Neoadjuvant Chemoradiotherapy Plus Sintilimab for Intermediate/High Immunoscore Locally Advanced Rectal Cancer

6 cycles of mFOLFOX6 (oxaliplatin 85 mg/m2, and folinic acid 400 mg/m2 followed by bolus 5-fluorouracil 400 mg/m2 and 5-fluorouracil 2400mg/m2 as a 46-hour continuous infusion on day 1) followed by long course chemoradiotherapy (50 Gy in 25 fractions) followed by surgery. Patients will receive sintilimab 3mg/kg every 2 weeks during chemoradiotherapy (2nd-6th cycle).