Biochemical Characterization of CD1d Expression in the Absence of β2-Microglobulin

CD1d is a major histocompatibility complex class I-like molecule that exhibits a distinct antigen processing pathway that functions in the presentation of hydrophobic antigens to T cells. CD1d has been previously shown to be expressed on the cell surface of human intestinal epithelial cell lines in vivo and a transfected cell line in vitro independently of beta2-microglobulin (beta2m). To define the relationship between CD1d and beta2m and characterize the biochemical structure of CD1d in the absence of beta2m, we have used a newly generated series of CD1d transfectants and CD1d-specific antibodies. These studies show that in the absence of beta2m, CD1d is expressed on the cell surface as a 45-kDa glycoprotein that is sensitive to endoglycosidase-H and is reduced to 37-kDa after N-glycanase digestion. In contrast, in the presence of beta2m, CD1d is expressed on the cell surface as a 48-kDa endoglycosidase-H-resistant glycoprotein. Pulse-chase metabolic labeling studies demonstrate that acquisition of endoglycosidase-H resistance of CD1d is observed in the presence of beta2m but not in the absence of beta2m even after a 24-h chase period. Thus, CD1d is able to be transported to the cell surface independently of beta2m; however, in the absence of beta2m, the glycosylation pattern of CD1d is altered and consistent with an immature glycoprotein.