Inclusion Criteria:

          1. Signed and dated informed consent document, willing and able to comply with protocol
             requirements

          2. Histologically or cytologically proven PDAC

          3. Age ≥ 18 years

          4. ECOG Performance Status (Eastern Cooperative Oncology Group) 0-1

          5. HLA-A2 genotype (Human Leukocyte Antigen)

          6. Recurrent or advanced disease not amenable to surgery with curative intent (previous
             resection of primary tumor allowed)

          7. Measurable or evaluable (radiologically detectable disease which does not fulfill
             RECIST criteria for measurable disease) lesions according to RECIST v1.1 criteria
             (CT-scan < 4 weeks)

          8. Stable disease or tumor response according to RECIST v1.1 after a 4-month (8 cycles)
             course of first-line FOLFIRINOX or modified FOLFIRINOX induction chemotherapy

          9. Have archival tissue sample that has been identified and confirmed as available for
             study, or newly obtained core or excisional biopsy of a tumor lesion

         10. Adequate organ function, as defined by the following:

               -  Serum aspartate aminotransferase (AST) and serum alanine aminotransferase (ALT) <
                  3 x upper limit of normal (ULN)

               -  Total serum bilirubin < 1.5 ULN

               -  Prothrombin ratio > 70%

               -  Serum albumin ≥ 2.8 g/dL

               -  Hemoglobin ≥ 10,0 g/dl

               -  White blood cell count (WBC) ≥ 3,000/μL

               -  Absolute neutrophil count (ANC) ≥ 1,500/μL

               -  Platelets ≥ 100,000/μL

               -  Serum creatinine ≤ 1.5 ULN or creatinine clearance > 50 mL/min (MDRD)

         11. Life expectancy ≥ 3 months

         12. Women participants of childbearing potential must have a negative serum pregnancy test
             within the 3 days prior to the first treatment administration until 6 months after the
             last dose of FOLFIRI, and 90 days after the last dose of OSE2101. Both women
             participants of childbearing potential and men participants who are sexually active
             with women of childbearing potential must agree to use a reliable method of birth
             control (i.e. pregnancy rate < 1% per year); women of childbearing potential receiving
             nivolumab will be instructed to adhere to contraception for a period of 5 months after
             the last dose of nivolumab. Men receiving nivolumab and who are sexually active with
             women of childbearing potential will be instructed to adhere to contraception for a
             period of 7 months after the last dose of nivolumab.

         13. Registration in a national health care system (PUMA included).

        Exclusion Criteria:

          1. Obstructive jaundice (bilirubin > 1.5 ULN) without adequate biliary drainage

          2. Any systemic steroid therapy (> 10 mg daily dose of prednisone or equivalent) whatever
             the duration of this corticotherapy

          3. Allograft recipient

          4. Active HBV (hepatitis B virus), HCV (hepatitis C virus ), or HIV infection Note:
             Patients with past HBV infection or resolved HBV infection (defined as having a
             negative HBsAg test and a positive HBc (hepatitis B core antigen) antibody test are
             eligible.

             Note: Patients positive for HCV antibody are eligible only if polymerase chain
             reaction testing is negative for HCV ribonucleic acid (RNA).

          5. Diagnosis of any second malignancy within the last 5 years, except for adequately
             treated basal cell or squamous cell skin cancer, or in situ carcinoma of the cervix
             uteri

          6. Any unresolved toxicity NCI CTCAE Grade ≥ 2 from previous anticancer therapy with the
             exception of neuropathy, alopecia, and the laboratory values defined in the inclusion
             criteria

          7. Prior treatment with any immune checkpoint inhibitor, including anti-PD-1, anti-PD-L1,
             anti-PD-L2, or anti-CTLA-4 (Cytotoxic T-lymphocyte-associated antigen-4) antibody

          8. Known active central nervous system metastases and/or carcinomatous meningitis;
             patients with previously treated brain metastases may participate provided they are
             stable (without evidence of progression by imaging for at least 4 weeks prior to the
             first dose of trial treatment and any neurologic symptoms have returned to baseline),
             have no evidence of new or enlarging brain metastases, and are not using steroids at a
             dose > 10 mg/day of prednisone or equivalent for at least 14 days prior to trial
             treatment

          9. Uncontrolled massive pleural effusion or massive ascites

         10. 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, that has required systemic
             treatment (i.e. with use of disease modifying agents, corticosteroids or
             immunosuppressive drugs) Note: Patients with vitiligo, alopecia, or any chronic skin
             condition that does not require systemic therapy are exception to this criterion.

             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.

         11. Evidence of interstitial lung disease, any active, non-infectious pneumonitis, or
             known active tuberculosis

         12. Active uncontrolled infection, or current unstable or uncompensated respiratory or
             cardiac conditions, or bleeding

         13. Known psychiatric or substance abuse disorders that would interfere with cooperation
             with the requirements of the study

         14. Live vaccine administration within 30 days prior to the first dose of study treatment

         15. History or current evidence of any condition, therapy, or laboratory abnormality that
             might confound the results of the trial, interfere with participation for the full
             duration of the trial, or is not in the best interest of the participant, in the
             opinion of the treating investigator

         16. Known or suspected drug hypersensitivity to OSE2101 vaccine or nivolumab

         17. Radiotherapy treatment to more than 30% of the bone marrow or with a wide field of
             radiation within 4 weeks of the first dose of study drug

         18. Major surgical procedure (as defined by the Investigator) within 28 days prior to the
             first dose of investigational product Note: Local surgery of isolated lesions for
             palliative intent is acceptable.

         19. Treatment with any investigational medicinal product within 28 days prior to study
             entry

         20. Prior intolerance/severe toxicity with 5FU or irinotecan (including DPD (
             dihydropyrimidinedehydrogénase) and UGT1A1 deficiency)

         21. Pregnancy/lactation

         22. Tutelage or guardianship.