During biosynthesis, major histochompatibility complex class II molecules are transported to the cell surface through a late endocytic multilaminar structure with lysosomal characteristics. This structure did not resemble any of the previously described endosomal compartments and was termed MIIC. We show here that continuous protein synthesis is required for the maintenance of MIIC in B cells. Transfection of class II molecules in human embryonal kidney cells induces the formation of multilaminar endocytic structures that are morphologically analogous to MIIC in B cells. Two lysosomal proteins (CD63 and lamp-1), which are expressed in MIIC of B cells, are also present in the structures induced by expression of major histocompatibility complex class II molecules. Moreover, endocytosed HRP enters the induced structures defining them as endocytic compartments. Exchanging the transmembrane and cytoplasmic tail of the class II alpha and beta chains for that of HLA-B27 does not result in the induction of multilaminar structures, and the chimeric class II molecules are now located in multivesicular structures. This suggests that expression of class II molecules is sufficient to induce the formation of characteristic MIIC-like multilaminar structures.

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