The effect of active immunity against placental proteins on pregnancy in monkeys.

Squirrel monkeys were actively immunized against human placental lactogen (HPL) and/or an extract of human placental tissue, or squirrel monkey placenta. Only half of the monkeys immunized with HPL developed a detectable antibody titer to HPL prior to the subsequent mating season. During the subsequent breeding season, the pregnancy rate was cut in half. The titer developed against HPL did not seem to be related to their ability to become pregnant. When the group of animals which had previously been immunized with HPL were challenged with extracts of human placenta, all animals developed detectable circulating antibody. There was no increase in the effectiveness of this immunization as compared to HPL in pregnancy prevention, however. A third group of monkeys were immunized with extracts from their own placental extracts, and all developed some degree of antibody. The titer obtained did not seem to be related to whether or not they became pregnant. In this group, however, the pregnancy rate was decreased to one fourth of what it had been before immunization.