Chemotherapy and Immunotherapy as Treatment for MSS Metastatic Colorectal Cancer With High Immune Infiltrate

Inclusion Criteria: * Age ≥ 18 years * MSS and pMMR metastatic colorectal adenocarcinoma (metachronous or synchronous metastases), histologically proven * Patients who have had chemotherapy (neo-adjuvant or adjuvant) or radiotherapy (neo-adjuvant or adjuvant) for the treatment of primary tumor or metastatic resected disease R0 can be included if they have a recurrence more than 6 months after the end of this treatment. * High immune response defined as the immune infiltration scores obtained on the primary tumour (resection of primary tumour containing at least 2 mm of tumour-free margin between the tumour and non-tumour area) * Unresectable cancer with at least one measurable metastatic target according to RECIST v1.1 criteria * WHO PS ≤ 1 * Life expectancy ≥ 3 months * Adequate haematological function: neutrophils ≥ 1,500 /mm3, platelets ≥ 100,000/mm3, Hb \> 9 g/dL * Adequate liver function: AST/ALT ≤ 5xULN, total bilirubin ≤ 2xULN, alkaline phosphatase. ≤ 5xULN * Creatinine clearance \> 50 mL/min according to the MDRD formula * Proteinuria \<2+ (dipstick urinalysis) or ≤1g/24hour * Patient who is a beneficiary of the social security system * Information provided to patient and signature of the informed consent form Exclusion Criteria: * Active infection requiring intravenous antibiotics at day 1 of cycle 1 * Active or untreated central nervous system metastases * Another concomitant cancer or history of cancer during the last 5 years, except for carcinoma in situ of the uterine cervix or a basal cell or squamous cell skin carcinoma or any other carcinoma in situ considered as cured * Previous bone marrow allogenic stem cell transplantation or previous organ transplantation * History of idiopathic pulmonary fibrosis, medicinal product-related pneumonia or proof of active pneumonia or pneumonitis on a chest CT-scan prior to therapy * HIV infection, active hepatitis B or C infection, active tuberculosis * Colorectal cancer with microsatellite instability (dMMR and/or MSI) * Patient eligible for curative treatment (resection and/or thermal ablation according to the opinion of the local multidisciplinary tumour meeting board) * Patient with only primary tumour biopsies available or only a sample of a metastasis (no surgical resection of the primary tumour) * Previous treatment with anti-PD1 or anti-PDL1 or another immunotherapy * An auto-immune disease which may worsen during treatment with an immune-stimulating agent (patients with type I diabetes, vitiligo, psoriasis, hypo or hyperthyroidism not requiring immunosuppressant therapy are eligible) * Long-term immunosuppressant therapy (patients requiring corticosteroid therapy are eligible if administration at a dose ≤ 10 mg prednisone equivalent dose per day, administration of steroids by a route of administration resulting in minimal systemic exposure (cutaneous, rectal, ocular or inhalation) is authorised) * Known severe hypersensitivity to monoclonal antibodies, to one of the medicinal products used or to one of the excipients in the products used or a history of anaphylactic shock or of uncontrolled asthma * Vaccinations (live vaccine) within 30 days prior to start of treatment * Dihydropyrimidine Dehydrogenase (DPD) deficiency defined by uracilemy level ≥ 16 ng/mL * QT/QTc interval \> 450 msec in men and \> 470 msec in women * One of the following disorders during the 6 months prior to inclusion: myocardial infarction, unstable/severe angina pectoris, coronary artery bypass grafting, NYHA class II, III or IV congestive heart failure, stroke or transient ischaemic attack * All uncontrolled progressive disorders during the last 6 months: hepatic insufficiency, renal insufficiency, respiratory insufficiency, arterial hypertension * History of an inflammatory digestive disease, obstruction or sub-obstruction not resolved with symptomatic treatment * Peptic ulcer disease not healed before the treatment * Not controlled HTA * Patient already enrolled in another therapeutic trial with an ongoing investigational drug or whose treatment ended less than 4 weeks before inclusion * Absence of effective contraception in patients (male and/or female patients) of childbearing potential, a pregnant or breastfeeding woman, women of childbearing potential and who have not had a pregnancy test * Impossibility to submit to medical follow-up of the trial due to geographic, social or psychological reasons