Chemokines MCP-1 and RANTES are induced when authentic bone resorbing human osteoclasts differentiate from monocyte precursors in vitro. In addition, MCP-1 and RANTES can stimulate the differentiation of cells with the visual appearance of osteoclasts, being multinuclear and positive for tartrate resistance acid phosphatase (TRAP +). We show here that MIP1alpha is also potently induced by RANKL during human osteoclast differentiation and that this chemokine also induces the formation of TRAP + multinucleated cells in the absence of RANKL. MIP1alpha was able to overcome the potent inhibition of GM-CSF on osteoclast differentiation, permitting the cells to pass through to TRAP + multinuclear cells, however these were unable to form resorption pits. Chemokine receptors CCR2b and CCR4 were potently induced by RANKL (12.6- and 49-fold, P = 4.0 x 10(-7) and 4.0 x 10(-8), respectively), while CCR1 and CCR5 were not regulated. Chemokine treatment in the absence of RANKL also induced MCP-1, RANTES and MIP1alpha. Unexpectedly, treatment with MCP-1 in the absence of RANKL resulted in 458-fold induction of CCR4 (P = 1.0 x 10(-10)), while RANTES treatment resulted in twofold repression (P = 1.0 x 10(-4)). Since CCR2b and CCR4 are MCP-1 receptors, these data support the existence of an MCP-1 autocrine loop in human osteoclasts differentiated using RANKL. (c) 2005 Wiley-Liss, Inc.