Abstract 5074: Omentum immune microenvironment: Metastatic niche for ovarian cancer

The status of the tumor-immune microenvironment plays a critical role in determining the response to immunotherapy. The omentum, a multifunctional organ that serves as the central regulator of peritoneal homeostasis and plays a central role in immune surveillance, is the most common site of metastasis in early stage ovarian cancer. We analyzed the human omental immune microenvironment that support ovarian cancer metastases. In experimental models, ovarian cancer cells that colonize the omentum are invariably found in association with immune aggregates known as milky spots. We hypothesize that ovarian cancer cells exploit the unique environment of these omental-immune clusters to promote their own survival and growth. Thus, we formulated a systematic approach to evaluate normal and ovarian cancer-populated omental tissues to characterize the cellular components of human omental milky spots. We performed immunohistochemistry and flow cytometry to profile immune cell populations present in omental immune clusters. To facilitate the quantitation of immunohistochemical staining, we developed a computational image processing workflow that enables quantitation of multiple immune-biomarkers from serial sections of formalin-fixed paraffin-embedded tissue. In a complementary approach, we used comprehensive multi-color flow cytometry staining panels detecting 28 markers that collectively encompass all innate and adaptive immune cell populations. We analyzed eight human omental samples (four omenta involved with serous carcinomas and four omenta from benign disease) by immunohistochemistry and found that M2-subtype macrophage (CD163+CD68+) infiltration predominate in high-grade cancer-involved omentum. Conversely, T-regulatory cells (CD25+) predominate cell type in the uninvolved omentum. Hi-D flow cytometry of three cancer-involved omentum samples revealed tumor-associated macrophages (20-40%), PMN myeloid-derived suppressor cells (2-3%), “double positive” CD3+CD4+CD8+ T-cells (3-4%), and “double negative” CD3+CD4-CD8- T-cells (0.9-1%). In addition, we found omental T-cells (Th, Tc, Treg) express immune checkpoint receptors CTLA4 and PD-1, and that omental B-cells express IgD, IgA, and/or IgM, and may be enriched for Ig lambda light chain expression. As this study is ongoing, we expect to present data for omenta from a larger cohort of patients with either benign or neoplastic disease. However, our findings are already sufficient to show that activated T cells, B cells and macrophages infiltrate omental milky spots in patients with high grade serous ovarian cancers. Citation Format: Venkatesh Krishnan, Paul A. Raju, Koah Vierkoetter, Sonia Patel, Justin Youngyunpipatkul, Supreeti Tallapragada, Bruce Schaar, Ann K. Folkins, Leonore A. Herzenberg, Oliver Dorigo. Omentum immune microenvironment: Metastatic niche for ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5074.