Zona pellucida: current status as a candidate antigen for contraceptive vaccine development

The porcine zona pellucida represents a unique immunocontraceptive target antigen. Initial active immunization studies using crude or partially purified zona components as immunogens resulted in adverse, nonreversible effects on ovarian folliculogenesis, but more recent findings in nonhuman primates indicate that such effects are not as severe and are reversible. Consequently, current efforts are directed toward identifying a zona immunogen that elicits contraceptive antibodies that produce no adverse effects on the ovary. Other studies are in progress to determine optimal injection/immunization format, proper immunogen dosage, and appropriate adjuvant for use in the zona-antizona immunocontraceptive system. Recent data have been very encouraging and emphasize the contraceptive efficacy of this approach to reproduction control.

Recent developments in immunocontraception

The possibility of controlling fertility by antibodies inactivating key reproductive hormones has been amply demonstrated by active and passive immunization in primates. Four birth control vaccines directed against human chorionic gonadotropin are currently in early clinical trials. The nature of these vaccines and the underlying principles are described, as are the available results from clinical studies. The alpha- and beta-subunits of human chorionic gonadotropin and the ovine gonadotropins have been cloned by recombinant deoxyribonucleic acid methods. A new breed of vaccines that combines the genes of gonadotropins linked to hepatitis B surface protein has been developed. The next generation of birth control vaccines is likely to be polyvalent and to have the ability to intercept fertility at more than one point. A number of monoclonal antibodies against human sperm have shown the presence of tissue-specific antigens and the possibility of preventing the fertilization of the egg. Inclusion of more than one carrier in the vaccine increases the percentage of high responders and accords immunoprophylactic benefits against more than one disease. Conjugates have also been developed to obtain high titers of antibodies against gonadotropin-releasing hormone with permissible adjuvants. This vaccine may have therapeutic applications in hormone-dependent cancers and precocious puberty.

Enhancement of antigonadotropin response to the beta-subunit of ovine luteinizing hormone by carrier conjugation and combination with the beta-subunit of human chorionic gonadotropin

Previous studies have demonstrated the control of fertility in monkeys on immunization with the beta subunit of ovine luteinizing hormone (beta-oLH) along with Freund's complete adjuvant (CFA). An eventual birth control vaccine would demand immunization without CFA. Beta-oLH linked to tetanus toxoid (TT) adsorbed on alum-induced antibody response in bonnet monkeys with detoxified sodium phthalyl derivative of Salmonella enteritidis lipopolysaccharide added only in the first injection. The antibodies reacted with both LH and human chorionic gonadotropin (hCG). A higher immune response was obtained with the use of a conjugate of beta-oLH and beta-hCG linked to a common carrier (TT) or with a mixture of beta-hCG/TT and beta-oLH/TT. In both cases, the antibodies were of high affinity with Ka ranging from 5 X 10(9) to 6 X 10(10) M-1. The antibodies were devoid of reactivity with human thyroid-stimulating hormone and human follicle-stimulating hormone. Monkeys with titers above 120 ng/ml hCG binding capacity (greater than 60 ng/ml in most cases) were protected from becoming pregnant during repeated mating with males of proven fertility in ovulatory cycles.

Recent developments in female contraception: LHRH

During the last few years new approaches to female contraception based on LHRH and its analogs have been developed. The physiological significance of pulsatile LHRH release and its stimulation of the pituitary has been elucidated by recent studies in rhesus monkey. Immunization against LHRH results in complete inhibition of reproductive function in animals and may find as a useful method of long-term fertility control in domestic animals. Clinical studies have utilized this knowledge to treat infertile hypogonadal women with chronic intermittent low-dose of LHRH. The superactive stimulatory LHRH analogs, used to treat infertility, paradoxically proved to have antifertility effects. They induce desensitization of the processes responsible for gonadotropic and gonadal hormone secretion, mediated by specific LHRH receptors in the pituitary and gonad. While contraceptive effectiveness of luteolytic approach remains to be proven, inhibition of ovulation by intranasal LHRH analog administration or continuous LHRH infusion by programmed minipumps seem to provide safe and effective contraception in women.

Effect of early postpartum use of the contraceptive implants, NORPLANT, on the serum levels of immunoglobulins of the mothers and their breastfed infants

Ten breastfeeding women had the contraceptive implants NORPLANT inserted between days 30 and 39 postpartum. The immunoglobulins IgG, IgM and IgA were measured in the serum of the mothers and the infants before insertion and five months later. A control group of breastfeeding mothers who did not use any contraception or used local barrier methods were similarly studied. Although there were changes in the immunoglobulin levels with time in both mothers and infants, there were no group differences; this indicates that the use of NORPLANT does not influence these factors of humoral immunity.

Production of monoclonal antibodies to porcine zona pellucida and their inhibition of sperm penetration through human zona pellucida in vitro

Two hybridoma cell lines producing murine monoclonal antibodies to antigens common to the zona pellucida (ZP) of pigs and humans were obtained by immunization of mice with solubilized porcine zona antigen. Indirect immunofluorescence tests showed that both these monoclonal antibodies stained the entire layer of porcine ZP but stained different regions of human ZP, one staining the entire layer and the other only the outer surface. At high concentrations, these two monoclonal antibodies directed against antigens common to porcine and human ZP prevented sperm binding and penetration into human ZP in vitro, whereas a monoclonal antibody directed against an antigen restricted to porcine ZP did not have these inhibitory effects. It is concluded that human and porcine ZP share at least two antigens with different locations in the ZP, and that these influence or are essential for interaction of human sperm with the ZP. These results provide a rationale for using porcine ZP clinically as a vaccine for human immunocontraception.

Current trends in antifertility vaccine research

We review the major advances that have recently occurred in the area of antifertility vaccines by examining the immunogenic potential of gamete, embryonic and placental antigens. In human trials using beta-human chorionic gonadotropin coupled with tetanus toxoid as the immunogen, the major problems with antifertility vaccines relate to specificity and maintaining an adequate antibody titer to disrupt gestation. Possible complications include cross-reaction with other body tissues, immune complex deposition, cytotoxicity, impaired immunologic tumor surveillance and nonreversibility.

Fertility following discontinuation of different methods of fertility control

The possibility of permanent sterility following use of modern contraceptive agents is a concept which causes great concern amongst members of the lay public. However, a thorough review of the literature gives a great deal of reassurance on this topic. The combined oral contraceptive pill causes slight delay, and the injectable contraceptives a moderate delay in the return of fertility following discontinuation, but there is no evidence of a greater likelihood of permanent infertility than in the normal population. The progestogen-only minipill should be risk-free, but final data are lacking. Epidemiological studies do not indicate an increased risk of infertility following IUD use, but it seems likely that there must be a very small number of IUD users who develop permanent infective damage to the Fallopian tubes. Barrier methods and periodic abstinence probably do not have any significant effect on subsequent fertility. Barrier methods could be protective. Therapeutic abortion may have a small deleterious effect on long-term reproductive success, but the data are inconclusive. Detailed information on fertility following use of other methods is not available.