Enzyme-linked immunosorbent determination of autoantibodies to zona pellucida as a possible cause of infertility in women

The status of anti-ovarian antibody immunoassays: Valid or invalid, in or out?

There has been a paradign shift in the status of immunoassays. There used to be a time where immunoassays had a very narrow role in clinical medicine, but that is not the case in today's world. Immunoassays have taken a central role in helping us better understand and treat human diseases. The literature around anti-ovarian antibodies (AOA) immunoassay testings have been conflicting. Researchers challenged the specificity of the reported assays, but a systematic study was never elucidated on what/who the trouble maker was in rendering these tests so nonspecific. Attempts were made by our group in Mumbai, India, to throw light on the culprit behind the nonspecificity casative factor in the immunoassays and a method to overcome this was reported and published. This review highlights the stories back five and a half decades to date, to demonstrate where the status of AOA testing was, is and will be.

The Human Egg's Zona Pellucida

Human zona pellucida (ZP) matrix, a delicate network of thin interconnected filaments, is primarily composed of four glycoproteins, namely, ZP1, ZP2, ZP3, and ZP4. All four zona proteins share common structural elements such as signal peptide, "ZP domain," consensus furin cleavage site, transmembrane-like domain, and short cytoplasmic tail. In addition, ZP1 and ZP4 also have "Trefoil domain." Recombinant/native human zona proteins have been used to investigate their binding characteristics to the capacitated and/or acrosome-reacted spermatozoa. These investigations revealed that ZP1, ZP3, and ZP4 primarily bind to the head region of the capacitated human spermatozoa, whereas ZP2 binds to the acrosome-reacted sperm. However, using transgenic mice, N-terminal region of human ZP2 has also been shown to play an important role in binding of sperm to the egg. ZP1, ZP3, and ZP4 lead to dose-dependent increase in acrosome reaction, suggesting that in humans more than one ZP glycoprotein is responsible for induction of acrosome reaction. Glycosylation of these proteins, in particular, N-linked glycosylation as well as sialyl-Lewis^x, is essential for inducing acrosome reaction. Studies delineating downstream signaling events associated with induction of acrosome reaction reveal subtle differences between ZP3 and ZP1/ZP4 with respect to activation of Gi protein-coupled receptor and protein kinase A. The role of mutations in the zona proteins and ZP autoantibodies leading to infertility in women is suggestive and needs more rigorous experimentations for confirming their role in female infertility. The above-mentioned aspects of the human ZP glycoproteins have been discussed in this review.

Role of zona pellucida glycoproteins during fertilization in humans

In the last decade, scientific investigations pertaining to the role of zona pellucida (ZP) glycoproteins during fertilization in humans have led to new insights. This has been achieved using purified native/recombinant human zona proteins and transgenic mice expressing human ZP glycoproteins. The proposed model in mice of ZP glycoprotein-3 (ZP3) acting as primary sperm receptor and ZP glycoprotein-2 (ZP2) as secondary sperm receptor has been modified for sperm-egg binding in humans. ZP glycoprotein-1 (ZP1), ZP3, and ZP glycoprotein-4 (ZP4) have been shown to bind to the capacitated human sperm. ZP2 binds to the acrosome-reacted human spermatozoa. Further, the eggs obtained from transgenic mice expressing human ZP2 alone or in conjunction with other human instead of mouse zona proteins showed binding of human sperm, suggesting that ZP2 might also play a role in sperm-egg binding. This function has been mapped to a domain corresponding to amino acid residues 51-144 of ZP2. In contrast to mice, where ZP3 is the primary agonist for inducing the acrosome reaction, in humans, the acrosome reaction can be mediated by ZP1, ZP3, and ZP4. The effect of mutations in the genes encoding zona proteins on the ZP morphology and infertility has not been established. Further, the role of autoantibodies against ZP in women with 'unexplained infertility' leading to poor outcome of in vitro fertilization is currently controversial and needs further investigations. Understanding the role of ZP glycoproteins during human fertilization facilitates the development of new contraceptives and strategies to overcome the problem of infertility.

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.

Autoantigens in ovarian autoimmunity associated with unexplained infertility and premature ovarian failure

OBJECTIVE

To identify ovarian autoantigens associated with ovarian autoantibodies.

DESIGN

Hypothesis-generating prospective study.

SETTING

Urban infertility referral centers and academic research institution.

PATIENT(S)

Seventy-four patients with infertility, 19 patients with premature ovarian failure (POF), and 16 healthy control women.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Identification of autoantigens.

RESULT(S)

To identify major antigens for ovarian autoimmunity, sera from 74 women with unexplained infertility were screened for ovarian autoantibodies (AOAs) by immunoassay and one-dimensional Western blot. The majority of sera had immunoreactions at 50-56 kDa. Six representative positive infertility sera were used to identify antigens between 40 and 60 kD by two-dimensional Western blot and mass spectrometry. Antigens included aldehyde (retinal) dehydrogenases (ALDH1A1, ALDH1A2, and ALDH7A1), protein disulfide isomerase A3, vimentin, α-enolase, phosphoglycerate dehydrogenase, and selenium-binding protein 1 (SBP1). Sixty percent (24 out of 40) of infertility and POF sera were positive for recombinant ALDH1A1, SBP1, or enolase; 80.7% (21 out of 26) of AOA-positive sera had antibodies to one or more of the three antigens, and only 7% (1 out of 14) of AOA-negative sera had antibodies to recombinant proteins.

CONCLUSION(S)

ALDH1A1 and SBP1 are unique to ovarian autoimmunity associated with infertility and POF, and may provide the basis for specific tests to identify patients with ovarian autoimmunity.

Sperm-zona pellucida interaction and immunological infertility

Immune reactions against gametes appear to be physiologically important for the maintenance of homeostasis in reproduction. In contrast, aberration of the immune homeostasis might give rise to 'immunological infertility'. Antisperm antibodies cause infertility by blocking fertilization. The mechanism can be explained as inhibiting the acrosome reaction of sperm by their blocking effect on capacitation through inhibiting an increase of fluidity of the sperm membrane. Autoantibodies against zona pellucida also cause infertility by blocking sperm-zona pellucida interaction, though the definitive mechanism has not been elucidated. Pretreatment of spermatozoa with D-mannnose completely inhibited sperm penetration through, but not binding to, the zona pellucida. Furthermore, very rapid kinetics between sperm extracts and D-mannnose by a BIAcore apparatus suggest that a D-mannose ligand of the sperm surface is easy to bind to and dissociate from a D-mannose residue in the sperm receptor site on the zona pellucida. Thus, D-mannnose on the human zona pellucida might be an essential molecule acting as a second sperm receptor, through which sperm penetrate into the zona pellucida. Because these antibodies appear to not cause any deleterious clinical symptoms, sperm and zona pellucida antigens are promising candidates in the development of an immunocontraceptive. (Reprod Med Biol 2006; 5: 95-104).

Sperm-zona pellucida interaction and immunological infertility

Immune reactions against gametes appear to be physiologically important for the maintenance of homeostasis in reproduction. In contrast, aberration of the immune homeostasis might give rise to 'immunological infertility'. Antisperm antibodies cause infertility by blocking fertilization. The mechanism can be explained as inhibiting the acrosome reaction of sperm by their blocking effect on capacitation through inhibiting an increase of fluidity of the sperm membrane. Autoantibodies against zona pellucida also cause infertility by blocking sperm-zona pellucida interaction, though the definitive mechanism has not been elucidated. Pretreatment of spermatozoa with D-mannnose completely inhibited sperm penetration through, but not binding to, the zona pellucida. Furthermore, very rapid kinetics between sperm extracts and D-mannnose by a BIAcore apparatus suggest that a D-mannose ligand of the sperm surface is easy to bind to and dissociate from a D-mannose residue in the sperm receptor site on the zona pellucida. Thus, D-mannnose on the human zona pellucida might be an essential molecule acting as a second sperm receptor, through which sperm penetrate into the zona pellucida. Because these antibodies appear to not cause any deleterious clinical symptoms, sperm and zona pellucida antigens are promising candidates in the development of an immunocontraceptive. (Reprod Med Biol 2006; 5: 95-104).

Ovarian autoimmune disease and ovarian autoantibodies

Detection of specific autoantibodies remains the most practical clinical and research marker of autoimmune disease. The lack of consensus on ovary specific antibodies as a marker for ovarian autoimmunity has clinical and research consequences. The objective of this review is to summarize the evidence for ovarian autoimmunity and the detection of ovary specific autoantibodies in humans. Evidence favors the presence of an autoimmune disease of the ovary. Ovarian autoantibodies are associated primarily with premature ovarian failure (POF) and unexplained infertility. Variations in detection of ovarian autoantibodies are likely to be due to study design elements such as antibody test format, antigen preparation, and criteria for study and comparison groups. In addition, multiple targets appear to be involved in ovarian autoimmunity including ovarian cellular elements and oocyte related antigens. Many studies only assess one target antigen, leaving individuals with ovarian autoimmunity unidentified. The next most significant advance in characterizing ovarian autoimmunity will be definitive identification of the specific antigens and development of standardized tests based on use of specific antigens.

Antizona and antisperm antibodies in women with endometriosis and/or infertility

OBJECTIVE

To measure the levels of antigamete antibodies in serum and peritoneal fluid of women with endometriosis and/or infertility.

DESIGN

Antibody activity against human sperm and porcine oocytes was analyzed in selected subgroups of women.

SETTING

Clinic of reproduction.

PATIENT(S)

Women with endometriosis and/or infertility.

INTERVENTION(S)

No treatment was implemented before peritoneal fluid and blood sample collection.

MAIN OUTCOME MEASURE(S)

Quantitative ELISA.

RESULT(S)

Four groups of women (n = 98) were analyzed for the presence of antizona and antisperm antibodies: infertile with endometriosis (n = 30), idiopathic infertility (n = 28), fertile with endometriosis (n = 20), and healthy fertile controls (n = 20). Antibodies were analyzed simultaneously in serum and peritoneal fluid. No statistically significant differences in antibody levels were detected in serum samples among the analyzed groups. The median values for antizona and antisperm antibodies in peritoneal fluid were significantly higher in women with idiopathic infertility than in the control group. In women with unexplained infertility, a high degree of correlation (Spearman) was found between the presence of antizona antibodies in peritoneal fluid and serum (r = 0.579). A positive predictive value of 80% was calculated for the presence of antizona antibodies (>5 ng/oocyte) in the peritoneal fluid of patients with infertility.

CONCLUSION(S)

Antizona antibodies locally produced in the peritoneal fluid have diagnostic value for infertility status; however, they cannot be treated as a marker or prognostic factor for minimal endometriosis and/or its treatment.

Immunization of chickens with quail and turkey perivitelline membrane proteins: production of antibodies and their effects on fertility

The cross-reactivity of antibodies developed against zona pellucida proteins and their subsequent deleterious effect on fertility in heterologous species is well documented. However, similar investigations have not been undertaken in avian species. In Experiment 1, White Leghorn hens were immunized with intact germinal discs (GD) of quail and chickens. Chicken GD proteins did not elicit an immune response in chicken hens, whereas quail GD proteins were highly immunogenic. Anti-quail GD antibodies did not bind with chicken inner perivitelline membrane (IPM) proteins as determined by immunoblot analysis. To examine the fertility of immunized hens, artificial insemination was performed at weekly intervals for 4 wk following the booster immunization. No significant differences were detected in fertility or hatchability of immunized hens when compared with unimmunized control hens. In Experiment 2, White Leghorn hens were immunized with intact turkey GD, solubilized turkey perivitelline membrane (PM) modified with dinitrophenol (DNP), and solubilized chicken IPM modified with DNP. High titers of antibodies were detected against the turkey GD and the DNP-modified turkey PM proteins. A weak immune response was observed in hens immunized with modified chicken IPM proteins. The fertility and hatchability of eggs laid by immunized hens, however, were not significantly different from those of unimmunized hens. Antibodies from immunized hens were further analyzed using an in vitro assay that assesses sperm penetration of intact IPM. Sperm penetration of intact IPM was inhibited to the same extent when IPM was preincubated with preimmune as well as anti-PM immunoglobulins. Collectively, these results suggested that the antibodies developed in these hens did not cross-react with the chicken IPM proteins involved in sperm-egg interaction and thus did not influence the fertility.

Etiological implication of autoantibodies to zona pellucida in human female infertility

PROBLEM

Autoantibodies to zona pellucida (ZP) have been implicated as a cause of infertility in woman by three lines of clinical and laboratory evidence.

METHOD

First, positive anti-zona activities, as assessed by the passive hemagglutination reaction (PHAR) using bovine red blood cells sensitized with porcine zona antigen, were detected in 45 of 1,872 serum samples collected from infertile women, but in only three (two nonpregnant and one pregnant) of 592 serum samples from control subjects. Their incidence in 320 women with unexplained infertility (5.6%) was much higher than that in 1,552 women with infertility of known cause (1.7%, P < 0.01), which was comparable to the incidence observed in 193 age-matched fertile nonpregnant and pregnant women (1.5%, P < 0.05). None of the serum samples from 292 age-matched adult men and 100 children of between 5 and 10 years old gave a positive PHAR. Second, follow-up study for a minimum of 2 years with treatment revealed that no pregnancy occurred in 11 infertile women with a consistently positive PHAR, and only three pregnancies in 19 infertile women with a fluctuating positive PHAR. Third, three of seven serum samples from women who gave a consistently positive PHAR produced strong immunofluorescence reactions on human ZP even after adsorption with porcine and human AB blood cells. Pre-exposure of human ZP to the sera showing positive immunofluorescence, including that of one patient undergoing an in vitro fertilization program, greatly diminished the number of spermatozoa of normal quality that bound to and penetrated across human ZP.

CONCLUSIONS

The observed high incidence of anti-zona activities and long-term resistancy to treatment in women with unexplained infertility may be closely correlated with inhibition of sperm-egg interaction by anti-zona autoantibodies produced in these women.

Solid-phase antigen immunoassay for the detection of anti-zona pellucida-antibodies in clinically defined sera

A new solid-phase antigen immunoassay was developed for the detection of anti-zona pellucida-antibodies. The assay was validated in a double blind study with clinically defined sera. The new assay is easy to perform and large numbers of sera can be processed in one day. The antigen can be prepared in advance and stored until use. This represents an advantage over other methods, such as immunofluorescence and RIA, which require fresh antigen preparation for each set of assays. With this new test ca. 7% of females and ca. 20% of males with unexplained infertility have shown elevated antibody concentrations compared with fertile controls. The estimated intra-assay and inter-assay CV were 8.5% and less than 10% respectively.