Endocrine response in rabbits immunized with native versus deglycosylated porcine zona pellucida antigens

Infertility and Immunocontraception based on zona pellucida

The zona pellucida (ZP) is an extracellular matrix surrounding ovarian oocytes, ovulated eggs and preimplantation embryos. It plays several important roles at different stages of reproduction. Its constituent glycoproteins are expressed specifically in the ovary. It is thus possible to produce autoantibodies to ZP proteins that interfere with reproductive functions including folliculogenesis, fertilization and implantation. First, this article describes the history of anti-ZP antibodies detected in women with idiopathic infertility. Second, the current relationship between anti-ZP antibodies and infertility is discussed in relation to assisted reproductive medicine. Third, we introduce the latest studies of animal experiments involving the ZP. Finally, immunocontraceptive vaccine development using various ZP antigens is reviewed.

Is there a role for immunocontraception?

The world's population is continuing to grow at an alarming rate and yet no novel methods of contraception have been introduced since 1960s. The paucity of our current contraceptive armoury is indicated by the 46 million abortions that are performed each year, largely in developing countries where population growth is greatest. Thus, whatever new forms of fertility control we develop for the next millennium, the particular needs of developing countries should be borne in mind. Contraceptive vaccines have the potential to provide safe, effective, prolonged, reversible protection against pregnancy in a form that can be easily administered in the Third World. In this review we consider the contraceptive targets that might be pursued, how vaccines might be engineered and the problems generated by inter-individual variations in antibody titre. We conclude that the specifications for a safe, effective, reversible vaccine are more likely to be met in animals than man.

Contraceptive vaccines

The world's population is growing at a tremendous rate, affecting growth and development. Apart from this population growth, unintended pregnancies resulting in elective abortions continue to be a major public health issue. In over half of these unintended pregnancies, the women have used some type of contraception. Thus, there is an urgent need for a better method of contraception that is acceptable, effective and available. The contraceptive choices available to women at this time include steroid contraceptives, intrauterine devices, barrier methods, spermicides, natural family planning, male and female sterilisation, and recently available emergency contraceptives. Contraceptive vaccines (CVs) may provide viable and valuable alternatives that can fulfill most, if not all, properties of an ideal contraceptive. Since both the developed and most of the developing nations have an infrastructure for mass immunisation, the development of vaccines for contraception is an exciting proposition. The molecules that are being explored for CV development either target gamete production (gonadotropin releasing hormone, follicle-stimulating hormone and luteinising hormone), gamete function (zona pellucida [ZP] proteins and sperm antigens) or gamete outcome (human chorionic gonadotropin [hCG]). Disadvantages of CVs targeting gamete production are that they affect sex steroids and/or show only a partial effect in reducing fertility. CVs targeting gamete function are better choices. Vaccines based on ZP proteins are quite efficacious in producing contraceptive effects. However, they invariably induce oophoritis affecting sex steroids. Sperm antigens constitute the most promising and exciting targets for CVs. Several sperm-specific antigens have been delineated in several laboratories and are being actively explored for CV development. Antisperm antibody-mediated immunoinfertility provides a naturally occurring model to indicate how an antisperm vaccine will work in humans. Vaccines targeting gamete outcome primarily focus on the hCG molecule. The hCG vaccine is the first vaccine to undergo phase I and II clinical trials in humans. Both the efficacy and the lack of immunotoxicity have been reasonably well demonstrated for this vaccine. The present studies focus on increasing the immunogenicity and efficacy of this birth control vaccine.

Expression in Escherichia coli and immunological characterization of three zona pellucida proteins (ZP1, ZP2, and ZP3) from a marsupial, the brushtail possum (Trichosurus vulpecula)

The brushtail possum (Trichosurus vulpecula) zona pellucida (ZP) is composed of three major glycoproteins, designated ZP1, ZP2, and ZP3 based on their size and homology with eutherian ZP proteins. These proteins are candidate antigens for the development of an immunocontraceptive vaccine to control the fertility of the brushtail possum in New Zealand, where it is an introduced pest. In order to further their immunological and functional characterization, recombinant possum ZP proteins were produced in Escherichia coli (E. coli) strain JM109, M15, SG13009, or BL21 codon plus. Each of the proteins produced possessed a N-terminal six histidine tag (His)(6) to facilitate purification and consisted of amino acid (aa) residues 18-471 of possum ZP1, aa residues 40-311 of ZP2 (ZP2-N), aa residues 305-634 of ZP2 (ZP2-C), and aa residues 23-342 of ZP3. Immunoblot using anti-RGS(His)(4) antibodies and polyclonal rabbit anti-porcine ZP antibodies detected major bands at 54 kDa for ZP1, 32 kDa for ZP2-N, 39 kDa for ZP2-C, and 40 kDa for ZP3. Immunization of male and female rabbits with ZP2-N, ZP2-C, and ZP3 purified on Ni-NTA resin under denaturing conditions generated antibodies reactive with recombinant ZP proteins on Western blot and with native ZP proteins in possum ovarian sections using immunofluorescence. Antibodies generated against ZP1 in the same way were reactive with recombinant ZP proteins on Western blot only. The recombinant possum ZP proteins and specific antibodies produced in this study give an indication of the antigenic relationship of the possum ZP proteins and are vital tools for future studies of sperm-ZP binding in marsupials and for the evaluation of ZP-based contraceptive vaccines in possums and other marsupials.

Immunocontraceptive vaccines for human use

The development of safe, effective, reversible contraceptive vaccines for the regulation of human fertility would be a significant addition to our contraceptive armamentarium. However, because we are such an out-bred species, immunological responsiveness to any given vaccine is certain to exhibit a high level of inter-individual variation that will impact upon the efficacy, reversibility and feasibility of the approach. Nevertheless a role for vaccines undoubtedly exists as an aid to birth spacing, particularly in developing countries, and as a non-surgical means of inducing sterility in men. Currently vaccines are being researched that target one of two strategic points in the reproductive process, fertilization and the maternal recognition of pregnancy. Our ability to engineer vaccines that target fertilization is hampered by deficiencies in our knowledge of the molecular mechanisms that regulate this process. However, anti-hCG vaccines have advanced to the stage of clinical trials and appear promising.

The contraceptive potential of ZP3 and ZP3 peptides in a primate model

It has been known for some time that antibodies raised against ZP3, the major component of the glycoprotein shell that surrounds all mammalian oocytes, can successfully inhibit sperm-egg interaction in vitro. In our own studies using the non-human primate Callithrix jacchus, active immunisation was successfully achieved when homologous or heterologous ZP3 was used as an immunogen. However this long-term suppression of fertility was at the expense of ovarian function. An ovarian pathology was observed which was characterised by a disruption of folliculogenesis and depletion of the primordial follicle pool. Adverse auto-immune reactions have also been observed in mice following induction of immunity to mouse ZP3. Following careful selection of B-cell epitopes on mouse ZP3, peptide vaccines were formulated which could circumvent these adverse side effects and induce reversible infertility in actively immunised mice. To identify similar epitopes on primate ZP3, epitope mapping studies were performed and several candidate regions of the molecule were identified. These were incorporated into chimeric peptide vaccines and administered as single or triple peptide vaccines. Active immunisation successfully induced antibodies that bound exclusively to the zona pellucida of marmoset and human ovarian sections. These antibodies were able to suppress human sperm-egg binding by up to 60% in vitro. Encouragingly, no adverse side effects on ovarian function were observed following long-term immunisation however, no loss of fertility was consistently observed in vivo. Thus considerable research is still required to identify a combination of ZP epitopes that will induce reversible infertility in the absence of any ovarian dysfunction.

Analysis of B cell epitopes of a glycoprotein porcine zona pellucida (pZP1)

The zona pellucida (ZP) of mammalian oocytes forms an extracellular matrix composed of three major glycoproteins and plays an important role in sperm-zona interactions. As ZP had a strong organ-specific but species-cross-reactive antigenicity and passive or active immunization with ZP antigens could impair fertilization, the possibility of developing a immunocontraceptive vaccine has been extensively studied. Studies on active immunization with porcine ZP (pZP) that contain B cell epitopes and T cell epitopes demonstrated that a temporary infertility could be induced along with the elevation of antibody titers, but it was always associated with ovarian failure. This could be due to the oophoritis by activation of pathogenic T cell immunity. It is the general consideration that any adverse effects by vaccination should be avoided for an immunocontraception. From this point of view, the analysis of B cell epitopes of pZP protein would be helpful for construction of a safe immunocontraceptive vaccine with zona antigens. We determined the amino acid sequence of the B epitope in the pZP1 protein by using a monoclonal antibody (MAb-5H4) that possesses a fertilization blocking ability. In addition, antiserum raised to the epitope sequence was revealed to block in vitro fertilization of homologous animal species.

Expression of bonnet monkey (Macaca radiata) zona pellucida-3 (ZP3) in a prokaryotic system and its immunogenicity

An internal fragment (978 bp) corresponding to the bonnet monkey (Macaca radiata) ZP3, excluding the N-terminus signal sequence and the C-terminus transmembrane-like domain, was amplified by PCR from a full-length cDNA clone. The amplified Bam HI and SacI restricted fragment was cloned in frame downstream of the T5 promoter under lac operator control for expression in the pQE-30 vector. Recombinant ZP3 (r-ZP3) was expressed as a poly-histidine fusion protein in E. coli strains SG13009[pREP4] and BL-21(DE3). Immunoblot with a murine monoclonal antibody, MA-451 (raised against porcine ZP3 beta-a homologue of bonnet ZP3, and cross-reactive with bonnet zona pellucida) revealed a predominant band of 50 kDa besides degraded fragments. Optimum expression of r-ZP3 was observed at 0.5 mM IPTG. Antisera generated in monkeys against synthetic peptides from the N-(23-45 aa residues) and C-(300-322 and 324-347 aa residues) termini of the deduced bonnet monkey precursor ZP3 sequence reacted with the r-ZP3 protein in ELISA. The r-ZP3 expressed in SG13009[pREP4] was purified on Ni-NTA resin under denaturing conditions and conjugated with diphtheria toxoid (DT). Immunization of a female rabbit and six female bonnet monkeys with the r-ZP3-DT conjugate generated antibodies reactive with r-ZP3 in ELISA. Rabbit r-ZP3 antiserum reacted with porcine ZP3 beta and bonnet r-ZP3 but failed to react with porcine ZP3 alpha in a Western blot. Moreover, antisera when tested by indirect immunofluorescence on bonnet monkey ovarian sections, showed positive fluorescence with zona pellucida. The availability of r-ZP3 will further help in evaluating its efficacy for fertility regulation and understanding the autoimmune oophoritis associated with ZP3 immunization in nonhuman primates.

Cloning and characterization of zona pellucida genes and cDNAs from a variety of mammalian species: the ZPA, ZPB and ZPC gene families

Full length zona pellucida cDNAs from cat, dog and pig that are homologous to the ZP2/rc75 genes from mouse, human and rabbit, a full length zona pellucida cDNA from cat and a gene and full length cDNA from human that are homologous to the rc55/ZP3 alpha genes from rabbit and pig, and full length zona pellucida cDNAs from cat, cow, dog, pig and rabbit that are homologous to the ZP3 genes from mouse, hamster, human and marmoset have been cloned and characterized. The members of these gene families are herein referred to as ZPA, ZPB and ZPC genes to avoid the confusion that currently exists in the zona pellucida of nomenclature. This report is the first to describe the presence all three major zona pellucida genes within individual mammalian species. Within the ZPA, ZPB and ZPC gene families, the DNA and deduced amino acid sequences are highly homologous to each other, and are most homologous between members of the same order within the class mammalia. These results imply that all or most mammalian species express the ZPA, ZPB and ZPC proteins, which form the zona pellucida layer surrounding the oocyte.

Immunoregulation of mammalian fertility

Fertility management is a global issue of agricultural, medical, economic, and social consequence. Although many methods have been devised to both inhibit and assist reproduction, more acceptable alternatives are needed. Regulation by immune intervention is a promising technology as applied to livestock, pets, wildlife, and human beings. Outcome is dictated by site within the reproductive axis that is targeted. Fertility is suppressed by immunization against gonadotropin-releasing hormone, gonadotropins, prostaglandin F2 alpha, oxytocin, gonadotropin receptors, and gamete/embryonic antigens. It also is possible to lyse gonadal cells with ligand-antibody hybrid molecules. Ovulation rates are enhanced by vaccination with inhibin. Antibodies to sex steroid hormones have yielded mixed results. Perhaps recombinant viral vectors can be used to deliver reproductive immunogens. A new and simple technique to generate sustained autoimmune reactions to hormones and cellular antigens entails direct gene transfer into somatic cells. Evolving advances in reproductive immunology and biotechnology should furnish us with novel nonsurgical contraceptives and profertility agents that can be efficiently and safely implemented.

The cellular immune response to immunization with zona pellucida antigens

The cellular immune response of mice to porcine and rat zona pellucida and cynomolgus macaques to porcine zona pellucida antigens was evaluated. Mice mounted a vigorous cellular response to both antigens, as determined by the T cell proliferation response in vitro. There was poor cross-reactivity to murine zonae by T cells or serum antibodies from mice immunized with rat zona pellucida. Nevertheless, ovaries from the mice immunized with rat zona had significantly fewer antral follicles than adjuvant-treated controls, suggesting that the immune response to the zona antigen disrupted follicular development. T cells from two macaques that had been immunized with porcine zona pelludica proteins proliferated in vitro in response to this antigen. Both macaques also had strong antibody responses. The patterns of urinary steroid metabolites in these animals provided clear evidence of ovarian malfunction following immunization. The data indicate that a significant cellular immune response is generated upon immunization of animals with zona pellucida antigens regardless of whether the antigens are cross reactive with the host zona antigens. Whether impaired ovarian function and follicular development are related to the cellular response must be determined in future studies.