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Although female sterility was induced in experimental animals by active immunization with sperm (1, 2), and antibodies that react with human sperm were found to be present in sera of infertile women (3), no strong correlation has been found between the presence of such antibodies and naturally occurring human female infertility (4, 5). However, Iscjima et al. (6) reported the novel finding that sera of infertile women inactivate human sperm motility in the presence ofguinea pig complement. Approximately 17% of infertile women with unknown cause had antibodies in their sera that inactivated human sperm motility in the presence of complement . No such antibody was detectable in sera of parous women. These findings led to a generally accepted hypothesis that the presence of such antibodies in sera of infertile women can be regarded as the cause of femal infertility (7). Thus, the specificities and immunobiological properties of antibodies present in sera of infertile women showing complement-dependent inactivation of sperm motility are of crucial importance for understanding the biological mechanism of female infertility. However, the activity of such antibodies in sera of infertile women is generally lower than a sperm immobilization titer of 1 :200 (200 SI5o) (8), and certain serum components of normal subjects showed nonspecific binding with human sperm. Therefore, immunobiological studies on infertility have been difficult to perform in further detail . To overcome this difficulty, a human mAb H6-3C4, inducing complement-dependent sperm immotility, was established after fusion of lymphocytes from an infertile patient (patient MI), showing high-titer antisperm antibodies, with mouse myeloma NSl (9). The antibody is an IgM with X light chain, and reacts with ejaculated sperm, but not with testicular sperm. The antibody showed a strong inactivation of human sperm motility in the presence of complement . The binding of H6-3C4 antibody to sperm was inhibited by the serum antibodies of patient MI; thus the serum antibodies of MI and antibody H6-3C4 must be directed to the same epitope structure present at the surface of ejaculated human sperm (9). It is essential, therefore, to elucidate the chemical properties of antigen defined by mAb H6-3C4.

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