Abstract 1514: Myeloid cell derived myeloperoxidase links cellular innate immunity to inhibition of NF-κB signaling in melanoma tumor cells

Myeloid-derived cells are considered the primary mediators of the cellular innate immune system where myeloperoxidase (MPO) is the major enzyme present in these cells. MPO is essential for fighting infections but little is known about the role of MPO in cancer. Using the murine melanoma cell line, B16F10, in an immune-competent orthotopic tumor model, tumor growth was compared in wild type (MPO+/+) and syngeneic MPO-deficient host (MPO-/-) mice. Survival studies demonstrated that B16F10 tumors grew slower in MPO+/+ animals (mean survival 27.4 days ± 4.5 days) compared to MPO-/- animals (mean survival 23.4 days ± 1.6 days; p ≤ 0.005). Using a specific and potent inhibitor of MPO, 4-aminobenzoic acid hydrazide (4ABAH), we pharmacologically mimicked the MPO-/- phenotype by continuously dosing MPO+/+ animals. B16F10 tumors grew faster in MPO+/+ animals treated with 4ABAH (mean survival 23.8 days ± 2.5 days; p ≤ 0.017). Utilizing intravital imaging with skinfold window chamber animal models, we evaluated in real-time the recruitment of MPO-expressing myeloid-derived cells during melanoma progression in MPO+/+ and MPO-/- animals. B16F10 tumor NF-κB signaling, and MPO-mediated activation from immune infiltrates were imaged simultaneously using a multi-spectral, multi-modal imaging strategy. Because NF-κB signaling is a central coordinator of the immune system and cancer development, the dynamics of NF-κB signaling in B16F10 tumor in vivo were assessed using a transcriptionally-activated NF-κB-promoter-driven Firefly luciferase reporter enabling real-time bioluminescent imaging and quantitative monitoring of NF-κB transcriptional activation. Mean NF-κB transcriptional activation within the tumor compartment in MPO-/- mice was 42.2% ± 1.6% greater compared to MPO+/+ animals. Intravital microscopy demonstrated heterogeneous activation levels of NF-κB within tumor cells in MPO+/+ animals. To explore whether the spatial heterogeneity was a consequence of myeloid-derived cell distribution within the microenvironment, myeloid-derived cells were labeled in vivo using an i.v. injection of fluorophore-labeled dextran and fluorophore-labeled αGr1 antibody. Remarkably, tumor cells in contact with myeloid cells in vivo demonstrated decreased NF-κB transcriptional activation compared to tumor cells not in contact with myeloid cells. These in vivo studies demonstrate that MPO-expressing myeloid-derived cells suppress NF-κB transcriptional activation within B16F10 tumors in a tight spatially-localized and proximity-dependent manner where MPO, expressed broadly by myeloid-derived cells, contributes to host protection and decreased tumor progression. We demonstrate that MPO-expressing myeloid-derived cells function as an anti-tumor component of the cellular innate immune response during early melanoma progression in a NF-κB-dependent manner. Citation Format: Tracy W. Liu, Seth T. Gammon, Ping Yang, David T. Fuentes, David Piwnica-Worms. Myeloid cell derived myeloperoxidase links cellular innate immunity to inhibition of NF-κB signaling in melanoma tumor cells [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 1514.