Immune Checkpoint Inhibitors for Organ Preservation in Non-metastatic dMMR/MSI-H Gastric or Colon Cancers

Inclusion Criteria: * Subjects are able to comprehend the informed consent form, and voluntarily sign the informed consent form. * Subjects are ≥18 years old on the day of signing the informed consent form, with no gender restrictions. * Histologically confirmed gastric cancer or colon cancer, without distant metastasis based on CR or MR. * ECOG performance status of 0-2. * dMMR confirmed by immunohistochemistry or MSI-H confirmed by PCR and NGS. If MSI status and MMR status were not consistent, whether to enroll this patient should be determine by investigators. Patients with MMR heterogeneity in tumors could not be included. * Patients who are about to receive or are receiving 24 weeks of PD1/PDL1 antibody monothearpy and have not had the first efficacy assessment. * Archived tumor tissue samples or freshly obtained tumor tissue samples are available. * Female subjects of childbearing potential or male subjects with partners of childbearing potential agree to use highly effective contraception from 7 days before the first dose until 120 days after the last dose. Female subjects of childbearing potential must have a negative serum pregnancy test within 7 days before the first dose. * Subjects have the ability and willingness to comply with the study protocol's visits, treatment plan, laboratory tests, and other study-related procedures. * For patients who are about to receive combination of Sintilimab, IBI310 and Lenvatinib. subjects should have good organ function within the first 7 days of initial dosing: HGB ≥ 80g/L, NEU ≥ 1.0\*10\^9/L, PLT ≥ 75\*10\^9/L, Cr≤1.5×ULN or CrCl≥50mL/min（Cockcroft-Gault method), TBiL ≤ 1.5×ULN, ALT and AST ≤3 ×ULN; urine protein \<2+; if urine protein ≥ 2+, 24 hour urinary protein quantity \<2g; INR, APTT, PT ≤ 1.5 ×ULN Exclusion Criteria: * Distant metastasis; * Previous treatment including CTLA4 blockade; * Subjects with interstitial lung disease or a history of non-infectious pneumonia requiring oral or intravenous corticosteroid treatment. * Subjects with active autoimmune diseases requiring systemic treatment before the start of the study or those considered at risk of recurrence or planned treatment for autoimmune diseases as judged by the investigator. Except for these conditions: a) skin diseases that do not require systemic treatment (e.g., vitiligo, alopecia, psoriasis, or eczema); b) hypothyroidism caused by autoimmune thyroiditis, requiring stable doses of hormone replacement therapy; c) type 1 diabetes requiring stable doses of insulin replacement therapy; d) childhood asthma fully resolved with no need for intervention in adulthood; e) the investigator judges that the disease will not relapse without external triggering factors. * Subjects with a history of other malignant tumors within 5 years, excluding cured skin squamous cell carcinoma, basal cell carcinoma, non-invasive bladder carcinoma, localized low-risk prostate cancer (defined as stage ≤T2a, Gleason score ≤6, and prostate-specific antigen (PSA) ≤10 ng/mL (if measured) in patients who have undergone curative treatment and have no biochemical recurrence of prostate-specific antigen (PSA)), in situ cervical/breast carcinoma, or Lynch syndrome. * Subjects with uncontrolled comorbidities, including but not limited to: a) active HBV or HCV infection; b) subjects who are HBsAg positive and/or HCV antibody positive during screening must undergo HBV DNA and/or HCV RNA testing. Only subjects with HBV DNA ≤500 IU/mL (or ≤2000 copies/mL) and/or HCV RNA negative can be enrolled; HBV DNA monitoring will be at the discretion of the investigator based on the subject's condition during the trial; c) known HIV infection or AIDS history; d) active tuberculosis; e) uncontrolled hypertension (resting blood pressure ≥160/100 mmHg), symptomatic congestive heart failure (NYHA II-IV), unstable angina or myocardial infarction within 6 months, or the presence of QTc prolongation or the risk of arrhythmia (baseline QTc \>470 msec \<Fridericia method correction\>, refractory hypokalemia, long QT syndrome, atrial fibrillation with resting heart rate \>100 bpm, or severe valvular heart disease); f) active bleeding that cannot be controlled after medical treatment. * History of allogeneic bone marrow or organ transplantation. * Previous history of allergic reactions, hypersensitivity reactions, or intolerance to antibody drugs (e.g., severe allergic reactions, immune-mediated hepatotoxicity, immune-mediated thrombocytopenia, or anemia). * Pregnant and/or lactating females. * For patients who are about to receive combination of Sintilimab, IBI310 and Lenvatinib: a) Subjects with a history of gastrointestinal perforation or fistula within 6 months before the first dose. If the perforation or fistula has been treated with resection or repair, and the disease is judged to be recovered or improved by the investigator, then enrollment is allowed. b) Subjects who have undergone major surgery within 28 days before the first dose (e.g., major abdominal or thoracic surgery; excluding drainage, diagnostic puncture, or peripheral vascular access replacement). c) Subjects who require systemic corticosteroids (≥10 mg/day prednisone or equivalent) or immunosuppressive therapy for a continuous 7-day period within 14 days before the first dose. Inhaled or locally applied steroids and physiological replacement doses of steroids due to adrenal insufficiency are allowed. Short-term (≤7 days) corticosteroids for prophylaxis (e.g., contrast dye allergy) or treatment of non-autoimmune diseases (e.g., delayed hypersensitivity reaction caused by exposure to allergens) are allowed. d) Toxicity from previous antitumor treatments has not recovered to Grade ≤2 (NCI-CTCAE v5.0) or baseline, except for alopecia, skin pigmentation (allowed at any level), and immune-related adverse reactions requiring physiological replacement (e.g., hypothyroidism, hypopituitarism, type 1 diabetes).