Pre-operative Nivolumab and Ipilimumab, Followed by Post-operative Nivolumab, for MSI/dMMR Oeso-gastric Adenocarcinoma.

Inclusion Criteria: 1. Signed and dated informed consent, 2. Age ≥18 years to ≤75 years of age, 3. Histologically proven non-metastatic gastric adenocarcinoma or of the oeso-gastric junction T2 to T4, Nx, M0 after thoraco-abdominopelvic computed tomography (CT) and echo-endoscopy, 4. Subjects must be willing and able to comply with scheduled visits, treatment schedule, laboratory tests, tumor biopsies, and other requirements of the study, 5. dMMR (protein expression by immunohistochemistry \[ICH\] and/or MSI by polymerase chain reaction \[PCR\]), MMR and/or MSI tumors should be assessed per local guidelines: ICH with two (anti-MLH1 and anti-MSH2 or anti-MSH6, and antiPMS2) or four antibodies (anti-MLH1, anti-MSH2, anti-MSH6, and anti-PMS2) and/or PCR (with PROMEGA: BAT- 25, BAT-26, NR-21, NR-24, and NR-27) by the investigators prior to screening, Extinct MLH1 (+/- PMS2), MSH2 (+/- MSH6), MSH6, or PMS2 alone protein expression by IHC (dMMR), and/or tumor with ≥ 2 instable MSI-H markers on PCR: BAT25, BAT26, NR21, NR24, and NR27 (pentaplex panel is recommended), 6. The Eastern Cooperative Oncology Group Performance status (ECOG PS) 0-1, for patients over 70 years ECOG PS of 0; 7. Hematological status: absolute neutrophil count (ANC) ≥1.5 x 109/L; platelets ≥100 x 109/L; hemoglobin ≥9 g/dL, 8. Adequate renal function: serum creatinine level \<120 µM, clearance \> 50ml/min (Modification of the Diet in Renal Disease \[MDRD\] or Cockcroft and Gault), 9. Adequate liver function: serum bilirubin ≤1.5 x upper normal limit (ULN), alkaline phosphatase \<5 x ULN, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) ≤3.0 x ULN, 10. No prior therapy for localized oeso-gastric cancer, 11. Radiological tumor assessment within 21 days before the start of treatment according to RECIST version 1.1 by Chest Abdomen and Pelvis CT, 12. For female patients of childbearing potential, negative pregnancy test within 7 days before starting the study drug, 13. Men and women are required to use adequate birth control during the study (when applicable), Female participants of childbearing potential and male participants with partners of childbearing potential must agree to use a highly effective method of birth control (i.e., pregnancy rate of less than 1% per year) during the period of treatment and during 5 and 7 months, woman and men, respectively, from the last treatment administration. Men must refrain from donating sperm during this same period, Contraceptive methods that result in a low failure rate when used consistently and correctly include methods such as combined hormonal contraception associated with inhibition of ovulation (oral, intravaginal, transdermal), progestogen-only hormonal contraception associated with inhibition of ovulation (oral, injectable, implantable), some intrauterine devices, intrauterine hormone- releasing stem, true sexual abstinence (when this is in line with the preferred and usual lifestyle of the participant), bilateral tubal occlusion, or a female partner who is not of childbearing potential or a male partner who has had a vasectomy. Women and female partners using hormonal contraceptive must also use a barrier method i.e. condom or occlusive cap (diaphragm or cervical/vault caps), A woman is considered to be of childbearing potential if she is postmenarcheal, has not reached a postmenopausal state (\>12 continuous months of amenorrhea with no identified cause other than menopause), and has not undergone surgical sterilization (removal of ovaries and/or uterus), 14. Subject willing to comply to provide primary tumor tissue (archival or fresh biopsy specimen), including possible pre-treatment biopsy for PD-L1 expression analysis and other biomarker correlative studies 15. Registration in a National Health Care System (PUMa - Protection Universelle Maladie included) Exclusion Criteria: Non-eligible to clinical trial if one of following parameter is reported: 1. Concomitant unplanned antitumor therapy (e.g. chemotherapy, molecular targeted therapy, immunotherapy), 2. Treatment with any investigational medicinal product within 28 days prior to study entry, 3. Major surgical procedure within 4 weeks prior to initiation of study treatment, 4. Other serious and uncontrolled non-malignant disease (including active infection), 5. Other concomitant or previous malignancy, except: i/ adequately treated in-situ carcinoma of the uterine cervix, ii/ basal or squamous cell carcinoma of the skin, iii/ cancer in complete remission for \>5 years, 6. Metastases (M stage disease) whatever the location, 7. Pregnant or breastfeeding women, 8. Human immunodeficiency virus (HIV), 9. Active hepatitis B virus (HBV, defined as having a positive hepatitis B surface antigen \[HBsAg\] test prior to inclusion) or hepatitis C virus (HCV). Note: Patients with past HBV infection or resolved HBV infection (defined as having a negative HBsAg test and a positive antibody to hepatitis B core antigen antibody test) are eligible. Note: Patients positive for HCV antibody are eligible only if PCR testing is negative for HCV RNA 10. Patient on tutelage or guardianship or under the protection of justice. 11. Impossibility of submitting to the medical follow-up of the study for geographical, social or psychiatric reasons. Non-eligible to immunotherapy: 1. History of autoimmune disease including, but not limited to myasthenia gravis, myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, vascular thrombosis associated with antiphospholipid syndrome, Wegener's granulomatosis, Sjögren's syndrome, Guillain-Barré syndrome, multiple sclerosis, vasculitis, or glomerulonephritis, Note: History of autoimmune-related hypothyroidism on a stable dose of thyroid replacement hormone may be eligible. Note: Controlled Type 1 diabetes mellitus on a stable insulin regimen may be eligible. 2. History of idiopathic pulmonary fibrosis (including pneumonitis), drug-induced pneumonitis, organizing pneumonia (i.e., bronchiolitis obliterans, cryptogenic organizing pneumonia), or evidence of active pneumonitis on screening chest imaging, 3. Administration of a live, attenuated vaccine within 4 weeks prior to start of treatment or anticipation that such a live attenuated vaccine will be required during the remainder of the study, 4. Prior treatment with CD137 agonists, anti-CTLA4, anti-PD-1, or antiPD-L1 therapeutic antibody or pathway-targeting agents, 5. Prior allogeneic bone marrow transplantation or prior solid organ transplantation, 6. Treatment with systemic corticosteroids or other systemic immunosuppressive medications (including, but not limited to prednisone, dexamethasone, cyclophosphamide, azathioprine, methotrexate, thalidomide, and anti-tumor necrosis factor agents) within 2 weeks prior to start of adjuvant treatment, or requirement for systemic immunosuppressive medications during the remainder of the study. Inhaled or topical steroids and adrenal replacement doses \>10 mg daily prednisone equivalents are permitted in the absence of active autoimmune disease. Note: Patients who have received acute, low-dose, systemic immunosuppressant medications (e.g., a one-time dose of dexamethasone for nausea) may be enrolled into the study after approval of the Medical Contact. Subjects are permitted the use of topical, ocular, intra- articular, intranasal, and inhalational corticosteroids (with minimal systemic absorption). Adrenal replacement steroid doses including doses \>10 mg daily prednisone is permitted. A brief (less than 3 weeks) course of corticosteroids for prophylaxis (e.g. contrast dye allergy) or for treatment of non-autoimmune conditions (e.g. delayed-type hypersensitivity reaction caused by a contact allergen) is permitted