The mechanism of action of an antifertility vaccine in the rhesus monkey: reversal of the effects of antisera to the beta-subunit of ovine luteinizing hormone by medroxyprogesterone acetate.

Active immunization of female rhesus monkeys with the beta-subunit of ovine luteinizing hormone )oLH beta) significantly reduced their fertility. To determine whether the major action of the vaccine was interruption of pregnancy, by suppression of "corpus luteum rescue," or inhibition of ovulation, we administered the progestational agent medroxyprogesterone acetate (MPA) from day 4 through day 40 after mating. In the untreated immunized group, the pregnancy rate was significantly below that of control monkeys. MPA treatment restored the fertility rate of immunized animals to that of the control group. These results strongly support the assumption that the antifertility action of antibodies of oLH beta is due to prevention of corpus luteum rescue. Whether the lack of corpus luteum rescue resulted because of neutralization of rhesus monkey chorionic gonadotropin or from a defective corpus luteum, as is found in animals with short luteal phases, cannot be determined from these studies. The successful reversal of the antifertility effect, however, suggests that the circulating antibodies do not interfere with normal ovulation or with the normal development and implantation of the blastocyst.