Immune checkpoint therapy (ICT) shows encouraging results in a subset of patients with metastatic castration-resistant prostate cancer (mCRPC) but still elicits a sub-optimal response among those with bone metastases. Analysis of patients' bone marrow samples revealed increased Th17 instead of Th1 subsets after ICT. To further evaluate the different tumor microenvironments, we injected mice with prostate tumor cells either subcutaneously or intraosseously. ICT in the subcutaneous CRPC model significantly increases intra-tumoral Th1 subsets and improves survival. However, ICT fails to elicit an anti-tumor response in the bone CRPC model despite an increase in the intra-tumoral CD4 T cells, which are polarized to Th17 rather than Th1 lineage. Mechanistically, tumors in the bone promote osteoclast-mediated bone resorption that releases TGF-β, which restrains Th1 lineage development. Blocking TGF-β along with ICT increases Th1 subsets and promotes clonal expansion of CD8 T cells and subsequent regression of bone CRPC and improves survival.
Keywords:T(h)1 subset; TGF-β blockade; anti-CTLA-4; anti-PD-1; bone metastases; castration-resistant prostate cancer.