Commensal dysbiosis modulates the tumor microenvironment in breast cancer and diminishes efficacy of PD-L1 inhibition

Abstract

Abstract For patients with HR+ breast cancer, non-responsiveness to PD-1/PD-L1 blockade cannot be attributed to poor immunogenicity, as immune-stimulatory gene signatures are associated with greater long-term survival and enhanced responsiveness to therapy. Targeting myeloid-driven immune suppression has also been shown to enhance efficacy of PD-L1 blockade in non-responsive tumors. However, we find that in HR+ breast cancer, synergy with PD-L1 blockade is negatively influenced by a pre-established loss of commensal homeostasis, or commensal dysbiosis. Using a clinically relevant non-responsive HR+ breast tumor model, establishing commensal dysbiosis prior to breast tumor initiation drives significant and irreversible T cell dysfunction within primary tumors. Additionally, commensal dysbiosis alters the cellular composition within the tumor microenvironment, culminating in the recruitment and polarization of M2-like macrophages with distinct tumor-suppressive attributes. Systemically, tumor-bearing animals with established commensal dysbiosis have significantly higher levels of prostaglandin E2 with the serum, which is independent of tumor volume. However, blockade of COX-2 only synergizes with PD-L1 blockade in animals without commensal dysbiosis. Unexpectedly, we found that commensal dysbiosis enhances COX-1/PGE2 signaling within the mammary epithelium during early tumor progression. Inhibition of COX-1-mediated PGE2 synthesis during early stages of tumor progression resulted in delayed tumor progression only in dysbiotic tumor-bearing mice, suggesting that dysbiosis-induced COX-1 mediated tissue inflammation is sufficient to drive irreversible immune suppression during advanced breast cancer.