Background: Interleukin (IL)-11, a cytokine produced by breast cancer, has been implicated in breast cancer-induced osteolysis (bone destruction) but the mechanism(s) of action remain controversial. Some studies show that IL-11 is able to promote osteoclast formation independent of the receptor activator of NF-κB ligand (RANKL), while others demonstrate IL-11 can induce osteoclast formation by inducing osteoblasts to secrete RANKL. This work aims to further investigate the role of IL-11 in metastasis-induced osteolysis by addressing a new hypothesis that IL-11 exerts effects on osteoclast progenitor cells.Methods: To address the precise role of breast cancer-derived IL-11 in osteoclastogenesis, we determined the effect of breast cancer conditioned media on osteoclast progenitor cells with or without an IL-11 neutralizing antibody. We next investigated whether recombinant IL-11 exerts effects on osteoclast progenitor cells and survival of mature osteoclasts. Finally, we examined the ability of IL-11 to mediate osteoclast formation in tissue culture dishes and on bone slices in the absence of RANKL, with suboptimal levels of RANKL, or from RANKL-pretreated murine bone marrow macrophages (BMMs).Results: We found that freshly isolated murine bone marrow cells cultured in the presence of breast cancer conditioned media for 6 days gave rise to a population of cells which were able to form osteoclasts upon treatment with RANKL and M-CSF. Moreover, a neutralizing anti-IL-11 antibody significantly inhibited the ability of breast cancer conditioned media to promote the development and/or survival of osteoclast progenitor cells. Similarly, recombinant IL-11 was able to sustain a population of osteoclast progenitor cells. However, IL-11 was unable to exert any effect on osteoclast survival, induce osteoclastogenesis independent of RANKL, or promote osteoclastogenesis in suboptimal RANKL conditions.Conclusions: Our data indicate that a) IL-11 plays an important role in osteoclastogenesis by stimulating the development and/or survival of osteoclast progenitor cells and b) breast cancer may promote osteolysis in part by increasing the pool of osteoclast progenitor cells via tumor cell-derived IL-11. However, given the heterogeneous nature of the bone marrow cells, the precise mechanism by which IL-11 treatment gives rise to a population of osteoclast progenitor cells warrants further investigation. © 2013 McCoy et al.; licensee BioMed Central Ltd.