Recent studies have focussed on the role of tumour infiltrating macrophages (TIMs) in the osteolysis associated with bone metastasis. TIMs indirectly promote pathological bone resorption by releasing local factors that stimulate host osteoclast activity. TIMs alone can also directly resorb bone in low grade fashion. Moreover, in the presence of stromal cells and 1,25 dihydroxyvitamin D3, TIMs differentiate into osteoclast-like cells that are capable of extensive, high-grade lacunar bone resorption. These two patterns of TIM-mediated osteolysis could account for differences in the rate of pathological bone resorption after the metastasis is established, and point to a central role for TIMs in effecting tumor osteolysis.

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