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* Basel Institute for Immunology, Grenzacherstrasse 487, Basel, Switzerland; and Major histocompatibility complex (MHC)
class II molecules are transported to intracellular MHC
class II compartments via a transient association with
the invariant chain (Ii). After removal of the invariant
chain, peptides can be loaded onto class II molecules, a
process catalyzed by human leukocyte antigen-DM
(HLA-DM) molecules. Here we show that MHC class
II compartments consist of two physically and functionally distinct organelles. Newly synthesized MHC class II/Ii complexes were targeted to endocytic organelles
lacking HLA-DM molecules, where Ii degradation occurred. From these organelles, class II molecules were
transported to a distinct organelle containing HLA-DM, in which peptides were loaded onto class II molecules. This latter organelle was not directly accessible
via fluid phase endocytosis, suggesting that it is not part
of the endosomal pathway. Uptake via antigen-specific
membrane immunoglobulin resulted however in small
amounts of antigen in the HLA-DM positive organelles. From this peptide-loading compartment, class
II-peptide complexes were transported to the plasma
membrane, in part after transit through endocytic organelles. The existence of two separate compartments, one involved in Ii removal and the other functioning in
HLA-DM-dependent peptide loading of class II molecules, may contribute to the efficiency of antigen presentation by the selective recruitment of peptide-receptive MHC class II molecules and HLA-DM to the same
subcellular location.
Department of Biochemistry, University of
Dundee, Dundee, UK
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