M-CSF–driven TAM remodelling and actomyosin reactivation underlie BRAFi resistance in melanoma

Abstract Background Resistance to BRAF inhibitors remains a major challenge in melanoma, limiting long-term clinical benefit. Increasing evidence suggests that interactions between tumour cells and the tumour microenvironment, particularly immune components, contribute to therapeutic failure. This study aimed to investigate how BRAF inhibitor-resistant melanoma cells remodel the immune microenvironment and to identify actionable mechanisms driving resistance. Methods BRAF-mutant melanoma cell lines with acquired resistance were analysed using cytokine profiling, gene expression sequencing, and functional assays. Human monocytes were differentiated into macrophages using conditioned media from melanoma cells to assess immune reprogramming. In vivo studies were performed using syngeneic mouse models to evaluate tumour growth, metastasis, and immune infiltration. Patient tumour samples and public datasets were analysed to assess clinical relevance. Statistical analyses included normality testing, one-way and two-way analysis of variance with post-hoc correction, non-parametric tests, correlation analyses, and Kaplan–Meier survival analysis with log-rank testing. Results We revealed that resistant melanoma cells orchestrate immune evasion through coordinated tumour-intrinsic and microenvironmental reprogramming. BRAFi-resistant cells reactivate actomyosin contractility and adopt an altered secretory phenotype characterized by increased M-CSF together with additional soluble mediators. This remodelled secretome promotes monocyte differentiation toward immunosuppressive tumour-associated macrophages, with M-CSF acting as a contributor to this process. This drives formation of a therapy-refractory microenvironment that is enriched in CD206+ macrophages, regulatory T cells, and checkpoint molecules. In the clinical cohort, higher CD206-positive macrophage infiltration was associated with poorer survival. Public-cohort analyses showed a shift towards M2 macrophage enrichment at progression on BRAF inhibition, whereas macrophage measures did not distinguish anti-PD-1 responders from non-responders. Mechanistically, M-CSF blockade reprogrammed the immune landscape, reduced immunosuppressive macrophage infiltration and enhanced BRAFi efficacy in vivo. Conclusions These findings establish M-CSF-driven immune remodelling as a hallmark of therapeutic resistance, and position M-CSF/CSF1R as a rational target to prevent or overcome resistance in BRAF-mutant melanoma.