TAPBPL (TAP binding protein like) encodes TAPBPR, a peptide editor in the antigen processing and presentation pathway that binds to MHC class I molecules coupled with β2-microglobulin 1. TAPBPR functions as a second peptide editor alongside tapasin, shaping the final peptide repertoire displayed on cell surfaces 2, 3. The protein contains an IgC domain critical for MHC class I binding; alternatively spliced isoforms lacking exon 5 lose this binding capacity 1. Beyond its classical role in antigen presentation, TAPBPL functions as a novel T cell co-inhibitory molecule that negatively regulates T cell proliferation, activation, and cytokine production 4. Disease relevance spans multiple conditions: TAPBPL was identified as a TWAS risk gene in atopic dermatitis within inflammatory pathways 5, shows altered cerebrospinal fluid levels in restless legs syndrome 6, and demonstrates upregulation in multiple sclerosis immune cells 7. Therapeutically, recombinant TAPBPL protein ameliorated collagen-induced arthritis in mice by reducing activated CD4 T cells and promoting regulatory T cells 8. These findings suggest TAPBPL has potential for treating autoimmune diseases through dual mechanisms: optimizing antigen presentation and suppressing pathogenic T cell responses.