Bone microenvironment-suppressed T cells increase osteoclast formation and breast cancer bone metastases

Abstract

Bone metastases is a frequent (>75%) complication of advanced breast cancer (BCa). BCa cells in bone are supported by cytokines released during the osteoclastic resorption, and remain untreatable. T cells during immunotherapy could turn against BCa cells in bone but could also increase osteoclasts and bone metastases. Comparing the development of bone metastases from 4T1 BCa cells between Balb/C and SCID or T cell-depleted mice, the absence of T cells decreased bone metastases (−72% and −38%, respectively), while orthotopic tumors were increased. Histology confirmed that the osteoclast number was decreased at the tumor-bone interface of mice lacking T cells. Ex vivo addition of non-activated T cells increased osteoclast formation, and flow cytometry confirmed that >85% of T cells in bone metastases were not expressing activation markers. In sharp contrast, activated T cells were potent inhibitors of osteoclast formation, which could be beneficial for the treatment of patients. Although activated T cells expressed the anti-osteoclastic Ifng and Il4, their effect was not reversed by neutralizing antibodies, or due to a bystander effect by cytotoxic T cells. Thus, we sought to activate T cells of bone metastases. However, it was not possible to activate T cells in bone marrow cultures ex vivo. This could be due to an increase of MDSCs in 4T1 bone metastases, including M-MDSCs that were PD-L1+ (>85%) and could suppress PD-1+ T cells (>70%) in bone. Our results suggest that while T cells under the influence of the bone microenvironment promote BCa bone metastases, immunotherapy-activated T cells could suppress bone resorption and eliminate BCa cells in patients with bone metastases.