The fertilization antigen (FA-1) causes a reduction of fertility in actively immunized female rabbits

Female rabbits were actively immunized against the fertilization antigen (FA-1) isolated from lithium diiodosalicylate (LIS)-solubilized murine testis. Three trials were performed in order to check the effect of immunization on fertility. In all of these trials, there was a significant (P less than 0.001) reduction of fertility as determined by the percentage of 9-day implants/corpora lutea ratio (FA-1, 0-26.3%; adjuvant control, 79.4-100%). A complete block was observed in animals which received intravenous booster immunization with the antigen. Antisera collected from FA-1-immunized rabbits were negative in the agglutination and the immobilization techniques, and demonstrated modal titers of greater than or equal to 1:2560 in the enzyme linked immunosorbent assay (ELISA) using FA-1. Antisera were tissue-specific and showed binding to the specific protein bands of 47,000 and 23,000 Mr, dimeric and monomeric forms of FA-1, respectively, in the Western blot procedure. Ova collected from rabbits inseminated with sperm which had been treated with antiserum from immunized rabbits showed reduced fertilization rates (anti-FA-1, 3.9-27.7%; control rabbit serum, 87.8%). There was again a reduction in percentage of the 9-day implants/corpora lutea ratio in the rabbits inseminated with treated sperm (anti-FA-1, 10.7%; control rabbit serum, 72.7%). It is concluded that active immunization with FA-1 resulted in a tissue-specific immune response which caused a reduction of fertility in rabbits, by a mechanism(s) involving an inhibition of the fertilization process.