PDCD1 and inflammatory bowel disease: The PD1/PD-L1 immune checkpoint axis plays a crucial role in peripheral tolerance by attenuating T-cell receptor signaling through SHP2-mediated dephosphorylation of CD3ζ, ZAP70, and CD28, leading to reduced PI3K, AKT, and ERK activation and cytokine release.6 Clinical blockade of this axis can trigger colitis, which resembles IBD, emphasizing its role in intestinal immune homeostasis.7 In the gut, PDL1 expressed by epithelial and myeloid cells contributes to mucosal tolerance, and altered expression correlates with disease activity.