Epitope analysis of immunoglobulins against gp20, a GPI-anchored protein of the human sperm surface homologous to leukocyte antigen CD52

Gp20 is a sialylglycoprotein of the human sperm surface related to maturation and capacitation and is homologous to CD52, a glycosylphosphatidyl- inositol (GPI)-anchored protein highly expressed in lymphocytes, monocytes, eosinophils, and epididymal cells, described by the monoclonal antibody family CAMPATH. The CAMPATH antigen is characterized by a very short peptide (12 amino acids) and an N-linked oligosaccharide chain bound to the asparagine located in the third position and a GPI anchor bound to the C-terminal serine. The CAMPATH epitope includes three amino acids at the C-terminus and part of the GPI anchor. It has been suggested that anti-gp20 interacts with the same peptide recognized by CAMPATH antibodies but with a different epitope, since it describes the corresponding antigen in a different way. For example, it localizes the corresponding antigen in the equatorial region of the sperm head when sperm are capacitated, whereas CAMPATH antibodies bind all over the sperm surface. Our results indicate that the anti-gp20 epitope does not include the peptide backbone, the GPI anchor, or the N-glycans but consists of O-linked oligosaccharide chains bound to a unique CD52 glycoform present both in sperm and leukocytes. This is suggested by results obtained using many different approaches, such as immunoblot analysis of gp20 after removal of N- and O-glycans and after jacalin (Artocarpus integrifolia agglutinin)-affinity chromatography.