Contribution of the immune bone marrow microenvironment to tumor growth and bone deconstruction: implications for improving immunotherapeutic strategies in bone metastasis

Bone metastases are frequent complications of many solid tumors, leading to painful skeletal morbidities and increasing mortality for patients with advanced cancer. Once in bone, cancer cells deregulate bone homeostasis, altering the functions of bone-forming (osteoblasts) and bone-resorbing (osteoclasts) cells, which results in skeletal deconstruction. Aside from bone cells, cancer cells in the bone marrow interact with other cell populations, including immune cells that also play an integral part in the regulation of bone homeostasis. In this respect, immune checkpoint inhibitors (ICIs) have become a standard of care in immunotherapy for the treatment of patients with advanced cancer. Strikingly, however, those with bone metastases have a shorter survival when treated with ICIs than ICI-treated cancer patients without bone metastases. In this Review, after presenting the immune cells involved in bone metastasis, we review preclinical and clinical findings assessing ICI efficacy both in bone and extraosseous metastases, and we discuss the clinical utility of using bone-targeted agents —including denosumab and bisphosphonates— to improve anti-tumoral efficacy of ICI treatments in patients with cancer and bone metastases.