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 until 60 days after the last dose of study drug Exclusion Criteria: * History of multiple primary cancers or concomitant malignant tumors other than rectal cancer * Receipt of any anti-cancer treatment (e.g., surgery, chemotherapy, radiotherapy, or other systemic therapies) within the past 5 years * History of recent major surgery * Conditions that may affect the gastrointestinal absorption of capecitabine (e.g., inability to swallow, persistent nausea or vomiting, chronic diarrhea) * Uncontrolled or severe concomitant diseases of any kind * Known allergy or hypersensitivity to any components of the study drugs * Estimated life expectancy ≤ 5 years due to any cause * Planned or prior organ or bone marrow transplantation * Use of immunosuppressive therapy or systemic corticosteroids for immunosuppressive purposes within 1 month prior to enrollment * History of central nervous system disorders that may impair the ability to provide informed consent or affect compliance with oral medication (investigator discretion) * Other conditions that may interfere with study results or lead to premature discontinuation of study treatment (e.g., alcoholism or drug abuse) * Pregnant or breastfeeding women, or women planning to become pregnant during the treatment period