Single-cell analysis deciphered a pivotal role of SPP1-mediated interactions contributing to lymph node metastasis in acral melanoma

Summary: Background: Acral melanoma (AM), a highly aggressive cutaneous malignancy with frequent lymph node metastasis, remains poorly understood in terms of myeloid-driven immunosuppression and metastatic niche formation. Objectives:<b> </b>To delineate myeloid cell heterogeneity and SPP1-mediated interactions driving lymph node metastasis in AM. Methods: We performed single-cell RNA sequencing from tumor and adjacent normal tissues of 13 treatment-naïve AM patients (3 with lymph node metastasis, 6 without and 4 from adjacent normal tissues). Prognostic and experimental validation utilized an independent cohort of 51 AM patients data. Results:<b> </b>Through an unbiased analysis of 106 pathways, myeloid-derived SPP1 signaling emerged as predominant driver in LN⁺. SPP1-high myeloid subcluster was more abundant in LN⁺ and displayed elevated expression of FTL and CCL2. This subcluster promoted formation of self-sustaining niches and directed interactions with melanoma cells through SPP1-CD44 axis, maintaining DPEP3⁺/S100A8⁺ melanoma subsets. Moreover, comparative cellchat mapping highlighted SPP1 signaling shifting from melanocyte-driven interactions in LN^- to myeloid-dominated networks in LN⁺, engaging five distinct cellular subsets. Clinically, the SPP1 axis was associated with CD8⁺ LAG3⁺ T cell exhaustion and a significant reduction of CD8⁺ T cell infiltration in LN⁺. Spatial analysis further revealed mutually exclusive distribution patterns of SPP1⁺ myeloid cells and CD8⁺ T lymphocytes. In addition, high SPP1 expression independently predicted poorer 5-year survival. Conclusions: Our study uncovers an SPP1-driven immunosuppressive axis in AM metastasis, characterized by myeloid-melanoma cell cooperativity and CD8⁺ T cell dysfunction. Targeting SPP1 signaling may restore anti-tumor immunity and synergize with existing immunotherapies, providing a rationale for SPP1⁺myeloid-directed therapeutic strategies in advanced AM. Yuli Zheng², Rui Xu¹, Fang Wang³, Xiaolong Cao² 1. The First Affiliated Hospital of Sun Yat-sen University, Guangzhou, Guangdong, China. 2. Zhujiang Hospital of Southern Medical University, Guangzhou, Guangdong, China. 3. Southern Medical University, Guangzhou, Guangdong, China. Pigmentation, Melanoma, and Melanoma Immune Surveillance