Inhibin immunoneutralization by antibodies raised against synthetic peptide sequences of inhibin α subunit: effects on gonadotrophin concentrations and ovulation rate in sheep

Abstract

Two experiments were conducted to explore the effectiveness of synthetic peptide-based vaccines for active and passive autoimmunization of sheep against inhibin. In the first experiment, adult Romney ewes (n = 20) were actively immunized against a synthetically produced peptide that corresponded to the N-terminus of the alpha-subunit of bovine inhibin (bI alpha(1-29)-Tyr30). This peptide was conjugated to tuberculin purified protein derivative (PPD) to increase its antigenic properties. Control groups comprised non-immunized (n = 10) and PPD-immunized (n = 10) ewes. Primary immunization (400 micrograms conjugate/ewe) was followed by two booster immunizations (200 micrograms conjugate/ewe), given 5 and 8 weeks later. Following synchronization of oestrus using progestagen sponges, ovulation rates were assessed by laparoscopy. Weekly blood samples were taken throughout the experiment. All inhibin-immunized ewes produced antibodies which bound 125I-labelled bovine inhibin (Mr 32,000), and ovulation rate in inhibin-immunized ewes (2.15 +/- 0.22; mean +/- S.E.M.) was significantly (P less than 0.01) greater than in both non-immunized (0.90 +/- 0.23) and PPD-immunized (1.20 +/- 0.13) control groups. Immunization against the peptide, but not against PPD alone, resulted in a modest rise in plasma FSH, with mean levels after the second boost being significantly (P less than 0.025) higher (22%) than those before immunization. Moreover, when blood samples were taken (2-h intervals) from randomly selected groups of control (n = 7) and inhibin-immunized (n = 7) ewes for an 84-h period following withdrawal of progestagen sponges, the mean plasma concentration of FSH during the 48 h immediately before the preovulatory LH surge was 37% greater (P less than 0.025) in immunized than in control animals. However, more frequent blood sampling (every 15 min for 12 h) during follicular and mid-luteal phases of the oestrous cycle revealed no significant differences between treatment groups in mean plasma concentrations of FSH. In addition, neither mean concentrations of LH nor the frequency and amplitude of LH episodes differed between immunized and control ewes. However, the mean response of LH to a 2 micrograms bolus of gonadotrophin-releasing hormone, given during the luteal phase, was significantly (P less than 0.05) less in immunized than in control ewes. These findings indicate that active immunization of Romney ewes against a synthetic fragment of inhibin can promote a controlled increase in ovulation rate, but this response cannot be unequivocally related to an increase in plasma levels of FSH.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulatory Role of Inhibin in Follicle-Stimulating Hormone Secretion and Folliculogenesis in the Guinea Pig.

“…The inhibitory effects of exogenous progesterone on folliculogenesis are demonstrated in cyclic rats through the hypothalamic-pituitary axis, especially via lowering serum LH [34]. The present results concerning with FSH secretion compare favorably with the results of passive immunization against inhibin in other animals, such as rats [26], hamsters [16,17], cows [14], mares [24] and sheep [21,36], implying passive immunization against inhibin can induce superovulation in guinea pigs. Inhibin has the ability to suppress FSH secretion by directly inhibiting expression of the gene encoding the FSH β subunit in the pituitary gland [3,22].…”

Effects of active immunization of sheep against an amino terminal peptide of the inhibin alpha C subunit on intrafollicular levels of activin A, inhibin A and follistatin

“…About 1 year earlier, six of the ewes had been actively immunized against a synthetic peptide corresponding to the amino terminal region (amino acids 1-29) of the mature C subunit of bovine inhibin conjugated to tuberculin purified protein derivative (PPD) as described elsewhere (Wrathall et al 1990(Wrathall et al , 1992. At the same time four of the ewes were actively immunized against PPD alone to serve as carrierimmunized controls.…”

“…Plasma and FF samples (1:1000 dilution) were tested for their ability to bind 125 I-labelled bovine inhibin (M r 32 000) as described previously (Wrathall et al 1990). Bovine inhibin was isolated and radiolabelled as described by Knight et al (1989).…”

“…In contrast to the equivocal FSH response to active immunization, during which inhibin antibodies are present for prolonged time periods, short-term passive immunization experiments have shown that plasma FSH levels increase abruptly in response to administration of inhibin antibodies (Mann et al 1989, 1993, Wrathall et al 1990. However, it has been noted that FSH levels soon revert to pretreatment levels despite the presence of high antibody titres (Mann et al 1989, 1993, indicating that ovarian compensatory mechanisms, involving increased oestradiol and/or inhibin secretion, are operational.…”

“…The existence of such ovarian compensatory mechanisms is also indicated by two studies carried out in this laboratory (Knight et al 1991b, Fray et al 1994) in which we failed to detect raised FSH levels in groups of long-term (>6-month) actively immunized ewes. Previously, the same ewes had shown raised FSH levels during their initial active immunization regimen (see Wrathall et al 1990Wrathall et al , 1992. Such inconsistencies in the FSH response to long-term inhibin immunoneutralization indicate that inhibin antibodies may have additional direct stimulatory effects on follicle development that are independent of changes in FSH secretion.…”