MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II MOLECULES INDUCE THE FORMATION OF ENDOCYTIC MIIC-LIKE STRUCTURES

During biosynthesis, major histochompatibility complex class II molecu les are transported to the cell surface through a late endocytic multi laminar structure with lysosomal characteristics. This structure did n ot resemble any of the previously described endosomal compartments and was termed MIIC. We show here that continuous protein synthesis is re quired for the maintenance of MIIC in B cells. Transfection of class I I molecules in human embryonal kidney cells induces the formation of m ultilaminar endocytic structures that are morphologically analogous to MIIC in B cells. Two lysosomal proteins (CD63 and lamp-l), which are expressed in MIIC of B cells, are also present in the structures induc ed by expression of major histocompatibility complex class II molecule s. Moreover, endocytosed HRP enters the induced structures defining th em as endocytic compartments. Exchanging the transmembrane and cytopla smic tail of the class II alpha and beta chains for that of HLA-B27 do es not result in the induction of multilaminar structures, and the chi meric 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 structur es.