CD22 antigen: biosynthesis, glycosyiation and surface expression of a B lymphocyte protein involved in B cell activation and adhesion

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Abstract
The CD22 antigen, although present In the cytoplasm of early and Immature B cells, first appears on the cell surface of mature B lymphocytes. The phenotypic patterns and some functional properties of the surface-expressed CD22 antigen are well described, but little Is known about Its molecular structure. We have therefore Investigated the relationship of thetwo CD22 glycoproteins (140/130 kd), and the influence of their complex N-gtycosylatlon on surface expression and antibody recognition. Comparative peptide mapping of the 100 and 80 kd protein cores, obtained by endoglycosidase F treatment, revealed a common structure shared by both protein cores. In pulse-chase experiments the mature glycoproteins originated from two separate precursor molecules, Indicating that the two proteins may be generated by different RNA processing. In cell lysates a CD22 specific polyclonal anti-serum recognized both molecules the size of the protein cores and glycosylated CD22 molecules, whereas In membrane preparations only the glycosylated forms were detected. The CD22 mAb HD39 reacted exclusively with the glycoprotein froms in either cellular preparation. The Influence of glycosyiation on surface expression and epitope recognition was Investigated in more detail by applying various Inhibitors of the glycosylation pathway. 1-Deoxymannojirlmycin and Swalnsonlne, which block glycosylation at the hlgh-mannose and hybrid-type stages respectively, modulated the CD22 antigen but did not alter its surface expression. Tunicamycin blocked de novo glycosyiation and led to reduced surface recognition of the CD22 antigen. Together, these results suggested that comparatively simple oligosaccharide structures of hlgh-mannose type are sufficient for surface expression of the CD22 antigen and for epitope recognition by mAb HD39. It is most likely that glycosylation is required to stabilize epitopes In the protein moiety recognized by CD22 mAb. Finally, we demonstrated the presence of glycosylated, cytoplasmic CD22 antigen In CD22 surface negative B-llneage ALL cells. This finding led us to conclude that complex glycosylation does not provide the determining signal for the switch from cytoplasmic to surface expression of the CD22 antigen.

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