The transporter associated with antigen processing (TAP): Structural integrity, expression, function, and its clinical relevance

Background: The transporter associated with antigen processing (TAP), a mem ber of the family of ABC transporters, plays a crucial role in the processi ng and presentation of the major histocompatibility complex (MHC) class I r estricted antigens. TAP transports peptides from the cytosol into the endop lasmic reticulum, thereby selecting peptides matching in length and sequenc e to respective MHC class I molecules. Upon loading on MHC class I molecule s, the trimeric MHC class I/beta (2)-microglobulin/ peptide complex is then transported to the cell surface and presented to CD8(+) cytotoxic T cells. Abnormalities in MHC class I surface expression have been found in a numbe r of different malignancies, including tumors of distinct histology, viral infections, and autoimmune diseases, and therefore represent an important m echanism of malignant or virus-infected cells to escape proper immune respo nse. In many cases, this downregulation has been attributed to impaired TAP expression, which could be due to structural alterations or dysregulation. This review summarizes the physiology and pathophysiology of TAP, thereby focusing on its function in immune responses and its role in human diseases .