Standard Chemotherapy vs Immunotherapie in 2nd Line Treatment of MSI Colorectal Mestastatic Cancer

Inclusion Criteria: * Histologically proven colorectal adenocarcinoma with metastasis(es) non-resectable * MSI-H determined by immunohistochemistry (loss of expression of MLH1, MSH2, MSH6 and/or PMS2) or by molecular biology * At least one measurable target (primary tumor or metastasis) according to RECIST v1.1 * Mutational status RAS and BRAF * Age ≥ 18 * OMS ≤ 2 * Life expectancy \< 3 months * Patient failure (progression or unacceptable toxicity) of chemotherapy containing fluoropyrimidine (capecitabine or 5FU) +/- irinotecan +/- oxaliplatin with or without cetuximab, bevacizumab and panitumumab (patients in progression during or within 3 months after discontinuation of adjuvant chemotherapy are eligible) * PNN \> 1500/mm3, platelets \> 100 000/mm3, Hb \> 9 g/dL * Total bilirubin \< 25 µmol/L, ASAT \< 5x LSN, ALAT \< 5 x LSN, PT \> 60%, , PAL\<2.5 x LSN ( \< 5 x LSN in case of hepatic metastasis) * Creatinine clearance \> 50 ml/min according to MDRD formula (≥ 30 ml/min according to the Cockcroft-Gault formula) * Patient belonging to a social security scheme * Patient information and signature of the informed consent Exclusion Criteria: * Patient immediately eligible for a curative therapy (surgical and/or percutaneous) after discussion in CPR * Patient treated with FOLFIRINOX or FOLFOXIRI in 1st line * Cerebral metastasis * Previous treatment with anti-PD1 or anti-PDL1 * Autoimmune disease that might be aggravated during treatment with an immuno-stimulating agent (patients with type I diabetes, vitiligo, psoriasis, hypo- or hyperthyroid disease not requiring immunosuppressive treatment are eligible) * Immunosuppressive long-term treatment (patients necessitating a corticotherapy are eligible if they are administered in doses \< or = to the equivalent of 10 mg of prednisone daily, administration of steroids by a route resulting in minimal systemic exposure (local, intra-anal, intraocular or inhalation) are eligible). * Transplant patients (including stem cell transplants), HIV positive or other immune deficiency syndromes * Active infection by HBV or HCV * Known severe hypersensitivity to monoclonal antibodies or history of anaphylactic shock, or uncontrolled asthma * Any known specific contraindication or allergy to the treatments used in the study * Persistence of toxicities related to 1st line chemotherapy grade \> 2 (NCI-CTC v4.0) (except alopecia and neuropathy sequelae of oxaliplatin) * Vaccination during the 4 weeks preceding the start of treatment * Known deficit in DPD * QT/QTc interval \> 450 msec for men and \> 470 msec for women * K+ \< LIN, Mg2+ \< LIN, Ca2+ \< LIN * Following alterations in the 6 months prior to inclusion: myocardial infarction, angina, severe/unstable angina, coronary artery bypass surgery, congestive heart failure NYHA class II, III or IV, stroke or transient ischemic attack * Any progressive pathology not stabilised over the past 6 months: hepatic failure, renal failure, respiratory failure * Patient with interstitial pneumonitis or pulmonary fibrosis * History of chronic diarrhea or inflammatory disease of the colon or rectum, or unresolved occlusion or sub-occlusion in symptomatic treatment * History of malignant pathologies during the past 5 years except basocellular skin carcinoma or in situ cervical carcinoma, properly treated * Patient already included in another clinical trial during treatment with an experimental molecule for L2 * Lack of effective contraception in patients (men and/or women) of childbearing age, pregnant or breastfeeding women, women of childbearing age not having had a pregnancy test * Persons deprived of liberty or under supervision * Impossibility of undergoing medical monitoring during the trial for geographic, social or psychological reasons