In this prospective, multicentre, randomized phase III trial, 612 locally advanced rectal cancer (LARC, T3-4/N+M0) patients with at least one high-risk features (lower location (≤5cm), cT4, cN2, MRF+, EMVI+, TD+) will be included, and randomly assigned to TNT group and iTNT group (1:1). TNT group receives long-course chemoradiotherapy (50Gy/25Fx concurrent with oral capecitabine) followed by 6 cycles of CAPOX. iTNT group receives short-course radiotherapy (25Gy/5Fx) followed by 6 cycles of Serplulimab combined with CAPOX. After the efficacy evaluation, the patients who achieves clinical complete response (cCR) will be managed by a watch and wait (W&W) protocol and non-cCR patients will be recommended surgery. The primary endpoint is 3-year event-free survival rate (3yEFS%). The secondary endpoints include the complete response (CR, pathological complete response [pCR] plus cCR) rate, 3-year organ preservation rate, 3-year disease-free survival rate (3yDFS%), 3-year local recurrence free survival rate (3yLRFS%), 3-year distant metastasis free survival rate (3yDMFS%), 3-year overall survival rate (3yOS%), grade 3-4 acute adverse effects (AE) rate, rate of surgical complications, anal functions and quality of life, etc.

Inclusion Criteria

1. Age 18-75 years old, female and male;
2. Pathological confirmed adenocarcinoma;
3. The distance from anal verge ≤ 10 cm;
4. MSI/MMR status: MSS/pMMR;
5. Clinical stage T3-4 and/or N+, without distance metastases;
6. At least one of the following factors is present: distance from the anus ≤5 cm, cT4, cN2, positive cMRF, positive cEMVI, positive tumor deposit or positive lateral lymph node;
7. KPS ≥ 70;
8. No radiotherapy, chemotherapy, immunotherapy, or any other anti-tumor therapy had been administered prior to enrollment;
9. Baseline blood and biochemical indicators meet the following criteria: neutrophils ≥ 1.5 × 10^9/L, Hb ≥ 90 g/L, PLT ≥ 100 × 10^9/L, ALT/ AST ≤ 2.5 ULN, Cr ≤ 1 ULN;
10. With good compliance and signed the consent form.

Exclusion Criteria

1. Pregnancy or breast-feeding women;
2. Known history of other malignancies within 5 years;
3. Known history of severe neurological or mental illness (such as schizophrenia, dementia or epilepsy);
4. Current severe cardiac disease (cardiac dysfunction and arrhythmia), renal dysfunction and liver dysfunction;
5. Acute cardiac infarction or cerebral ischemic stroke occurred within 6 months before recruitment;
6. Uncontrolled infection which needs systemic therapy;
7. Active autoimmune disease or immunodeficiencies, known history of organ transplantation or systematic use of immunosuppressive agents;
8. Known history of human immunodeficiency virus (HIV) infection (i.e., HIV 1 to 2 antibody positive), active syphilis infection, active pulmonary tuberculosis infection;
9. Allergic to any component of the therapy.