Patients and TMAs
Formalin-fixed, paraffin-embedded tumor samples from 487 patients with stage III or stage IV GC who underwent gastrectomy at our hospital from January 2002 to December 2010 were examined. None of the patients underwent systematic chemotherapy before surgery. The construction of the TMAs with these tumors has been described elsewhere [11]. Briefly, two representative tumor cores (2 mm in diameter) mostly located in the invasive area of each tumor were obtained from the same formalin-fixed, paraffin-embedded tissue block in each case. Serial 4-μm sections were prepared and used for hematoxylin and eosin staining, immunohistochemistry (IHC), and in situ hybridization (ISH). Clinicopathological characteristics, including age, sex, tumor location, histological type, lymphatic invasion, venous invasion, depth of invasion, lymph node metastasis, presence of residual tumor on the resection margin, and adjuvant chemotherapy, were reviewed from medical records. The disease stage was classified according to the TNM criteria of the Union for International Cancer Control (seventh edition). Histological types were classified according to the Japanese classification of gastric carcinoma (third English edition). The study protocol was approved by the Institutional Review Board at the National Cancer Center.
Immunohistochemistry
The primary antibodies used for IHC were anti-PD-L1 (SP142) rabbit monoclonal antibody, CONFIRM anti-CD3 (2GV6) rabbit monoclonal antibody, CONFIRM anti-CD4 (SP35) rabbit monoclonal antibody, anti-CD8 (C8/144B) mouse monoclonal antibody, anti-forkhead box P3 (FOXP3; 236A/E7) mouse monoclonal antibody, anti-mutL homolog 1 (MLH1; ES05) mouse monoclonal antibody, anti-mutS homolog 2 (MSH2; FE11) mouse monoclonal antibody, anti-postmeiotic segregation increased 2 (PMS2; EP51) rabbit monoclonal antibody, and anti-mutS homolog 6 (MSH6; EP49) rabbit monoclonal antibody. Antibodies against PD-L1, CD3, and CD4 were purchased from Ventana (Tucson, AZ, USA), the antibody against FOXP3 was from Abcam (Cambridge, MA, USA), and all the other antibodies were from Dako (Copenhagen, Denmark). CD3, CD4, and CD8 IHC was performed with a BenchMark ULTRA fully automated slide processing system (Ventana) according to the manufacturer’s instructions. MLH1, MSH2, PMS2, and MSH6 IHC was performed with a Dako autostainer according to the manufacturer’s instructions. For PD-L1 and FOXP3 staining, the sections were deparaffinized in xylene, dehydrated with graded ethanol, and then immersed in methanol with 0.3 % hydrogen peroxidase for 20 min to block endogenous peroxidase. Antigen retrieval was performed at 121 °C for 10 min in a pressure cooker (Pascal pressurized heating chamber) followed by placement of sections in Dako target retrieval solution for PD-L1 or citrate buffer for FOXP3. The sections were immersed in 2 % normal swine serum in phosphate-buffered saline to block nonspecific binding for 30 min at room temperature. The slides were then incubated overnight at 4 °C with the primary antibody. After they had been washed five times with phosphate-buffered saline, the slides were incubated with labeled polymer horseradish peroxidase rabbit/mouse antibody for 30 min (Envision Plus detection system; Dako). After extensive washing with phosphate-buffered saline, the color reaction was developed in 2 % 3,3′-diaminobenzidine in 50 mmol/L tris(hydroxymethyl)aminomethane buffer (pH7.6) containing 0.3 % hydrogen peroxide for 4 min. Background staining was performed with Mayer’s hematoxylin solution, and sections were then dehydrated through ascending alcohols to xylene and mounted on slides.
Evaluation of PD-L1 expression
All tissue cores immunohistochemically stained with the anti-PD-L1 antibody were evaluated by two pathologists (A.K. and T.K.). Specimens were scored on the basis of the percentage of stained TCs or TIICs by IHC: 0, less than 1 %; 1+, from 1 % to less than 10 %; 2+, from 10 % to less than 20 %; 3+, 20 % or more (Fig. 1). PD-L1-positive cases on TCs or TIICs were defined by the presence of at least 1 % of TCs or TIICs with membrane staining. A higher score was selected if two cores from the same case exhibited different PD-L1 expression scores.
Measurement of TILs
Stained slides were digitized with a NanoZoomer HT scan system (Hamamatsu Photonics, Japan). The densities of TILs were assessed in accordance with a previous report with some modification [12]. Briefly, 0.30 mm2 of the representative tumor area in each core was selected, and the number of positive cells was counted in a total area of 0.60 mm2. The median values were used for cutoff points for high densities of TILs (vs low densities of TILs).
Evaluation of MMR status
Tumors were considered negative for MLH1, MSH2, PMS2, or MSH6 expression only if there was a complete absence of nuclear staining in the TCs, and normal epithelial cells and lymphocytes were used as an internal control. Tumors lacking MLH1, MSH2, PMS2, or MSH6 expression were considered to be MMR deficient, whereas tumors that maintained expression of all markers were considered to be MMR proficient.
EBV ISH
Chromogenic ISH for EBV-encoded RNA (EBER) was performed with fluorescein-labeled oligonucleotide probes (INFORM EBER probe, Ventana) with enzymatic digestion (ISH protease 3, Ventana) and an iViewBlue detection kit (Ventana) with use of the BenchMark ULTRA staining system.
Statistical analysis
Comparisons of categorical variables were done by a chi-square test or Fisher’s exact test as appropriate. We performed survival analyses in the patients with R0 resection. OS was defined as the time from the date of surgery until death from any cause. Patients who were alive were censored at the last follow-up date. OS rates were estimated by the Kaplan–Meier method, and differences between the groups according to PD-L1 expression, densities of TILs, MMR status, and EBV status were identified by univariate and multivariate analyzes by Cox proportional hazards models and are presented as hazard ratios with 95 % confidence intervals. Confounders in univariate and multivariate analyses included age, histological type, depth of invasion, lymphatic invasion, venous invasion, TNM stage, and adjuvant chemotherapy. Statistical analyses were performed with IBM SPSS Statistics version 21 (IBM, Armonk, NY, USA). All tests were two-sided, and differences were considered significant when P < 0.05.