# Comprehensive tumour‐immune profiling reveals TREM2+ tumour‐associated macrophages facilitating lymph node metastasis in head and neck squamous cell carcinoma

## TL;DR

This study identifies TREM2+ tumor-associated macrophages as key drivers of lymph node metastasis in head and neck cancer by suppressing immune cells.

## Contribution

The study reveals a novel mechanism by which TREM2+ TAMs promote metastasis via the SPP1–CD44–BHLHE40 axis and CD8+ T cell exhaustion.

## Key findings

- TREM2+ TAMs promote CD8+ T cell exhaustion through the SPP1–CD44–BHLHE40 axis in HNSCC.
- CD8+ T cell exhaustion induced by TREM2+ TAMs facilitates tumor immune escape and lymph node metastasis.

## Abstract

Lymph node (LN) metastasis is a well‐established independent prognostic factor in head and neck squamous cell carcinoma (HNSCC). Formation of suppressive tumour immune microenvironment (TIME) is a major contributor to tumour immune evasion and metastasis. However, the TIME landscape underlying LN‐metastatic HNSCC remains poorly elucidated.

A total of 688 866 single‐cell transcriptomes across 212 HNSCC samples were integrated. Comprehensive bioinformatic analyses on single‐cell RNA sequencing and microarray datasets revealed a TREM2+ tumour‐associated macrophage (TAM) cluster associated with LN metastasis. The functional role of TREM2+ TAMs was investigated through multiplex immunohistochemistry (mIHC) staining in clinical HNSCC cohort and in vitro co‐culture experiments. Furthermore, machine learning algorithms were employed to construct a prognostic model for HNSCC.

Integrative single‐cell analysis revealed the immunosuppressive TIME of LN‐metastatic HNSCC, characterised by high infiltration of exhausted CD8+ T cells (CD8+ Tex). We identified a specific TREM2+ TAM cluster that was strongly associated with CD8+ Tex infiltration and LN metastasis. In vitro experiment confirmed that TREM2+ TAMs promoted CD8+ T cell exhaustion. Mechanistically, TREM2+ TAMs exhibited a terminally differentiated phenotype driven by ETV5, and secreted SPP1 to interact with CD44 on CD8+ T cells, thus upregulating BHLHE40 to promote CD8+ Tex formation. Clinically, a prognostic model based on TREM2+ TAM signature genes was trained to independently predict HNSCC outcomes.

This study delineates the mechanism that TREM2+ TAMs promote LN metastasis in HNSCC by facilitating CD8+ T cells exhaustion via SPP1–CD44–BHLHE40 axis, proposing TREM2+ TAMs as potential therapeutic target for HNSCC.

(1) TREM2+ TAMs promote CD8+ T cell exhaustion by upregulating BHLHE40 through the SPP1–CD44 axis, contributing to a suppressive tumour immune microenvironment in HNSCC. (2) CD8+ Tex induced by TREM2+ TAMs facilitates immune escape and promotes tumour cell metastasis via the lymphangion.