Open Access

Background A subgroup of patients with HER2-positive metastatic gastric and gastroesophageal junction cancers shows long-term response under trastuzumab maintenance monotherapy. Obviously, HER2 status alone is not able to identify these patients. We performed this study to identify potential new prognostic biomarkers for this long-term responding patient group. Patients and methods Tumor samples of 19 patients with HER2-positive metastatic gastric and gastroesophageal junction cancer who underwent trastuzumab treatment were retrospectively collected from multiple centers. Patients were divided into long-term responding (n=7) or short-term responding group (n=12) according to progression-free survival (PFS≥12 months vs. PFS<12 months). Next generation sequencing and microarray-based gene expression analysis were performed along with HER2 and PD-L1 immunohistochemistry. Results Long-term responding patients had significantly higher PD-L1 combined positive scores (CPS) and CPS correlated with longer progression-free survival. PD-L1 positivity (CPS≥1) was further associated with an increased CD4+ memory T-cell score. The ERBB2 copy number as well as the tumor mutational burden could not discriminate between short-term and long-term responding patients. Genetic alterations and co-amplifications in HER2 pathway associated genes such as EGFR, which were connected to trastuzumab resistance, were present in 10% of the patients and equally distributed between the groups. Conclusion The study highlights the clinical relevance of PD-L1 testing also in the context of trastuzumab treatment and offers a biological rational by demonstrating elevated CD4+ memory T-cells scores in the PD-L1-positive group.