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 Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 6810.