Low Dose MSH2-/- tumor cell vaccine

The goal of this clinical trial is to evaluate the safety and tolerability of an MSH2-/- tumor cell vaccine and to explore its preliminary antitumor activity and immunogenicity in adults with advanced proficient mismatch repair (pMMR) colorectal cancer who have failed, are intolerant of, or decline standard systemic therapies at West China Hospital, Sichuan University. The main objectives are to determine the incidence of dose-limiting toxicities (DLTs) and other treatment-emergent adverse events (TEAEs) related to the vaccine (n/N, %, graded per NCI CTCAE v5.0), to assess preliminary antitumor activity (objective response per RECIST v1.1, progression-free survival, and overall survival), and to characterize the vaccine's immunogenicity profile. This study using a 3+3 dose-escalation design with three dose levels of the MSH2-/- tumor cell vaccine (1×10\^7, 2.5×10\^7, and 5×10\^7 cells per dose), manufactured under GMP conditions and administered by intradermal injection. Each participant will receive four induction vaccinations (three doses every 2 weeks and a fourth dose 1 month after the third), followed by up to eight booster doses every 4 weeks based on tumor response. Participants will undergo protocol-specified safety monitoring with clinical assessments, laboratory tests, and documentation of all AEs/SAEs, and tumor response will be evaluated regularly by imaging per RECIST v1.1. After treatment completion or discontinuation, participants will enter safety and long-term follow-up for disease status and survival.