CD26: A multifunctional integral membrane and secreted protein of activated lymphocytes

CD26 has proved interesting in the fields of immunology, endocrinology, can cer biology and nutrition owing to its ubiquitous and unusual enzyme activi ty. This dipeptidyl aminopeptidase (DPP IV) activity generally inactivates but sometimes alters or enhances the biological activities of its peptide s ubstrates, which include several chemokines. CD26 costimulates both the CD3 and the CD2 dependent T-cell activation and tyrosine phosphorylation of TC R/CD3 signal transduction pathway proteins. CD26 in vivo has integral membr ane protein and soluble forms. Soluble CD26 is at significant levels in ser um, these levels alter in many diseases and soluble CD26 can modulate in vi tro T-cell proliferation. CD26, being an adenosine deaminase binding protei n (ADAbp), functions as a receptor for ADA on lymphocytes. The focus of thi s review is the structure and function of CD26 and the influence of its lig and binding activity on T-cell proliferation and the T cell costimulatory a ctivity of CD26.