Antigen Remodeling in Colorectal Cancer: How Radiotherapy and Chemotherapy Enhance Immunotherapy Responsiveness

Colorectal cancer (CRC) is traditionally considered a "cold tumor" characterized by low immunogenicity and limited responsiveness to immune checkpoint inhibitors (ICIs). However, recent findings reveal that cytotoxic modalities can reprogram this immunologically inert landscape. This review integrates these evolving concepts to guide the optimization of future treatments. Radiotherapy induces extensive DNA double-strand breaks, which may generate de novo mutations through error-prone repair while simultaneously exposing cryptic antigens via increased transcriptional instability, alternative splicing, and enhanced proteasomal processing. Chemoradiation also amplifies epigenetic and epitranscriptomic sources of neoepitope diversity, including RNA editing and stress-induced splicing alterations, expanding the immunopeptidome beyond canonical mutation-driven neoantigens. These changes collectively enhance antigen presentation and facilitate T-cell priming. Chemotherapy further reduces immunosuppressive cell populations and promotes dendritic cell activation, creating a permissive milieu for subsequent immune engagement. Clinically, the VOLTAGE studies demonstrated that long-course chemoradiotherapy can sensitize even mismatch repair-proficient rectal cancers to PD-1 blockade, yielding clinically meaningful pathological responses. In contrast, mismatch repair-deficient rectal tumors may respond completely to ICIs alone. Short-course radiotherapy combined with chemotherapy and ICIs has also shown encouraging activity in the setting of total neoadjuvant therapy. Collectively, these findings support a paradigm in which radiotherapy, chemotherapy, and epigenetic/epitranscriptomic alterations-including RNA editing-act as potent modulators of tumor antigenicity. By expanding the neoantigen repertoire and reshaping the tumor microenvironment, these strategies can transform CRC from a cold tumor into one that is increasingly responsive to immunotherapy.