Single-cell profiling of PBMCS reveals an immune signature of irAEs in anti-PD-1-treated acral melanoma patients

Introduction: Immune checkpoint inhibitors (ICIs) targeting PD-1 have revolutionized melanoma treatment, yet their clinical efficacy is frequently limited by immune-related adverse events (irAEs). The underlying mechanisms of irAEs remain poorly defined, particularly in the acral melanoma subtype. Methods: To identify peripheral immune signatures associated with irAE development, we performed single-cell RNA sequencing (scRNA-seq) on peripheral blood mononuclear cells (PBMCs) from eight acral melanoma patients: three who developed irAEs on anti-PD-1 therapy (AE), three treated patients without irAEs (NAE), and two untreated controls (UNT). Cellular composition, transcriptional profiles, and differentiation trajectories were analyzed. Results: expression. Consequently, the ratio of transitional to cytotoxic CD8+ T cells was decreased in the AE group. Pseudotime trajectory analysis confirmed that GZMK+ transitional cells represent an intermediate differentiation state between naïve and terminal cytotoxic phenotypes. Additionally, AE patients exhibited an elevated proportion of proliferating T cells and enrichment of cell-killing gene pathways. Discussion: Our findings propose a model wherein an imbalance in CD8+ T cell differentiation, favoring aggressive cytotoxic effectors over a putative buffering transitional population, underpins irAEs pathogenesis in acral melanoma patients receiving anti-PD-1 therapy. The transitional-to-cytotoxic CD8+ T cell ratio emerges as an exploratory candidate biomarker for irAEs risk, warranting validation in larger prospective cohorts.