Pregnancy-blocking progesterone antibody targets specifically the uterus through its progesterone-binding sites

Passive immunization with a mouse monoclonal antibody against progesterone, designated DB3, blocks pregnancy in several species. We have previously reported that DB3 localizes in the mouse uterine epithelium shortly before normal implantation. This phenomenon is pregnancy dependent and specific for the progesterone antibody. In this study we demonstrate that DB3 is present in the lumen of the uterus 36 h after an i.p. injection; this correlates with the time of maximum antibody reaction on the uterine epithelium. Incubation of DB3 with free progesterone, progesterone-hemisuccinate or progesterone-bovine serum albumin before administration prevented its localization on the epithelium, indicating that the localization requires free progesterone-binding sites and thus probably depends upon progesterone binding. In addition, studies in vitro show that DB3 can effectively bind to progesterone carried by high-affinity progesterone-binding protein purified from coypu plasma. We suggest that specific targeting of DB3 may be through progesterone associated with a progesterone-binding molecule on the membrane of the uterine epithelia. This may be an important part of the mechanism of antibody action against implantation.