Neoadjuvant Chemoradiotherapy Plus Tislelizumab With or Without Probio-M9 in pMMR/MSS Locally Advanced Rectal Cancer

Inclusion Criteria: * Age ≥ 18 years * ECOG performance status 0-2 * Biopsy-proven rectal adenocarcinoma * Distal tumor margin within 10 cm of the anal verge * No distant metastasis * Clinical stage II or III based on MRI (T4b excluded) * Maximum diameter of rectal tumor ≥ 10 mm on baseline CT or MRI (measurable lesion according to RECIST 1.1) * Willing and able to comply with the study protocol * Willing to provide blood and tissue specimens for research purposes * No prior anti-tumor treatment (e.g., radiotherapy, chemotherapy, immunotherapy, biological therapy, or herbal therapy) * No history of immune system disorders (e.g., systemic lupus erythematosus, rheumatoid arthritis, systemic vasculitis, scleroderma, pemphigus, dermatomyositis, mixed connective tissue disease, autoimmune hemolytic anemia, thyroid autoimmune disease, ulcerative colitis, HIV infection, etc.) * No significant dysfunction of major organs (heart, lung, liver, kidney) * No jaundice or gastrointestinal obstruction * No acute or ongoing infection * Adequate hematologic and biochemical function including: * Neutrophils ≥ 1.5 × 10\^9/L * Hemoglobin ≥ 80 g/L * Platelets ≥ 100 × 10\^9/L * Serum creatinine ≤ 1.5 × ULN * Total bilirubin ≤ 1.5 × ULN * ALT and AST ≤ 2.5 × ULN * No social or psychiatric disorders that may interfere with study participation * Women of childbearing potential must have a negative pregnancy test before enrollment and must use effective contraception from study entry unt…