Introduction:
CD26 (dipeptidyl peptidase IV, DPP IV), adenosine deaminase (ADA) binding protein) is a homodimeric atypical serine protease belonging to the prolyl oligopeptidase family. CD26 is expressed on lymphocyte cells and is upregulated during T-cell activation. CD26 is also expressed on activated B cells and natural killer cells and abundantly on epithelia. CD26 is implicated in a variety of biological functions including T-cell activation, cell adhesion with extracellular matrix such as fibronectin or collagens, and in HIV infection. CD26 identical to adenosine deaminase complexing protein-2, and to the T-cell activation antigen CD26. Further, CD26 is an intrinsic membrane glycoprotein and a serine exopeptidase that cleaves X-proline dipeptides from the N-terminus of polypeptides. Alterations in CD26 peptidase activity are characteristic of malignant transformation, and the enzymatic activity increases dramatically with tumor grade and severity. CD26 is expressed in various blood cell types, but also in cells that are histogenetically related to activated fibroblasts. Alterations in CD26 density have been reported on circulating monocytes and CD4+ T cells during rheumatoid arthritis and systemic lupus erythematosus.
