CD13

This chapter focuses on the tissue distribution, structure, and functions of the leucocyte antigen CD13. CD13 is expressed by granulocytes and monocytes and their precursors. CD13 is a marker for most acute myeloid leukaemias and a smaller proportion of acute lymphoid leukaemias. Various nonhematopoietic cells express CD13, including epithelial cells from renal proximal tubules and intestinal brush border, endothelial cells, fibroblasts, brain cells, bone marrow stromal cells, osteoclasts, and cells lining the biliary caniculae. CD13 is a member of a group of type II integral membrane metalloproteases that includes the leucocyte antigens CD10, CD26, CD73, and BP-1. The CD13 glycoprotein has a short N-terminal cytoplasmic tail, a transmembrane region that functions as a signal peptide, and a large C-terminal extracellular region that contains 11 N-linked glycosylation sites and also O-linked glycosylation. CD13 is a receptor for coronaviruses, RNA viruses that cause respiratory diseases in humans and several species of animals. The binding site on CD 13 for the swine coronavirus TGEV (transmissible gastroenteritis virus) is distinct from the enzymatic site. CD13 is a zinc-binding metalloprotease that plays a role in cell surface antigen presentation by trimming the N-terminal amino acids from MHC Class II-bound peptides.