Modulation of the Tumour Microenvironment by HER2 in Oesophagogastric Adenocarcinoma: Implications for Tumour Progression, Therapeutic Resistance, and Clinicopathological Outcomes

HER2 (human epidermal growth factor receptor 2) is a receptor tyrosine kinase which is overexpressed in ~20% of patients with oesophagogastric adenocarcinoma (EGA). HER2 represents a targetable transmembrane glycoprotein receptor of the epidermal growth factor receptor (EGFR) family, which plays a crucial role in cell proliferation, survival, and differentiation. HER2 significantly influences the tumour microenvironment (TME) through various mechanisms, creating a niche that supports tumour progression, immune evasion, and therapeutic resistance. In HER2-positive EGA, aberrant signalling pathways, such as PI3K/AKT and MAPK/ERK, enhance tumour cell survival and proliferation, whilst upregulation of angiogenic factors like VEGF fosters vascularization, meeting a tumour's metabolic demands and facilitating its proliferation. HER2 also modulates the tumour immune microenvironment (TIME) by downregulating MHC molecules and recruiting immunosuppressive cells, including regulatory T-cells (T-reg) and tumour-associated macrophages (TAMs), which release cytokines that further inhibit anti-tumour immune responses. Together, these factors foster a pro-inflammatory, immunosuppressive microenvironment that underpins resistance to HER2-targeted therapies. As more HER2-directed treatments become available, such as trastuzumab-deruxtecan (T-DXd), gaining a deeper understanding of the multifaceted influence of HER2 on the TME in EGA will be crucial for the development of improved targeted treatments that can overcome these challenges and lead to advancements in targeted treatment for HER2-overexpressing EGA. This review provides a comprehensive overview of the impact of HER2 on the TME in EGA and highlights the challenge it represents as well as the opportunity for novel therapeutic development and the implications for patients in terms of clinicopathological outcomes.