CD81

CD81, officially named CD81 molecule, is a tetraspanin membrane protein also known as TAPA-1 (target of the antiproliferative antibody 1). It forms tetraspanin-enriched microdomains (TEMs) with partners like CD9, CD63, and integrins, regulating cell adhesion, migration, and immune synapse assembly. As a co-receptor for hepatitis C virus (HCV) E2 protein, it mediates viral entry into hepatocytes. In B cells, it associates with the B-cell receptor (BCR) complex to modulate antigen-dependent activation and signaling.
Dysregulation of CD81 links to HCV infection, B-cell lymphomas (overexpression correlates with aggressive disease), and autoimmune disorders like Sjögren’s syndrome. Therapeutically, anti-CD81 monoclonal antibodies (e.g., bilitacept) are in clinical trials for B-cell-mediated diseases. While HCV treatments now use direct-acting antivirals, CD81 remains a cancer immunotherapy target, with studies exploring its role in enhancing T-cell antitumor responses.