PD-1 Antibody Following Preoperative Chemoradiotherapy for Locally Advanced pMMR/MSS Rectal Cancer

Inclusion Criteria:

* Patients who personally provided written consent for participation in the study
* Treatment-naive patients with rectal cancer, in whom the inferior margin of the tumor was at a distance of 12 cm or less from the AV before CRT
* Primary rectal cancer histopathologically confirmed to be adenocarcinoma
* Clinical stage of T3,and T4 ,N any,M0,before CRT
* Macroscopic radical resection could be feasible, based on diagnostic imaging before CRT
* Patients with the ECOG performance status of 0 or 1 at the time of enrollment
* Patients without distant metastasis on the imaging test before CRT
* Preoperative biopsy of tumor immunohistochemistry TPS\>1% or CPS\>1
* Life expectancy of greater than 2 years
* No signs of intestinal obstruction; or the obstruction has been relieved after the proximal colostomy operation
* Hematology: WBC\>4000/mm3; PLT\>100000/mm3; Hb\>10g/dL
* Liver function: SGOT and SGPT are less than 1.5 times the normal value; bilirubin is less than 1.5mg/dL
* Renal function: creatinine \<1.8mg/dL Others: non-pregnant or breast-feeding women; no other malignant diseases (except for non-melanoma or cervical carcinoma in situ) within 5 years or during the same period; no mental illness that causes the inability to obtain informed consent; no other serious diseases that can shorten the survival time disease.
* Have not received rectal surgery in the past;
* Have not received chemotherapy or radiotherapy in the past;
* Have not received biological treatment in the past;
* Past endocrine therapy: unlimited.

Exclusion Criteria:

* Rectal cancer with unstable microsatellite (MSI or dMMR);
* Preoperative biopsy of tumor immunohistochemistry TPS≤1% or CPS≤1
* Known history of human immunodeficiency virus (HIV) or chronic hepatitis B or C (high copy viral DNA);
* Autoimmune diseases;
* Other active clinical serious infections (\>NCI-CTC version 3.0);
* Patients in clinical phase I;
* There is evidence that there is distant metastasis before surgery;
* Cachexia, decompensation of organ function;
* Have a history of pelvic or abdominal radiotherapy;
* Multiple primary cancers;
* Patients who need treatment for seizures (such as steroids or anti-epileptic treatment);
* Have a known additional malignant tumors within 5 years. Exceptions include basal cell carcinoma of the skin that has undergone potentially curative therapy or in situ cervical cancer;
* Chronic inflammatory bowel disease, intestinal obstruction;
* Drug abuse and medical, psychological or social conditions may interfere with patients' participation in research or have an impact on the evaluation of research results;
* Known or suspected to be allergic to the study drug or to any drug given in connection with this test;
* Any unstable conditions or situations that may endanger patient safety and compliance