Abstract 1347: A bone repair external state program drives cancer cell dedifferentiation and immune evasion in melanoma

Abstract The spatial organization of immune and stromal cells in the tumor microenvironment is emerging as an important predictor of effective cancer immunity. To systematically probe how multicellular circuits within the melanoma microenvironment drive cancer and immune cell state, we profiled 27 fresh-frozen human melanoma sections with Slide-tags snRNA-seq from 16 patients spanning common metastatic sites. We discovered six recurrent immune hub types representing conserved multicellular structures across melanoma, including an anti-inflammatory niche composed of SPP1+ macrophages, and POSTN+ fibroblasts, and an immune effector hub with NK cells and tumor-specific CD8+ T cells. Using spatial TCR sequencing, we observed a striking enrichment of expanded CD8+ T cell clonotypes in immune effector hubs and an enrichment of singleton CD8+ T cell clonotypes in anti-inflammatory hubs. Further, we found CD8+ T cells in the anti-inflammatory hubs to be more exhausted and less stem-like. Next, we sought to understand how immune niches interact with the malignant compartment. We identified 18 high-quality malignant transcriptional programs spanning the conventional de-differentiation trajectory in melanoma. We revealed that neural crest-like program usage is strongly predicted by the neighborhood presence of SPP1+ macrophages and POSTN+ fibroblasts, but negatively predicted by immune effector hub cells. We find that malignant cells that are more melanocytic-like and less de-differentiated are more immune infiltrated and have less SPP1+ macrophages and POSTN+ fibroblasts in their neighborhoods. Hypothesizing that ligands from the anti-inflammatory niche are shaping tumor state, we perturbed melanoma cell lines with SPP1 and POSTN. We found that when we treat melanocytic-like cells with POSTN, the tumor cells downregulate melanocytic markers and shift towards a more neural crest-like state. Further, when we treat neural crest-like cells with SPP1, the cells become protected from IFNg and TNFa killing. Our integrated spatial and functional analysis delineates six conserved immune hub architectures in metastatic melanoma and maps the spatial trajectory of CD8+ T-cell activation, expansion, and exhaustion. We demonstrate that anti-inflammatory niches composed of SPP1+ macrophages and POSTN+ fibroblasts not only exclude effector T cells but also drive tumor de-differentiation and cytokine resistance. In sum, our results illustrate how specific microenvironmental niches influence CD8 T cell responses, malignant de-differentiation, and tumor cell immune evasion in melanoma. Citation Format: Jackson A. Weir, Alex Wong, Pauling Liu, DK Alakwe, Julianne Wu, Moshe Sade-Feldman, Nir Hacohen, Fei Chen. A bone repair external state program drives cancer cell dedifferentiation and immune evasion in melanoma abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1347.