BSLD-03 MIMICKING CAFS, PIAL CELLS SUPPORTLEPTOMENINGEAL METASTASIS

Leptomeningeal metastasis (LM), the spread of cancer into cerebrospinal fluid (CSF)-filled coverings of the brain and spinal cord, is a dismal complication of cancer leading to rapid neurologic disability and death. While a diverse array of primary tumors may result in LM, recent work from our laboratory (Chi Y et al 2020 Science) demonstrates a conserved transcriptional signature, independent of primary tumor, suggesting that tumor-microenvironmental interactions play a dominant role in this pathology. We have proteomically interrogated CSF collected from breast (n = 45), lung (n = 30), and melanoma (n = 27) patients with and without LM. We detect robust expression of inflammatory cytokines in all LM+ samples, 15 of these cytokines including IL8, IL6, and CXCL1 are conserved across tumor types in the presence of LM. Because cancer cells in the CSF do not express mRNA associated with these cytokines, we hypothesized that these inflammatory cytokines may originate from the microenvironment. Within the leptomeninges, LM cells adhere to and interact with the innermost layer of the meninges, the pia mater, which contains fibroblast-like cells. Co-culture of cancer cells with pial cells induced changes in the pial cell transcriptome and secretome, leading to increased IL8, IL6, and CXCL1 production. This communication between cancer cells and pial cells is preserved in the absence of direct cell-cell contact. These pial cells support cancer cell growth in vivo, as measured by co-implantation experiments. In addition, bulk RNA sequencing of mouse pial cells shows an inflammatory signature upregulated in the presence of LM. Taken together, these observations support a cancer-associated-fibroblast like role for pial cells in the setting of LM. Our work extends the CAF phenotype beyond classical fibroblasts to include pial cells, suggesting novel therapeutic opportunities within CNS malignancies.