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

## TL;DR

This study identifies a specific immune cell pattern in blood samples that may predict severe immune side effects in melanoma patients treated with anti-PD-1 therapy.

## Contribution

The study reveals a novel immune signature linked to immune-related adverse events in acral melanoma patients treated with anti-PD-1 therapy.

## Key findings

- AE patients showed a reconfigured CD8+ T cell compartment with expanded cytotoxic cells and reduced transitional cells.
- The transitional-to-cytotoxic CD8+ T cell ratio was significantly lower in AE patients.
- AE patients had elevated T cell proliferation and enriched cell-killing gene pathways.

## Abstract

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.

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.

Analysis of 54,793 high-quality cells revealed a profound reconfiguration of the CD8+ T cell compartment specifically in AE patients. This was characterized by an expansion of cytotoxic CD8+ T cells (enriched for GZMB, GNLY, NKG7) and a concurrent contraction of a transitional CD8+ T cell population marked by GZMK 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.

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.