Bone metastatic prostate cancer (BM-PCa) significantly reduces overall patient survival and is currently incurable. Current standard immune therapies have shown promising results for patients with less advanced disease (fewer than 20 bone lesions) suggesting that PCa growth in bone contributes to immunotherapeutic response. Gaining a better understanding of the immunogenicity of BM-PCa may present a new therapeutic approach for treating mCRPC. In bone, prostate cancer cells hijack the behavior of bone resident cells, inducing excessive bone degradation and release of bone-sequestered growth factors, such as transforming growth factor beta (TGFβ), that promote a “vicious cycle” of tumor progression. Findings from my lab revealed that: 1) BM-PCa stimulates bone stromal expression of Interleukin 8 (IL-8), a potent chemoattractant for polymorphonuclear neutrophils (PMNs), innate immune cells generated in bone marrow, and 2) PMNs heavily infiltrate the prostate tumor-bone microenvironment. Recent studies demonstrated the existence of two distinct PMN populations: anti-tumoral (N1) and immunosuppressive pro-tumoral (N2) PMNs, with emergence of the latter regulated by TGFβ. RealTime qPCR revealed that bone metastatic C42B prostate cancer significantly increases neutrophil expression of TGFβ receptor and MMP9, a trend previously identified as a marker of pro-tumoral neutrophils. Based upon these findings, we hypothesized that bone metastatic prostate cancer skews neutrophils to a pro-tumoral phenotype in a TGFβ-dependent manner. To test this, bone marrow-derived primary neutrophils were treated with conditioned media from metastatic metastatic prostate cancer cells (C42B) and poorly metastatic LNCaP and gene expression and oxidative burst were measured. Similar to previous findings, C42B stimulated reactive oxygen species (ROS) production by PMNs, a mechanism of immunosuppression utilized by N2 PMNs, and induced anti-inflammatory gene expression. In contrast, LNCaP cells failed to stimulate this response. C42B media promoted PMN expansion, a phenomenon that is reversed by TGFβ-blockade. Surprisingly, direct contact with neutrophils significantly increased luminescence of luciferase-expressing C42B cells independently of cell growth, when compared to LNCaP, demonstrating that neutrophils activate alternate protein signaling in metastatic cells compared to non-metastatic. In vivo intratibial models utilizing antibody-mediated neutrophil depletion validated our hypothesis that neutrophils are critical mediators of prostate cancer growth in bone. These results demonstrate the significant role of neutrophils in the progression of bone metastatic prostate cancer.

Citation Format: Leah M. Cook, Diane Costanzo-Garvey, Tyler Keeley. Bone-resident neutrophils are mediators of prostate cancer growth in bone [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1087.