Impact of immune microenvironment on immune checkpoint inhibitor response in HER2-overexpressing urothelial carcinoma.

[BACKGROUND] The biological interaction between human epidermal growth factor receptor 2 (HER2) overexpression and the tumor immune microenvironment (TIME) in urothelial carcinoma (UC) has not been comprehensively elucidated, which limits the rational integration of HER2-targeted therapies with Immune checkpoint Inhibitors (ICIs). This study aimed to characterize the immune landscape associated with HER2 expression and evaluate its clinical implications.

[METHODS] Using transcriptomic datasets from The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO), patients were stratified based on an exploratory threshold, with the highest 20% of ERBB2 mRNA expression defined as HER2-high. Integrated bioinformatic analyses were validated via immunohistochemistry (IHC) in 21 UC specimens. Additionally, a real-world cohort of 29 patients with locally advanced or metastatic UC (la/mUC) treated with disitamab vedotin (RC48), as monotherapy or combined with PD-1 inhibitors, was evaluated for clinical efficacy.

[RESULTS] In this exploratory analysis, HER2-high tumors were associated with an immunologically suppressed phenotype, characterized by reduced immune/stromal infiltration and increased tumor purity. Notably, these tumors exhibited a selective enrichment of FOXP3 + regulatory T cells (Tregs) (P < 0.001) alongside significantly lower expression of canonical immune checkpoints, including CTLA-4, PD-1, and PD-L2 (all P < 0.001). These findings suggest a distinct immune regulatory pattern characterized by regulatory cell enrichment and relatively low expression of classical immune checkpoint molecules.

[CONCLUSIONS] HER2 expression in urothelial carcinoma was associated with a distinct immune microenvironmental profile characterized by Treg enrichment and relatively low immune checkpoint expression, as supported by our protein-level validation. These findings provide additional biological context for the potential therapeutic benefit of combining RC48 with ICIs. Given the exploratory nature of the HER2-high definition and the limited sample size of the validation cohorts, further prospective studies are warranted to confirm these observations.