TMIC-81. AN IMMUNOSUPPRESSIVE MACROPHAGE NICHE MAY DRIVE T CELL DYSFUNCTION AND IMMUNOTHERAPY RESISTANCE IN MELANOMA BRAIN METASTASES

Abstract Nearly half of metastatic melanoma patients develop brain metastases (MBM). Local therapies like surgery and radiotherapy provide limited survival benefit. While targeted therapies and immune checkpoint inhibitors (ICI) improved outcomes in patients with asymptomatic MBM, the prognosis is still grim for the majority of patients. To decipher resistance mechanism to immunotherapy in MBM, we integrated datasets from both single-cell RNAseq (scRNAseq) and spatial transcriptomics (Visium). Our scRNAseq analysis of 25 MBM patients (7 treated pre-surgery with ICI) showed that lymphoid cells increased by 8.1-fold post-ICI (p=0.006). Notably, 34% of these were CXCL13+CXCR6+ putative tumor-reactive T cells (TR-T), known for their critical role as tumor-killing effectors. This led us to investigate why, despite their prevalence, TR-T don’t translate to effective immunotherapy in MBM. Our scRNAseq interactome analysis revealed that ICI intensified IFN-II interactions between TR-T and interferon-stimulated macrophage/microglia (Mφ.ISG). However, it also boosted immunosuppressive interactions, like the TIGIT-NECTIN2 interaction, which increased 20 fold, potentially diminishing TR-T’s effectiveness. To overcome the lack of spatial context in scRNAseq, we applied spatial transcriptomics on 27 MBM sections, including 12 post-ICI treatments. Remarkably, TR-T were primarily found (63%) in proximity with Mφ.ISG cells—not with tumor cells. These TR-T-Mφ.ISG niches were 1.7 times more prevalent in ICI-treated samples; these niches are rich in chemokine receptors like CCR1 and immunosuppressive molecules like NECTIN2. This suggests that Mφ.ISG cells are chemotactically attracted to TR-T, forming a barrier that impedes TR-T’s tumor interaction and impairs their functionality. Our current efforts focus on pinpointing key pathways enriched within TR-T-Mφ.ISG niches, particularly in ICI-treated MBM. We are also assessing the clinical impact of these niches in relation to patient survival. Our goal is to elucidate how specific immunosuppressive subsets and molecules within the MBM microenvironment restrict TR-T effectiveness, with the ultimate objective of enhancing ICI efficacy for these patients.