PROBLEM: To determine whether complement regulatory proteins are present on human spermatozoa and whether antibodies to these proteins adversely affect sperm motility.

METHOD OF STUDY: Human sperm membrane proteins were solubilized and subjected to Polyacrylamide gel electrophoresis followed by Western blot analysis against antibodies to complement component 1 inhibitor (C1‐INH), decay‐activating factor (DAF; CD55), membrane cofactor protein (MCP; CD46), and homologous restriction factor (HRF; CD59). Spermatozoa, obtained by a swim‐up technique, were incubated in medium (control 1) and medium supplemented with antibodies to human albumin (control 2) and antibodies to these complement regulatory proteins. We used a computerized sperm motion analysis to determine the effect of these antibodies on sperm motion characteristics.

RESULTS: Complement regulatory proteins such as C1‐INH, CD55, CD46, and CD59 were found in the sperm extracts as shown by Western blot analysis. The heat‐treated (56°C, 30 min) IgG fraction of antibodies to these proteins significantly reduced sperm motility in general and other motion parameters. Addition of complement did not affect these results except in the antibodies to CD46 in which the reducing action was further amplified.

CONCLUSIONS: Our data suggest that C1‐INH, CD55, CD46, and CD59 are present on the sperm surface. These proteins may have biological functions, such as affecting sperm motility, besides the complement regulatory functions. In infertile men and women with antibodies that recognize one or more of these complement regulatory proteins, there may be problems related to poor sperm motility and survival in the reproductive tracts.