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Background The predictive significance of programmed death ligand 1 (PD-L1) for programmed death 1 (PD-1) inhibitors remains unclear in gastric cancer (GC) due to the dynamic alteration by treatments. We aimed to elucidate the effects of trastuzumab (Tmab) on PD-L1 expression in GC. Methods PD-L1 expression was evaluated by multicolour flow cytometry analysis after co-culturing GG cell lines and immune cells with Tmab. IFN-γ in the co-culture experiments was quantified. Immunohistochemistry (IHC) for PD-L1 expression using clinical samples was also performed to confirm PD-L1 alteration by Tmab. Results PD-L1 expression was significantly upregulated by Tmab in HER2-amplified GC cell lines co-cultured with peripheral blood mononuclear cells (PBMCs). PD-L1 upregulation by Tmab was also observed in the GC cells co-cultured with NK cells in time-dependent manner, but not with monocytes. IFN-γ concentration in conditioned media from co-cultured PBMCs and NK cells with Tmab was significantly higher and anti-IFN-γ significantly suppress the Tmab-induced PD-L1 upregulation. IHC also suggested PD-L1 upregulation after Tmab treatment. Conclusions Tmab can upregulate PD-L1 expression on GC cells through interaction with NK cells. These results suggest clinical implications in the assessment of the predictive significance of PD-L1 expression for PD-1 inhibitors.

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Background IgG4-related disease (IgG4-RD) is an immune-mediated disorder in which abundant IgG4-positive plasma cells infiltrate affected organs. There have been reported four cases of probable IgG4-RD presenting as a submucosal tumor of the stomach. We herein report the first case of definite IgG4-RD presenting as a submucosal tumor of the stomach resected with laparoscopic endoscopic cooperative surgery (LECS). Case presentation A 70-year-old woman with a 6-year history of autoimmune pancreatitis was referred to our department because a 15-mm submucosal tumor in the greater curvature of the lower part of the stomach had been identified via upper gastrointestinal endoscopy. Endoscopic ultrasonography showed a 10-mm low-echoic lesion derived from the submucosal layer of the stomach. A fine-needle aspiration biopsy was attempted, but the tumor was too hard for sampling. F-fluorodeoxyglucose (FDG) positron emission tomography showed an FDG uptake, suggesting a possibility of malignant disease. As the diagnosis could not be confirmed, LECS for both the diagnosis and curative treatment was performed. A histopathological examination showed a tumor with IgG4-positive lymphoplasmacytic infiltration and fibrosis. The ratio of IgG4+/IgG+ lymphoplasmacytic cells was > 80%. A laboratory examination showed elevation of the serum IgG4 levels preoperatively. Thus, the final diagnosis was IgG4-RD of the stomach. No recurrence was observed within 1 year after surgery. Conclusions We encountered a case of definite IgG4-RD presenting as a gastric SMT in which a correct diagnosis was achieved by a minimally invasive LECS technique. IgG4-RD may present as a gastric lesion and should be taken into consideration as a differential diagnosis.