CD74, also known as the MHC Class II-associated invariant chain (Ii), is a type II transmembrane glycoprotein that plays a critical role in the antigen presentation process as a chaperone of MHC Class II proteins. It is expressed at high levels on B cells and to a lesser extent on numerous antigen presenting cell (APC) types including dendritic cells, Langerhans cells, monocytes, and macrophages as well as non-traditional APCs such as epithelial cells. CD74 was initially identified for its ability to regulate the folding and intracellular trafficking of newly synthesized MHC Class II molecules. Following expression, CD74 self-assembles as a trimer that serves as a scaffold for the assembly of MHC Class II molecules. Through this interaction, CD74 blocks the peptide binding cleft of MHC Class II molecules and prevents their premature association with endogenous polypeptides. Binding to CD74 also facilitates the translocation of MHC Class II molecules from the endoplasmic reticulum to the endocytic compartments during antigen presentation.
