The role of tumour-associated macrophages in bone metastasis

This overview addresses the recent research developments in the role of tumour-associated macrophages (TAM) in bone metastasis biology and management of breast and prostate cancer as well as in primary and lung metastatic osteosarcoma. Immunosuppressive M2-type TAMs have been shown to associate with poor prognosis. Throughout their life cycle, macrophages (Macs) can adapt to environmental cues and influence the surroundings by secreting different cytokines and enzymes crucial to matrix remodelling, infection fighting, immune regulation and/or inflammation. In general terms, there is a broad and complex spectrum of Mac polarization statuses from M1 (classically activated/inflammatory) to M2 (alternatively activated/wound healing/immune regulating) Macs. Often the activation status of TAMs resembles more the M2-type. Considering the physiological functions of M2 Macs, it is no surprise that TAMs appear to have a role in metastasis, participating in almost every step of the metastatic cascade, which we review and explore in selected bone tropic cancers.