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Eur J Immunol. 2005 Sep;35(9):2552-62.

In vivo control of endosomal architecture by class II-associated invariant chain and cathepsin S.

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Department of Pathology, Harvard Medical School, Boston, MA 02115, USA.


The invariant chain (Ii) is a chaperone that regulates assembly and transport of class II MHC molecules. In the absence of the lysosomal protease cathepsin S (CatS), degradation of Ii is impaired and an Ii remnant that extends from the N terminus to about residue 110 accumulates in class II MHC-positive endosomal compartments, which are enlarged in size and lack multivesicular morphology. In primary B cells examined in vitro and in lymph nodes examined by immuno-electron microscopy, CatS controls architecture of class II-positive endosomal compartments. In a compound mutant mouse that lacks both CatS and Ii, the normal size of endosomes in class II-positive cells is restored, although internal endosomal membranes are absent. Proper degradation of Ii is thus essential for normal endosomal morphology in antigen-presenting cells in vivo.

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