Direct vesicular transport of MHC class II molecules from lysosomal structures to the cell surface.

Abstract

Abstract. Newly synthesized MHC class II molecules are sorted to lysosomal structures where peptide loading can occur. Beyond this point in biosynthesis, no MHC class II molecules have been detected at locations other than the cell surface. We studied this step in intracellular transport by visualizing MHC class II molecules in living cells. For this purpose we stably expressed a modified HLA-DR1 13 chain with the Green Fluorescent Protein (GFP) coupled to its cytoplasmic tail (13-GFP) in class II-expressing Mel JuSo cells. This modification of the class II 13 chain does not affect assembly, intracellular distribution, and peptide loading of the MHC class II complex. Transport of the class II/ 13-GFP chimera was studied in living cells at 37°C. We visualize rapid movement of acidic class II/13-GFP containing vesicles from lysosomal compartments to the plasma membrane and show that fusion of these vesicles with the plasma membrane occurs. Furthermore, we show that this transport route does not intersect the earlier endosomal pathway. MHC class II molecules present peptides to CD4 ÷ T cells. Most bound peptides are derived from antigens degraded in the endosomal pathway. To allow association with these peptides, class II molecules are targeted to endosomal compartments by the invariant chain (or Ii) 1 (7,8). Here, Ii is degraded and Ii-degradation products are exchanged with antigenic peptides, a process catalyzed by 41,47,48). The endosomal compartments where class II molecules are loaded with peptide may be considered "special" lysosomes with a multilamellar and/or multivesicular appearance and were originally termed MIIC for MHC class II--containing compartments (31). This "unique" morphology appears to be induced by the expression of class II molecules (4). Although earlier endosomal compartments have been noted as "specialized class II loading compartments" as well (1, 50), HLA-DM and class II molecules are generally located in compartments with lysosomal proteins like CD63, lamp-l, and cathepsin D (13,24,32,42).

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Rab7: A Key to Lysosome Biogenesis

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