CTLA-4 as a Double-Edged Sword in Colorectal Cancer: Molecular Insights into Immune Checkpoint Suppression and Therapeutic Targeting.

Colorectal cancer (CRC) remains a major global health burden and ranks third among the most commonly diagnosed malignancies and the second leading cause of cancer-related mortality worldwide. Its development arises through successive molecular and cellular alterations driven by cumulative genetic mutations, environmental exposure, and immune dysregulation. Among immune regulators, cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) plays dual roles in CRC progression. On one hand, CTLA-4 preserves immune homeostasis by restraining excessive T-cell activation through competitive binding with CD80/CD86. This regulatory function is mediated by attenuated T-cell receptor signaling, dominance over CD28-mediated co-stimulation, and sustained activity of regulatory T cells to maintain peripheral tolerance. On the other hand, this inhibitory mechanism facilitates tumor immune evasion and contributes to cancer progression by limiting effector T-cell proliferation, reducing cytotoxic activity, and promoting immune exhaustion within the tumor microenvironment. Therapeutically, antibodies such as ipilimumab and tremelimumab leverage these roles by blocking CTLA-4 to restore antitumor immunity. However, the downstream signaling cascades following CTLA-4 activation and the detailed molecular basis of tremelimumab-mediated modulation are not yet fully understood. More detailed understanding of these dual-role mechanisms through integrative molecular, cellular, and structural analyses may help refine immune checkpoint blockade strategies and improve therapeutic outcomes in CRC patients.