Tissue distribution

The distribution of the CD52 antigen is one of its most distinctive features. In humans the antigen is abundantly expressed on virtually all lymphocytes at most stages of differentiation (except plasma cells)

Rat OQ-----Jji,S?T A VTTPANK A ATTA A ATTK A A ATT ATKTTTA VR KTPTrK

Figure 2 Comparison of CD52 protein sequences from different species showing the diversity of the mature sequences. Signal peptides (not shown) are much more similar. The human peptide has a single N-linked oligosaccharide. The rat peptide (SMemG) probably has, in addition, up to 14 O-linked oligosaccharides attached to the various Thr residues.

and also on monocytes, macrophages and eosinophils. T cells have approximately 450 000 molecules per cell. The antigen is not found on any other tissues except the male reproductive tract, where it is expressed very strongly on epithelial cells lining the epididymis, vas deferens and seminal vesicle. Antigen is shed from these cells in large quantities into seminal plasma, where it exists in small micellar particles and in larger membrane-bound structures called pro-stasomes. From these, it can be transferred to other cells, particularly spermatozoa. The CD52 antigen, along with other GPI-anchored antigens such as CD55 and CD59, is a major membrane protein of sperm, acquired during maturation. It is a matter of speculation whether transfer of GPI-anchored molecules from one cell to another is of more general physiological significance. Genes homologous to human CD52 have been described in monkey, dog, rat and mouse. They are all expressed in the male reproductive tissues and (where tested) in lymphocytes. Although the genes have substantial homology in the nontranslated regions and signal peptides, the short mature peptides are quite different (Figure 2).

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