Identification of the human sperm protein that interacts with sperm-immobilizing antibodies in the sera of infertile women

Objective: To identify the target antigen of sperm-immobilizing antibodies present in the circulation of infertile women. Design: Laboratory research. Setting: Academic research laboratory. Patient(s): Twenty-nine infertile women with sperm-immobilizing antibodies, 22 infertile women with other disorders, and 20 fertile women. Intervention(s): Titers of antibodies to the sperm protein, rSMP-B, were determined by ELISA using as substrate the synthetic peptide segment (rSMP-230) that corresponds with the hydrophilic domain of rSMP-B. Tests for sperm immobilization and zona pellucida penetration were performed using the human IVF system. Main Outcome Measure(s): Human sera with sperm-immobilizing activity were assayed for the presence of antibodies to rSMP-230. Polyclonal antibodies to rSMP-230 were assessed for the same biologic activities as sperm-immobilizing antibodies. Result(s): Antibodies to rSMP-230 were detected in 10 (34%) of 29 sera obtained from women with immunologic infertility. In contrast, only one serum sample (2%) from women without sperm-immobilizing activity had a low titer of antibodies to rSMP-230. Polyclonal antibodies to rSMP-230 completely immobilized human sperm in the presence of complement and blocked sperm penetration across the zona pellucida. Conclusion(s): The human sperm protein, rSMP-B, probably is the target antigen of sperm-immobilizing antibodies.