N-linked glycan of a sperm CD52 glycoform associated with human infertility

In a benchmark study, Isojima and colleagues established H6-3C4, the first successful heterohybridoma immortalized from the peripheral blood lymphocyt es of an infertile woman who exhibited high sperm-immobilizing antibody tit ers. The present report demonstrates the identity between the glycoprotein antigens recognized by the human H6-3C4 monoclonal antibody (mAb) and the m urine S19 mAb, generated in our laboratory to sperm agglutination antigen-1 (SAGA-1). Both mAb's recognize N-linked carbohydrate epitopes on the 15-25 kDa, polymorphic SAGA-I glycoprotein that is localized to all domains of t he human sperm surface. Treatment with phosphatidylinositol-specific phosph olipase C demonstrated that SAGA-I is anchored in the sperm plasmalemma via a GPI-lipid linkage. Immunoaffinity purification and microsequencing indic ated that the core peptide of the SAGA-1 glycoprotein is identical to the s equence of CD52, a GPI-anchored lymphocyte differentiation marker implicate d in signal transduction. Comparison of anti-SAGA-1 and anti-CD52 immunorea ctivities revealed that the sperm form of CD52 exhibits N-linked glycan epi topes, including the epitope recognized by the infertility-associated H6-3C 4 mAb, which are not detected on lymphocyte CD52. Thus, the two populations of the CD52 glycoprotein on lymphocytes and spermatozoa represent glycofor ms, glycoprotein isoforms with the same core amino acid sequence but differ ent carbohydrate structures. Furthermore, mAb's to the unique carbohydrate epitopes on sperm CD52 have multiple inhibitory effects on sperm function, including a cytotoxic effect on spermatozoa in the presence of complement. These results are the first to implicate unique carbohydrate moieties of a sperm CD52 glycoform as target epitopes in the anti-sperm immune response o f an infertile woman. Furthermore, localization of CD52 on all domains of t he sperm surface coupled with the multiple sperm-inhibitory effects of anti bodies to its unique carbohydrate moieties suggest opportunities for immuno contraceptive development.