Intracellular Pathway for the Generation of Functional MHC Class II Peptide Complexes in Immature Human Dendritic Cells

Abstract

Abstract Binding of antigenic peptides to MHC class II (MHC-II) molecules occurs in the endocytic pathway. From previous studies in B lymphocytes, it is believed that most but not all of the newly synthesized MHC-II molecules are directly targeted from the trans-Golgi network to endosomal compartments. By using pulse-chase metabolic labeling followed by cell surface biotinylation, we show here that in contrast to an EBV-transformed B cell line and human monocytes, the majority of newly synthesized MHC-II molecules (at least 55 ± 13%) are first routed to the plasma membrane of dendritic cells derived from human monocytes. They reach the cell surface in association with the invariant chain (Ii), a polypeptide known to target MHC-II to the endosomal/lysosomal system. Following rapid internalization and degradation of Ii, these αβIi complexes are converted into αβ-peptide complexes as shown by their SDS stability. These SDS-stable dimers appear as soon as 15 to 30 min after internalization of the αβIi complexes. More than 80% of αβ dimers originating from internalized αβIi complexes are progressively delivered to the cell surface within the next 2 h. Depolymerization of microtubules, which delays the transport to late endosomal compartments, did not affect the kinetics of conversion of surface αβIi into SDS-stable and -unstable αβ dimers. Altogether, these data suggest that newly liberated class II αβ heterodimers may bind peptides in different compartments along the endocytic pathway in dendritic cells derived from human monocytes.