Tumor Microenvironment Features of Response to Perioperative Gemcitabine, Cisplatin, and Pembrolizumab in Potentially Resectable Biliary Tract Cancers

Inclusion Criteria: * Must have a newly diagnosed, biopsy-proven biliary tract cancer (BTC) including gallbladder, intrahepatic, extrahepatic, and hilar cholangiocarcinoma. * Resectable BTC (biliary tract cancer) * Measurable disease per RECIST 1.1 as determined by the investigator. * Age ≥18 years. * ECOG (Eastern Cooperative Oncology Group) performance status ≤1 or Karnofsky ≥80 * Patients must have adequate organ and marrow function defined by study-specified laboratory tests. * Patients must have adequate liver function defined by study-specified laboratory tests. * Patients with chronic or acute HBV or HCV infection must have disease controlled prior to enrollment. * Women of childbearing potential (WOCBP) must have a negative urine or serum pregnancy test. * For both Women and Men, must use acceptable form of birth control while on study. * Ability to understand and willingness to sign a written informed consent document. Exclusion Criteria: * Receiving, or previously received, any systemic chemotherapy, or investigational agent for BTC. * Has received prior radiotherapy within 2 weeks of start of study intervention. * Patients with a history of prior treatment with anti-PD-1 and anti-PD-L1. * Have been diagnosed with another cancer or myeloproliferative disorder whose natural history or treatment has the potential to interfere with safety or efficacy assessment of this study's investigational drugs. * Has a known history of Human Immunodeficiency Virus (HIV)/AIDS * Has active co-infection with HBV and HDV. * Has a diagnosis of immunodeficiency. * Has active autoimmune disease that has required systemic treatment in the past 2 years. * Systemic or topical corticosteroids at immunosuppressive doses. * Prior allogeneic stem cell transplantation or organ transplantation. * Prior tissue or organ allograft or allogeneic bone marrow transplantation, including corneal transplants. * Uncontrolled intercurrent active medical and/or psychiatric illness/social psychosocial problems that that would limit compliance with study requirements. * Uncontrolled intercurrent illness including, but not limited to, uncontrolled infection, symptomatic congestive heart failure, unstable angina, cardiac arrhythmia, metastatic cancer, or psychiatric illness/social situations that would limit compliance with study requirements. * Evidence of clinical ascites. * Has a history of (non-infectious) pneumonitis/interstitial lung disease that required steroids or has current pneumonitis/interstitial lung disease. * Previously identified allergy or hypersensitivity to monoclonal antibodies or any component of the study treatment formulations. * Pregnant or breastfeeding. * WOCBP and men with female partners (WOCBP) who are not willing to use contraception. * Subjects unable to undergo venipuncture and/or tolerate venous access. * Patient is at the time of signing informed consent a regular user (including "recreational use") of any illicit drugs or had a recent history (within the last year) of substance abuse (including alcohol).