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© The Rockefeller University Press, 0021-9525/1997//1433 $5.00
The Journal of Cell Biology, Volume 139, Number 6, , 1997 1433-1446

Article

Distinct Intracellular Compartments Involved in Invariant Chain Degradation and Antigenic Peptide Loading of Major Histocompatibility Complex (MHC) Class II Molecules



Giorgio Ferrari*, Andrew M. Knight{ddagger}, Colin Watts{ddagger}, and Jean Pieters*

* Basel Institute for Immunology, Grenzacherstrasse 487, Basel, Switzerland; and {ddagger} Department of Biochemistry, University of Dundee, Dundee, UK

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.

Abbreviations used in this paper: CLIP, class II–associated invariant chain peptide; HLA-DM, human leukocyte antigen-DM; Ii, invariant chain; MHC, major histocompatibility complex.

The Basel Institute for Immunology was founded and is supported by F. Hoffmann-La Roche Ltd. (Basel, Switzerland).

Address all correspondence to Jean Pieters, Basel Institute for Immunology, Grenzacherstrasse 487, Postfach, CH 4005 Basel, Switzerland. Tel.: (41) 61 605 12 68. Fax: (41) 61 605 13 64. E-mail: pieters{at}bii.ch


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