Neoadjuvant Long-course Chemoradiotherapy Followed by Immunotherapy for Locally Advanced Mid-low Rectal Cancer

Inclusion Criteria: * aged 18\~75 * ECOG score 0\~2 * biopsy diagnosed rectal adenocarcinoma, distal margin within 10cm to anal verge * no distant metastasis, staged II/III (T4b excluded) by MRI * maximum diameter of rectal cancer lesion≥10mm according to baseline CT or MR * willing and able to comply with study protocol * consent to the use of blood and tissue specimens for study * no history of previous anti-tumor treatment (e.g. radiation, chemo, immuno, bio, herbal, etc.) * no disorders/diseases of immune system (e.g. systemic lupus erythematosus, rheumatoid arthritis, systemic vasculitis, scleroderma, pemphigus, dermatomyositis, mixed connective tissue disease, autoimmune hemolytic anemia, hyperthyroidism/hypothyroidism, ulcerative colitis, autoimmune hemolytic anemia, HIV infection, etc.) * no significant dysfunction of major viscera (e.g. heart, lung, liver, kidney, etc.) * no jaundice or gastrointestinal obstruction * no acute/ongoing infection * no significant irregularities in blood routine test and biochemical test results, particular requirements include: neutrophils≥1.5×109/L, HGB≥80g/L, platelet≥100×109/L, serum creatinine≤1.5×ULN, total bilirubin≤1.5×ULN, ALT、AST≤2.5×ULN * no social or mental disorder * for women of child-bearing age, a negative result of serological pregnancy test is required, and effective contraception measures from inclusion till 60 days after the last dose of study drug is required Exclusion Criteria: * multiple cancers, or with concomi…