Abstract 2261: Deciphering cross-resistance mechanisms associated to targeted therapies and immunotherapy in BRAFmutmelanoma: The role of tumor-associated macrophages and galectin-1

Both targeted therapies (TT) and immunotherapies have revolutionized the treatment of metastatic melanoma patients. Particularly, TT with BRAFi or BRAFi/MEKi leads to prompt but not durable responses in most patients due to the development of resistance within 1 year. Tumors refractory to TT derive little benefit from subsequent immunotherapy. Here we aim to elucidate how previous treatments with TT shape tumor-immune landscape and how it could be overcome therapeutically. Firstly, we analyzed the gene expression profile of a single-cell RNA-Seq dataset of 48 biopsies (16,291 cells) from melanoma patients before and during treatment with anti-PD1/anti-CTLA4. Our single-cell data analysis showed that macrophages and CD8+ T-cells of non-responders to immunotherapy upregulate a specific glycosylation-related signature unraveling a novel potential mechanism of resistance. Interestingly, we found a new subset of macrophages associated with resistance that arises during treatment. Macrophages of non-responders express high levels of galectin-1 (Gal-1), an immunosuppressive lectin implicated in tumor progression and immune escape, and display an M2-like phenotype regulated by the activation of specific transcription factors and signaling pathways including TGF-b, hypoxia, and VEGF. This observation was validated on bulk RNA-Seq datasets, where we observed that melanoma patients refractory to immunotherapy have a higher M2-macrophage infiltrate at baseline by MIXTURE (p-value