PROBLEM: An 88–92-kDa C1-inhibitor-like molecule (C1-INH-L) was previously identified to elicit cytotoxic sperm antibody response in infertile men and women. Here, we document that it is present on the human sperm surface and could be detected by an enzyme-labeled immunoglobulin G (IgG) fraction of anti-human C1-INH antibody. METHOD OF STUDY: Western blot analysis, enzyme-lined immunoadsorbent assay (ELISA) and computerized sperm motion analysis.
RESULTS: The existence of C1-INH-L on the sperm surface is calcium independent. Phosphatidylinositol-specific phospholipase C (PIPLC), EDTA, and acid (pH 3.0) could not remove the C1-INH-L from sperm, but trypsin did. Activated C1s was able to bind to the sperm surface. Immunofluorescence studies localized the protein to the head and midpiece of the sperm membrane. The C1-INH-L exists on both uncapacitated and capacitated sperm surfaces, which suggests that this protein is a sperm-surface protein. The heat-treated (56°C, 30 min) IgG fraction of anti-C1-INH greatly reduced the percentage of motile spermatozoa and the progressive and path velocities in the absence of complement. CONCLUSION: Our data suggest that C1-INH is a sperm membrane-anchored protein that may have complement and sperm motility regulatory function.