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