Peri-operative Sintilimab in Combination With SOX in Locally Advanced Gastric Cancer

Inclusion Criteria: * Male or female, 18 years old ≤ age ≤ 75 years old * ECOG PS score 0-1 * Treatment naive patients diagnosed as gastric adenocarcinoma or gastroesophageal junction adenocarcinoma by histopathology * No known HER2-positive status; * Clinical stage Ⅱ, Ⅲ (T1-4a N+ M0, T3-4a N0 M0, AJCC 8th) * The research center and the surgeon can complete D2 radical gastrectomy * Physical condition and organ function allow for larger abdominal surgery * Sufficient organ and bone marrow function, which is defined as follows: 1. Blood routine: absolute neutrophil count (ANC)≥1.5×109/L; platelet count (PLT)≥100×109/L; hemoglobin content (HGB)≥9.0 g/dL. 2. Liver function: Patients without liver metastasis require serum total bilirubin (TBIL) ≤1.5×upper limit of normal (ULN); alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤2.5 ×ULN; 3. Renal function: Creatinine clearance rate (Ccr) ≥50 mL/min (calculated by Cockcroft/Gault formula): 1. Female: Ccr= (140-years old) x weight (kg) x 0.85/(72 x serum creatinine (mg/dL)) 2. Male: Ccr= (140-years old) x weight (kg) x 1.00/(72 x serum creatinine (mg/dL)) 4. The coagulation function is adequate, defined as the international normalized ratio (INR) or prothrombin time (PT) ≤ 1.5 times ULN; if the subject is receiving anticoagulation therapy, as long as the PT is within the proposed range of anticoagulation drugs * LVEF≥50%; * Agree and be able to comply with the plan during the research period; * Provide written informed consent before entering the study screening, and the patient has understood that participants can withdraw from the study at any time during the study without any loss; Exclusion Criteria: * Complicated with upper gastrointestinal obstruction/bleeding or abnormal digestive function or malabsorption syndrome; * Complicated with severe uncontrolled concurrent infection or other severe uncontrolled concomitant disease, moderate or severe renal injury; * Received previous anti-tumor therapy, including chemotherapy, radiotherapy, targeted therapy or immunotherapy, etc.; * Suffered from other malignant tumors in the past 5 years (except basal cell or squamous cell carcinoma, superficial bladder cancer, cervical cancer in situ or breast cancer); * Uncontrollable pleural effusion, pericardial effusion or ascites; * Suffered from severe cardiovascular disease within 12 months before enrollment, such as symptomatic coronary heart disease, congestive heart failure ≥ Grade II, uncontrolled arrhythmia, and myocardial infarction; * Allergic reactions to the drugs used in this study; * Use steroids or other systemic immunosuppressive therapies 14 days before enrollment; * Patients who received study drug treatment within 4 weeks before enrollment (participate in other clinical trials); * Active autoimmune diseases; * History of primary immunodeficiency; * Have used immunosuppressive drugs within 4 weeks before the first dose of study treatment, excluding nasal spray, inhaled or other local glucocorticoids or physiological doses of systemic glucocorticoids (that is, no more than 10 mg/day Pred nisone or other glucocorticoids in equivalent doses), or use hormones to prevent allergy to contrast agents; * Within 4 weeks before the first dose of study treatment or plan to receive live attenuated vaccine during the study period; * Known to have active tuberculosis; * Known history of allogeneic organ transplantation and allogeneic hematopoietic stem cell transplantation; * HIV antibody positive, active hepatitis B or C (HBV, HCV); * Pregnant or lactating women