Biochemical and cellular aspects of antigen presentation by human CD1d

Members of the CD1 family of membrane glycoproteins can present antigenic lipids to T lymphocytes. Like MHC class I molecules, they form a heterodimeric complex of a heavy chain and β₂-microglobulin (β₂m) in the ER. Binding of lipid antigens, however, takes place in endosomal compartments, similar to class II molecules, and on the plasma membrane. The studies presented here try to understand the mechanisms governing the assembly of human CD1d in the endoplasmic reticulum (ER) and its intracellular trafficking to lysosomes, as well as identifying the endogenous ligands for CD1d.; CD1d associates in the ER with the chaperones calnexin and calreticulin and with the thiol oxidoreductase ERp57 in a manner dependent on glucose trimming of its N-linked glycans. Complete disulfide bond formation in the CD1d heavy chain was substantially impaired if the chaperone interactions were blocked by glucosidase inhibitors. The formation of at least one of the disulfide bonds in the CD1d heavy chain is coupled to its glucose-trimming-dependent association with ERp57, calnexin and calreticulin.; A subset of human CD1d molecules were found to be associated with MHC class II molecules, both on the cell surface and in the late endosomal/lysosomal compartments where class II molecules transiently accumulate during transport. The interaction is initiated in the ER with class II-invariant chain complexes and appears to be maintained throughout the class II trafficking pathway. A truncated form of CD1d which lacks its cytoplasmic domain and therefore the YXXZ internalization motif fails to be transported to late endosomal/lysosomal compartments in a mutant B cell line which lacks MHC class II expression. However, the same CD1d deletion mutant is targeted to lysosomal compartments in B cell lines expressing class II molecules and HeLa cells expressing class II molecules and invariant chain by transfection. The deletion mutant was also found in lysosomal compartments in HeLa cells expressing only the p33 form of the invariant chain. These data suggest that the intracellular trafficking pathway of CD1d may be altered by class II molecules and invariant chain induced during inflammation.; By using soluble secreted and ER-retained CD1d molecules, phosphatidylinositol was identified as a natural ligand that CD1d binds in the ER in vivo .