Pregnancy suppression in the bonnet monkey by active immunisation with chicken riboflavin carrier protein.

Five healthy female bonnet monkeys (Macaca radiata) of proven fertility and normal menstrual cyclicity have been actively immunised with the purified chicken egg white riboflavin carrier protein (cRCP). All the immunised animals exhibited specific antibodies to cRCP and the immunopotencies of their sera varied from 200 to 840 micrograms/ml at equivalent point. A definite fraction of antibodies in these sera specifically recognised the purified and 125I-labelled monkey RCP. Immunisation per se had no adverse effect on the animals' menstrual cyclicity, circulating levels of estrogen and progesterone and the riboflavin status as reflected by glutathione reductase activities and the total flavin contents of the erythrocytes. The fertility of these animals was monitored for a period extending up to 3 years after primary immunisation. Four out of the five animals exhibited termination of pregnancy once or more than once depending on their antibody titers. Towards the end of the study period, when the immune response was poor, all the animals delivered normal babies at term. Circulating anti-cRCP antibodies were monitored by 125I-labelled cRCP binding. The results show that pregnancy termination, owing to immuno-neutralisation of monkey RCP, occurred only in animals which had sufficiently high antibody titers. If the titers fell below a critical threshold level the pregnancies were carried to term.