Phase II Study of Short Course FOLFOX Chemotherapy With Either Nivolumab or Nivolumab + Radiation in the First Line Treatment of Metastatic or Unresectable Gastroesophageal Cancers (BMS Protocol CA209-76L)

Inclusion Criteria * Subjects with a diagnosis of advanced unresectable or metastatic gastroesophageal adenocarcinoma (eg. gastric, gastroesophageal junction, and esophageal adenocarcinoma) * Be willing and able to provide written informed consent/assent for the trial * Age \> 18 years * ECOG performance status ≤ 1 * Absolute neutrophil count ≥ 1,500/mL * Platelets ≥ 100,000/mL * Total bilirubin ≤ 1.5x upper limits of normal, unless the patient has known Gilbert's disease. * AST/ALT ≤ 2.5 upper limits of normal, or \< 5x ULN for subjects with liver metastases * Creatinine ≤ 1.5 mg/dl. If Creatinine \> 1.5 mg/dl, creatinine clearance \> 60 ml/min * Be willing to provide tissue from a newly obtained core or excisional biopsy of a tumor lesion. Newly-obtained is defined as a specimen obtained up to 28 days prior to initiation of treatment on Day 1. * For all males and females of childbearing potential, they should be agreeable to use an adequate method of contraception or birth control. For females of child bearing potential, a negative pregnancy test within 7 days of start of study drug is required Exclusion Criteria * Prior cytotoxic therapy for metastatic or incurable disease. * Patients may have had prior therapy with curative intent for localized disease, if their recurrence or disease progression was more than six months from completing prior therapy. * HER2 positive adenocarcinoma * Diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other for…