The biogenesis of the MHC class II compartment in human I-cell disease B lymphoblasts

The localization and intracellular transport of major histocompatibility complex (MHC) class II molecules and lysosomal hydrolases were studied in I- Cell Disease (ICD) B lymphoblasts, which possess a mannose 6-phosphate (Man- 6-P)-independent targeting pathway for lysosomal enzymes. In the trans-Golgi network (TGN), MHC class II-invariant chain complexes colocalized with the lysosomal hydrolase cathepsin D in buds and vesicles that lacked markers of clathrin-coated vesicle-mediated transport. These vesicles fused with the endocytic pathway leading to the formation of 'early' MHC class II-rich compartments (MIICs). Similar structures were observed in the TGN of normal β lymphoblasts although they were less abundant. Metabolic labeling and subcellular fractionation experiments indicated that newly synthesized cathepsin D and MHC class II-invariant chain complexes enter a non-clathrin- coated vesicular structure after their passage through the TGN and segregation from the secretory pathway. These vesicles were also devoid of the cation-dependent mannose 6-phosphate (Man-6-P) receptor, a marker of early and late endosomes. These findings suggest that in ICD B lymphoblasts the majority of MHC class II molecules are transported directly from the TGN to 'early' MIICs and that acid hydrolases can be incorporated into MIICs simultaneously by a Man-6-P-independent process.