Human Chorionic Gonadotropin Stimulates Luteal Function in Rhesus Monkeys Immunized against the β-Subunit of Ovine Luteinizing Hormone*

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Abstract
Active immunization against the β-subunit of ovine (o) LH (oLHβ) reduced fertility in rhesus monkeys significantly without interfering with ovulation and regular menstrual cycles. In immunized monkeys, there was an absence of corpus luteum rescue (CG-induced prolongation of progesterone production by the corpus luteum). In addition, progesterone production during the luteal phase was significantly reduced and menstrual cycles were shortened in immunized animals. Therefore, we suspected that an additional mechanism for the infertility was 1) a shortened luteal phase in which the corpus luteum regresses before endogenous CG is available, or 2) an insufficient luteal phase in which the corpus luteum does not respond to a gonadotropic stimulus. To distinguish between these two possibilities, we treated eight immunized and seven nonimmunized monkeys with human (h) CG for 5 days, beginning on day 17 of the cycle. During the luteal phase of the pretreatment cycles, plasma progesterone levels were significantly lower in immunized monkeys. Treatment with hCG increased progesterone to the same concentration in both groups, even though most of the hCG in the immunized animals was antibody bound. In addition, the overall cycle length was shorter in untreated immunized animals due to shorter luteal phases. In these animals, the cycle was lengthened by hCG-administration so that they were not significantly different from that in nonimmunized controls during the treatment cycles. These observations indicate that the corpora lutea of immunized monkeys are not insensitive to stimulation by hCG. Estradiol and 17α-hydroxyprogesterone levels were similar for both experimental groups, indicating that follicular steroidogenesis was not impaired by the oLHβ-antibodies. Plasma FSH concentrations were elevated during the follicular phase in immunized monkeys. These results suggest that oLHβ-antibodies induced short luteal phases in rhesus monkeys. The short luteal phases are associated with an increase in FSH concentrations during the first half of the cycle, which may in turn result in impaired folliculogenesis. The corpora lutea produce reduced amounts of progesterone, but are not unresponsive to hCG.