P49: The Immune and Transcriptomic Landscape of Appendix Cancer Peritoneal Disease

Improved understanding of the pathobiology of appendiceal mucinous neoplasms (AMN) is required for development of novel treatments. While the appendix is an immune-rich tissue, the tumor microenvironment in AMN is incompletely characterized. We therefore utilized spatial delineation of stroma from epithelium to molecularly profile each compartment.

Methods:

Cores of patient-matched adjacent normal appendix, primary appendix tumor, and peritoneal metastases tissue were included in a tissue microarray and subjected to GeoMx digital spatial profiling (Nanostring) using pancytokeratin (PCK) positivity to delineate stroma from epithelium. RNAseq was performed on photocleaved barcodes from PCK +/- areas to measure transcript abundance separately within the two compartments.

Results:

We identified 14 patients with matched normal, primary AMN, and peritoneal metastases that had suitable archival paraffin-embedded blocks at our institution. Ten tumors were low-grade and 4 high-grade. Differential expression and pathway analyses illustrated large-scale differences between stromal and epithelial cells in all tissue types.

RNAseq immune deconvolution demonstrated a significant increase in M1 macrophage signatures and in the M1:M2 macrophage ratio in tumor/metastatic stroma. A decrease in B-cell signatures was observed in stroma of tumor tissues (vs normal) and this decrease was mostly observed in patients with low peritoneal cancer incidence (PCI) score (Fig. 1). Conversely, Treg signatures were increased in low-PCI tumor/metastasis stroma compared to normal stroma and these changes are reflected in increased expression of inflammatory markers such as TGFB.

Ribosomal biogenesis pathways were upregulated in high-grade tumors relative to low grade tumors (p< 0.001). Peritoneal metastases exhibited upregulated cytoskeletal and collagen metabolism pathways compared to primary tumor. For example, COL1A1 has a log2 fold-change of 2.9 (p=9.00E-05).

Conclusions:

Taken together, our data indicate that inflammatory macrophages are enriched in AMN tumor/metastasis-adjacent stroma and B-cell signatures are decreased in AMN tumor compared to normal tissue. In addition, appendiceal peritoneal disease is transcriptomically distinct from the primary tumor, potentially secondary to changes in the stromal compartment of the microenvironment in the metastatic setting. Since current treatment options for AMN are similar to colorectal cancer, these findings may lead to better targeted therapeutics specific to AMN biology.