Eosinophil Chemotactic Factor-L (ECF-L): A Novel Osteoclast Stimulating Factor

Screening a cDNA library enriched for genes expressed in OCLs identitied ECF-L. ECF-L enhanced OCL formation without increasing RANKL levels. Anti-ECF-L inhibited RANKL-induced OCL formation. These results support a potent role of ECF-L in osteoclastogenesis. Introduction: To investigate the molecular mechanisms that control osteoclastogenesis, we developed an immortalized osteoclast (OCL) precursor cell line that forms mature OCLs in the absence of stromal cells and used it to form pure populations of OCLs. Materials and Methods: Polymerase chain reaction (PCR) selective cDNA subtraction was used to identify genes that are highly expressed in mature OCLs compared with OCL precursors employing OCL and OCL precursors derived from this cell line. Results: Eosinophil chemotactic factor-L (ECF-L), a previously described chemotactic factor for eosinophils, was one of the genes identified. Conditioned media from 293 cells transfected with mECF-L cDNA, or purified ECF-L Fc protein, increased OCL formation in a dose-dependent manner in mouse bone marrow cultures treated with 10^-10 M 1,25(OH)₂D ₃. OCLs derived from marrow cultures treated with ECF-L conditioned media formed increased pit numbers and resorption area per dentin slice compared with OCLs induced by 1,25(OH)₂D₃ (p < 0.01). Addition of an antisense S-oligonucleotide to mECF-L inhibited OCL formation in murine bone marrow cultures treated only with 10^-9 M 1,25(OH) ₂D₃ compared with the sense S-oligonucleotide control. Time course studies demonstrated that ECF-L acted at the later stages of OCL formation, and chemotactic assays showed that mECF-L increased migration of OCL precursors. mECF-L mRNA was detectable in mononuclear and multinucleated cells by in situ hybridization. Interestingly, a neutralizing antibody to ECF-L blocked RANKL or 10^-9 M 1,25(OH)₂D₃-induced OCL formation in mouse bone marrow cultures, although ECF-L did not induce RANKL expression. Conclusions: These data show ECF-L is a previously unknown factor that is a potent mediator of OCL formation, which acts at the later stages of OCL formation and enhances the effects of RANKL.