This multicenter, randomized, controlled, phase III trial evaluates whether adding iparomlimab and tuvonralimab injection to CAPOX consolidation chemotherapy after short-course radiotherapy improves tumor response in patients with treatment-naive, proficient mismatch repair/microsatellite-stable (pMMR/MSS) locally advanced rectal adenocarcinoma. Eligible patients will be randomly assigned in a 1:1 ratio to receive short-course radiotherapy followed by CAPOX plus iparomlimab and tuvonralimab, or short-course radiotherapy followed by CAPOX alone.

After total neoadjuvant therapy, patients with a clinical complete response may undergo a Watch-and-Wait strategy, whereas other patients will undergo total mesorectal excision according to standard clinical practice. The primary endpoint is complete response rate, defined as pathologic complete response after surgery or clinical complete response sustained for more than 1 year. Secondary endpoints include 3-year relapse-free survival, 3-year overall survival, sphincter preservation rate, and grade 3-4 acute adverse events. Exploratory analyses will assess tissue and blood biomarkers associated with treatment response.

Inclusion Criteria:

• Patients aged 18-75 years with histologically confirmed pMMR/MSS rectal adenocarcinoma are eligible if they have MRI-defined clinical stage II or III disease according to AJCC 8th edition, tumor located within 12 cm from the anal verge, and at least one high-risk feature, including cT4b, cN2, EMVI positivity, MRF positivity, lateral lymph node positivity, tumor deposits, or tumor located ≤5 cm from the anal verge. Patients must have no distant metastasis, ECOG performance status 0-1, life expectancy greater than 6 months, and adequate hematologic, hepatic, and renal function

Exclusion Criteria:

• active or prior autoimmune disease requiring systemic treatment, use of immunosuppressive therapy or systemic corticosteroids at immunosuppressive doses, severe hypersensitivity to monoclonal antibodies, uncontrolled cardiac disease, significant coagulopathy or bleeding tendency, active infection, interstitial lung disease or severe pulmonary dysfunction, HIV infection or active hepatitis, prior or concurrent malignancy except specified cured cancers, recent investigational drug use, planned use of other systemic antitumor therapy during the study, pregnancy or breastfeeding, and any other condition judged by the investigator to make participation unsuitable.