SPP1⁺ macrophage-driven interactions shape the tumor microenvironment in lymph node metastatic acral melanoma

Melanoma is a highly aggressive skin cancer with biologically distinct subtypes. Acral melanoma (AM), a rare but particularly aggressive form, often presents with lymph node (LN) metastasis at diagnosis. Despite its clinical severity, the mechanisms underlying its metastatic behavior remain unclear. Emerging studies suggest the tumor microenvironment as a key driver of metastatic niche formation, but its specific role in AM progression is not well characterized. To investigate the role of the tumor microenvironment in AM progression, we performed single-cell RNA sequencing (scRNA-seq) on tumor tissues and matched adjacent normal samples from treatment-naïve AM patients, comparing cases with (LN⁺) and without (LN^-) lymph node metastasis. Key transcriptomic findings were validated by immunofluorescence staining. Functional relevance was tested by conducting in vitro and in vivo assays. An independent validation cohort was employed to confirm key observations and evaluate prognostic associations. Our results reveal preferential SPP1⁺ signaling pathways, including autocrine amplification within secreted phosphoprotein (SPP) 1⁺ macrophages and their interactions with S100A8⁺ melanoma cells via the SPP1-CD44 axis. S100A8⁺ melanoma cells emerged as the predominant malignant subpopulation in LN metastatic tumors (56.3% versus 34.7% in non-metastatic cases). Clinically, elevated SPP1 expression emerged as an independent predictor of poor overall survival. In vivo, anti-SPP1 therapy induced a macrophage phenotype switch and significantly reduced tumor burden. Together, these findings indicate that LN⁺ AM is characterized by an SPP1⁺ macrophage-driven immunosuppressive microenvironment and highlight the SPP1-CD44 axis as a promising therapeutic target for limiting AM dissemination.