausal osteoporosis

Postmenopausal osteoporosis is a systemic disease characterized by the loss of bone mass and increased bone fracture risk largely resulting from significantly reduced levels of the hormone estrogen after menopause. Besides the direct negative effects of estrogen-deficiency on bone, indirect effects of altered immune status in postmenopausal women might contribute to ongoing bone destruction, as postmenopausal women often display a chronic low-grade inflammatory phenotype with altered cytokine expression and immune cell profile. In this context, it was previously shown that various immune cells interact with osteoblasts and osteoclasts either via direct cell-cell contact, or more likely via paracrine mechanisms. For example, specific subtypes of T lymphocytes express TNFα, which was shown to increase osteoblast apoptosis and to indirectly stimulate osteoclastogenesis via B cell-produced receptor-activator of NF-κB ligand (RANKL), thereby triggering bone loss during postmenopausal osteoporosis. T_h17 cells release interleukin-17 (IL-17), which directs mesenchymal stem cell differentiation towards the osteogenic lineage, but also indirectly increases osteoclast differentiation. B lymphocytes are a major regulator of osteoclast formation via granulocyte colony-stimulating factor secretion and the RANKL/osteoprotegerin system under estrogen-deficient conditions. Macrophages might act differently on bone cells dependent on their polarization profile and their secreted paracrine factors, which might have implications for the development of postmenopausal osteoporosis, because macrophage polarization is altered during disease progression. Likewise, neutrophils play an important role during bone homeostasis, but their over-activation under estrogen-deficient conditions contributes to osteoblast apoptosis via the release of reactive oxygen species and increased osteoclastogenesis via RANKL signaling. Furthermore, mast cells might be involved in the development of postmenopausal osteoporosis, because they store high levels of osteoclastic mediators, including IL-6 and RANKL, in their granules and their numbers are greatly increased in osteoporotic bone. Additionally, bone fracture healing is altered under estrogen-deficient conditions with the increased presence of pro-inflammatory cytokines, including IL-6 and Midkine, which might contribute to healing disturbances. Consequently, in addition to the direct negative influence of estrogen-deficiency on bone, immune cell alterations contribute to the pathogenesis of postmenopausal osteoporosis.