Opposing roles of IL-17 in bone turnover. IL-17 is produced by T cells (particularly memory T cells), and acts on a wide variety of target cells to trigger expression of inflammatory effectors. Most of these effectors have been shown to have an impact on bone metabolism. Those factors that promote osteoclastogenesis indirectly favor bone destruction. Conversely, chemotactic factors promote neutrophil recruitment and activation, which can exert both bone protective and bone destructive effects. G-CSF, granulocyte colony-stimulating factor; ICAM, intercellular adhesion molecule; IFN, interferon; IL, interleukin; LIX, LPS-inducible CXC chemokine; MCP, monocyte chemotactic protein; PGE2, prostaglandin E2; RANKL, receptor activator of nuclear factor-κB ligand; TNF, tumor necrosis factor.