Quantitative assessment of tumor immune microenvironment in context of HER2 expression in MIBC

Muscle-invasive bladder cancer (MIBC) carries high risks of relapse and death despite contemporary care. In this study we aimed to evaluate whether the pretreatment immune microenvironment varied with HER2 status and if the differences influenced the response to neoadjuvant chemotherapy (NAC) and survival. We conducted a retrospective study of 62 treatment-naiive MIBC patients diagnosed on pretreatment transurethral resection of bladder tumor (TURBT) specimens collected between 2008 and 2024. All patients received NAC followed by radical cystectomy. HER2 immunohistochemistry (IHC) was scored 0/1+/2+/3+ using gastroesophageal criteria. PD-L1 was assessed by combined positive score (CPS). CD3, CD20, and CD68 cell densities were quantified by digital image analysis. Overall survival was measured from the date of TURBT. HER2 IHC scores were 2+/3+ in 28 of 62 tumors (45%) and 0/1+ in 34 (55%). All seven IHC 2+ tumors lacked ERBB2 amplification by fluorescence in situ hybridization (FISH). NAC response occurred in 11 of 34 tumors with HER2 IHC 0/1+ (32%) and 9 of 28 tumors with HER2 IHC 2+/3+ (32%) (P = 1.00). PD-L1 expression was not associated with HER2 status. Immune-cell analyses showed trends toward higher CD3+ T-cell density, CD20+ B-cell density, and TM ratio in NAC responders, but no significant differences by HER2 status or combined PD-L1/HER2 subgroup status. After Bonferroni correction for multiple comparisons, none of the immune-cell metrics remained statistically significant. Overall survival did not differ between the HER2 IHC groups (P = 0.71). These findings indicate that HER2 IHC 2+/3+ expression is present in a substantial subset of MIBC but is not associated with NAC response or overall survival in this cohort. Further study is warranted to define the clinical relevance of MIBC with HER2 IHC 2+/3+ and its potential role in HER2-directed therapeutic strategies.