T cell suppression by the bone microenvironment increases bone metastases from breast cancer in mice

Abstract

Abstract Bone metastases are a frequent complication of breast cancer and cannot be cured. Immunotherapy could be used for their treatment. However, its efficiency could be limited by the ability of T cells to increase bone resorption that supports cancer cell proliferation and bone metastases. To characterize the effect of T cells on bone metastases, we used a syngeneic mouse model with 4T1 breast cancer cells. When inoculated in normal mice, 4T1 caused more osteolysis as measured on x-rays, compared to SCID and T cell-depleted mice (253% and 61% more, respectively). Histology confirmed that normal mice have an increased tumor burden and number of osteoclasts at the tumor/bone interface compared to SCID mice. T cells isolated from bone metastases suppressed osteoclast formation ex vivo, in contrast with in vivo data. However, this effect was due to the ex vivo activation of T cells, consistent with decreased levels of Rankl and increased expression of anti-osteoclastic Ifng and Il4 mRNA in activated T cells. In vivo, Ifng was not detected, and levels of pro-osteoclastic Rankl and Tnfa were higher in T cells from bone metastases, suggesting they are pro-osteoclastic, unlike activated T cells. Culturing bone metastasis T cells ex vivo with ConA or anti-CD3/CD28, failed to increase the expression of the activation marker CD69, confirming the presence of suppressive factors in this microenvironment. Accordingly, there was an increase of MDSCs, including monocytic MDSCs that are PD-L1+ (87%) in 4T1 bone metastases. They could suppress the activation of T cells that are PD-1+ (71%) in bone metastases. These results suggest that unactivated T cells increase bone metastases and their activation by immunotherapy could be used for the treatment of bone metastases.