title: Distinct antigen MHC class II complexes generated by separate processing pathways.
abstract: The peptide binding site of MHC class II molecules is open at both ends and, therefore, does not restrict the length of the bound ligand. Here we show that a partially folded protein antigen (*HEL) spontaneously formed SDS-unstable complexes with the purified MHC class II molecule I-Ak (Ak). These complexes were also detected on the surface of antigen-presenting cells (APCs) where they stimulated T cells. However, they rapidly disappeared after endocytosis. Intracellular processing of *HEL gave rise to SDS-stable, long-lived Ak complexes containing *HEL peptides and, unexpectedly, full-length *HEL. Both SDS-stable products were formed in low pH compartments and then transported to the plasma membrane. In contrast to *HEL peptides, the stable association of *HEL occurred in an alternative pathway that required mature class II molecules and did not involve HLA-DM or proteases. SDS-stable *HEL-Ak complexes were formed by a reaction of endosomal Ak with endocytosed *HEL, but not by direct conversion of SDS-unstable complexes derived from the plasma membrane. Our work establishes a fundamental difference between the two MHC class II loading pathways and for the first time demonstrates a full-length protein as a product of antigen processing.
authors.affiliation: Department of Pathology, Washington University School of Medicine, St. Louis, MO 63110, USA.
authors.forename: R
authors.initials: R
authors.lastname: Lindner
authors.forename: E R
authors.initials: ER
authors.lastname: Unanue
mesh_terms: D002460:Cell Line; D002738:Chloroquine / Q000494:pharmacology; D000949:Histocompatibility Antigens Class II / Q000378:metabolism*; D007700:Kinetics; D007976:Leupeptins / Q000494:pharmacology; D011485:Protein Binding; D017510:Protein Folding; D011499:Protein Processing, Post-Translational; D012967:Sodium Dodecyl Sulfate
chemical_list: D000949:Histocompatibility Antigens Class II; D007976:Leupeptins; D012967:Sodium Dodecyl Sulfate; D002738:Chloroquine; C032854:leupeptin