Leptomeningeal anti-tumor immunity follows unique signaling principles

Metastasis to the cerebrospinal fluid (CSF)-filled leptomeninges, or leptomeningeal metastasis (LM), represents a fatal complication of cancer. Proteomic and transcriptomic analyses of human CSF reveal a substantial inflammatory infiltrate in LM. We find the solute and immune composition of CSF in the setting of LM changes dramatically, with notable enrichment in IFN-gamma signaling. To investigate the mechanistic relationships between immune cell signaling and cancer cells within the leptomeninges, we developed syngeneic lung, breast, and melanoma LM mouse models. Here we show that transgenic host mice, lacking IFN-gamma or its receptor, fail to control LM growth. Overexpression of Ifn-gamma through a targeted AAV system controls cancer cell growth independent of adaptive immunity. Instead, leptomeningeal IFN-gamma actively recruits and activates peripheral myeloid cells, generating a diverse spectrum of dendritic cell subsets. These migratory, CCR7+ dendritic cells orchestrate the influx, proliferation, and cytotoxic action of natural killer cells to control cancer cell growth in the leptomeninges. This work uncovers leptomeningeal-specific IFN-gamma signaling and suggests a novel immune-therapeutic approach against tumors within this space. ### Competing Interest Statement A.B. holds an unpaid position on the Scientific Advisory Board for Evren Scientific and is an inventor on the following patents: 62/258,044, 10/413,522, and 63/052,139. D.P. is on the scientific advisory board of Insitro. Other authors declare no conflict of interest.