Characterization of fertilization-blocking monoclonal antibody 1G12 with human sperm-immobilizing activity

Antigenic epitope for sperm-immobilizing antibody detected in infertile women

CD52 is a glycosylphosphatidylinositol (GPI) anchor protein occurring in lymphocytes, the epididymis, seminal plasma and on ejaculated sperm surface. The molecular structure of male reproductive tract CD52 (mrtCD52) is quite different from that of lymphocyte CD52 except for a peptide sequence comprising 12 amino acids. The carbohydrate molecule may thus provide antigenic epitopes to females. The repeated N-acetyllactosamine units of the N-linked carbohydrate present in the mrtCD52 were identified as a pathogenic antigen for infertility using a monoclonal antibody (MAb H6-3C4) generated from an infertile patient's peripheral blood lymphocyte. We suggested also the possible presence of O-linked carbohydrate in human mrtCD52, although it is not known whether this carbohydrate is immunogenic or not. This article gives an overview of the structure and function of mrtCD52 as well as the fertilization-blocking mechanism of the antibody against this molecule.

Isolation and characterization of a human sperm antigen gene h-Sp-1

We isolated and characterized a human sperm antigen gene (h-Sp-1) from human testis complementary DNA using antiserum against the human sperm membrane. Northern blot analysis detected two transcripts (2.3 and 1.1 kb) of the h-Sp-1 gene. The 2.3-kb transcript is ubiquitous, whereas the 1.1-kb transcript is specific to the human testis with a high level of expression. Determination of the base sequence of h-Sp-1 showed a size of 2170 bp and 43.4% homology with human synaptophysin. The base sequence indicates a molecule consisting of 259 amino acids, with four hydrophilic and four hydrophobic regions. In order to further characterize the h-Sp-1 molecule, we synthesized the probable region of amino acids with high antigenicity based on the amino acid sequence (amino acid nos. 174-198) and immunized rabbits to prepare an antiserum. In our experimental model of fertilization between human sperm and zona pellucida-free hamster ova, partial inhibition of fertilization was observed. We were able to synthesize a large quantity of recombinant protein by inserting the h-Sp-1 gene into a baculovirus vector and infecting spodoptera frugiperda culture cells (sf9 insect cells). The synthesized protein had a molecular weight of 30 kDa. We then immunized Balb/c mice with this protein to prepare a monoclonal antibody (G3G9), which was used to localize the h-Sp-1 molecule in sperm and tissues (e.g. testis). The h-Sp-1 molecule was present in the cell membrane from the head to tail of human sperm. Staining of the testis and epididymis also showed h-Sp-1 to be present in spermatogonia, spermatocyte, sperm and epididymal duct epithelium. These findings suggest that the h-Sp-1 molecule is expressed in sperm and testes and plays a role in fertilization.