Abstract 6810: Macrophage secreted CCL9/CCL5 induces CCR1-mediated ovarian cancer metastasis to the omentum in the absence of CCL6

Abstract The tumor-microenvironment (TME) of the omentum plays an important role in providing a pre-metastatic niche for high-grade serous ovarian cancer (HGSC-OvCa) progression. Omentum, a layer of fatty adipose tissue within the peritoneal cavity, is the preferred primary metastatic site for OvCa. Omentum is a well-vascularized fatty tissue containing wide-spread immune clusters (milky spots). Milky spots in the omentum are primarily composed of B cells, T cells, macrophages, NK cells, and dendritic cells. Previous studies from our laboratory have highlighted the importance of CCL6 secreted by omental macrophages as a key chemokine in providing a pre-metastatic niche by activating the CCR1 axis on the cancer cells required for OvCa invasion. We engineered and characterized a CCL6-depleted in vivo mouse model using CRISPR/Cas9 to study the effect on OvCa progression and macrophage biology. We demonstrate that deletion of CCL6 in mice alters characteristics of omental tissue-resident (OM-Macφ) and bone marrow-derived macrophages (BMDM) derived from circulating monocytes. RNA sequencing of OM-Macφ and microarray study of BMDM revealed that CCL6 deletion produces enhanced TGFβ1+CSF-1+CCL5+ expression in OM-Macφ and an increase in TGFβ1+CSF-1+CCL9+ in BMDM. This acts as a rescue mechanism to maintain activation of their shared CCR1 receptor with a pro-M2 stimulation profile. Deep multiplexed imaging of the omentum tissue by CODEX showed that CCL6 deletion alters the architecture of milky spots, with enriched macrophage content and reduced B-cell and T-cell-rich neighborhoods. While our previous studies indicated that CCL6 deletion could potentially have an anti-tumoral effect, we found that it does not significantly affect murine ID8 ovarian cancer tumor burden in both short-term (7 days) and long-term in vivo studies and provides no added survival advantage. We report that CCL9+F4/80+ macrophages accumulate in the omentum post-tumor infiltration in the CCL6-KO model, and neutralizing CCL9 in the CCL6-KO BMDM macrophage-derived conditioned media significantly reduces migration of ID8 cancer cells in vitro. Furthermore, we demonstrate that murine CCL9 and its analog, human CCL15, promote the epithelial-mesenchymal transition (EMT) of mouse ID8 and human A2780 OvCa cells. Moreover, our findings indicate that human CCL15 is a prognostic biomarker for reduced survival in OvCa patient cohorts (Stages I&II and Stage III&IV). Further classification of HGSC-OV patients (n=375) from TCGA-OV based on CCL15 and CCR1-high expression correlated with a higher expression of TGFβ1, CCL5, and a T-cell exhaustion signature in the TME. Taken together, given the apparent redundancy of the chemokine network controlling CCR1 signaling, our findings support the strategy of therapeutically blocking the CCR1 receptor to potentially produce a favorable anti-tumor response in HGSC-OvCa patients. Citation Format: Supreeti Tallapragada, Justine Chan, Venkatesh Krishnan, Oliver Dorigo. Macrophage secreted CCL9/CCL5 induces CCR1-mediated ovarian cancer metastasis to the omentum in the absence of CCL6 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6810.