Response to Immunotherapy in MMR-deficient Localized Colon Cancer

Inclusion Criteria: 1. Histologically confirmed localized dMMR stage cT1N0M0 to cT4N2M0 (stage I to III) colon carcinoma. 2. Indication for elective curative intended surgery without neoadjuvant chemotherapy. 3. Age of ≥ 18 years. 4. Written informed consent. 5. Eastern Cooperative Oncology Group performance status of 0 or 1. 6. Adequate bone marrow function defined as: * Hemoglobin ≥ 6.2 mmol/L or ≥ 10 g/dL. * Absolute neutrophil count ≥ 1.5 × 109/L. * Platelet count ≥ 100 × 109/L. 7. Adequate kidney function defined as: o Estimated glomerular filtration rate ≥ 60 mL/min or creatinine ≤1.5 × upper limit of normal (ULN). 8. Adequate liver function defined as: * Total bilirubin: ≤ 1.5 × ULN. * Alanine aminotransferase: ≤ 2.5 × ULN. * Alkaline phosphatase: ≤ 2.5 × ULN. 9. Follow the conditions regarding fertility, pregnancy, and lactation: * Female and male participants of reproductive potential (PORP) must agree to avoid becoming pregnant or impregnating a partner, respectively, while receiving pembrolizumab and for 120 days after the dose. * PORPs must use, or have their partner use, an acceptable method of contraception e.g. intrauterine device, contraceptive rod implanted into the skin, or hormonal contraceptive and male condom during heterosexual activity, while receiving pembrolizumab and for 120 days after the dose. * Women of reproductive potential must have a negative serum or urine pregnancy test (minimum sensitivity 25 IU/L HCG) within 72 hours prior to receiving pembrolizumab. * Women must not be breastfeeding. Exclusion Criteria: 1. Any serious or uncontrolled medical disorder that, in the opinion of the investigator or treating physician, may increase the risk associated with study participation, impair the ability of the subject to receive protocol therapy, or interfere with the interpretation of study results. 2. Autoimmune disorders (except thyroiditis with replacement therapy and type I diabetes mellitus). 3. Prior treatment with ICIs or any other antibody/drug specifically targeting the T-cell co-stimulation or checkpoint pathways. 4. A known history of Human Immunodeficiency Virus, active chronic, or acute Hepatitis B or Hepatitis C. 5. A condition requiring systemic treatment with either corticosteroids (\> 10 mg daily prednisone equivalents) or other immunosuppressive medications within 14 days of study drug administration. Inhaled or topical steroids and adrenal replacement doses \> 10 mg daily prednisone equivalents are permitted in the absence of active autoimmune disease. 6. Prior participation in another trial with an investigational medicinal product. 7. Received live vaccines within 30 days prior to pembrolizumab trial treatment. Seasonal influenza vaccines for injection are allowed. 8. A history of allergy to study drug components or a history of severe hypersensitivity reaction to any monoclonal antibody.