Biochemical characterization of sperm agglutination antigen-1, a human sperm surface antigen implicated in gamete interactions

Identification of Sialyl-Lewis(x)-Interacting Protein on Human Spermatozoa

Capacitated spermatozoa initiate fertilization by binding to the zona pellucida (ZP). Defective spermatozoa-ZP binding causes infertility. The sialyl-Lewis(x) (SLeX) sequence is the most abundant terminal sequence on the glycans of human ZP glycoproteins involving in spermatozoa-ZP binding. This study aimed to identify and characterize the SLeX-binding proteins on human spermatozoa. By using affinity chromatography followed by mass spectrometric analysis, chromosome 1 open reading frame 56 (C1orf56) was identified to be a SLeX-binding protein of capacitated spermatozoa. The acrosomal region of spermatozoa possessed C1orf56 immunoreactive signals with intensities that increased after capacitation indicating translocation of C1orf56 to the cell surface during capacitation. Treatment with antibody against C1orf56 inhibited spermatozoa-ZP binding and ZP-induced acrosome reaction. Purified C1orf56 from capacitated spermatozoa bound to human ZP. A pilot clinical study was conducted and found no association between the percentage of capacitated spermatozoa with C1orf56 expression and in vitro fertilization (IVF) rate in assisted reproduction treatment. However, the percentage of C1orf56 positive spermatozoa in the acrosome-reacted population was significantly (P < 0.05) lower in cycles with a fertilization rate < 60% when compared to those with a higher fertilization rate, suggesting that C1orf56 may have functions after ZP-binding and acrosome reaction. A larger clinical trial is needed to determine the possible use of sperm C1orf56 content for the prediction of fertilization potential of sperm samples.

Latheef S, Sharun K, Khurana SK, K. A, Tiwari R, Bhatt P, K. V, Chaicumpa W. Role of Antisperm Antibodies in Infertility, Pregnancy, and Potential forContraceptive and Antifertility Vaccine Designs: Research Progress and Pioneering Vision

Sperm of humans, non-human primates, and other mammalian subjects is considered to be antigenic. The effect of changes in autoimmunity on reproductive cells such as spermatozoa and oocytes play a critical but indistinct role in fertility. Antisperm antibodies (ASAs) are invariably present in both females and males. However, the degree of ASA occurrence may vary according to individual and gender. Although the extent of infertility due to ASAs alone is yet to be determined, it has been found in almost 9-12% of patients who are infertile due to different causes. Postcoital presence of spermatozoa in the reproductive tract of women is not a contributory factor in ASA generation. However, ASA generation may be induced by trauma to the vaginal mucosa, or by anal or oral sex resulting in the deposition of sperm inside the digestive tract. It is strongly believed that, in humans and other species, at least some antibodies may bind to sperm antigens, causing infertility. This form of infertility is termed as immunological infertility, which may be accompanied by impairment of fertility, even in individuals with normozoospermia. Researchers target ASAs for two major reasons: (i) to elucidate the association between ASAs and infertility, the reason ASAs causes infertility, and the mechanism underlying ASA-mediated infertility; and (ii) to assess the potential of ASAs as a contraceptive in humans in case ASAs influences infertility. Therefore, this review explores the potential application of ASAs in the development of anti-spermatozoa vaccines for contraceptive purposes. The usefulness of ASAs for diagnosing obstructive azoospermia, salpingitis, and oligoasthenoteratozoospermia has been reviewed extensively. Important patents pertaining to potential candidates for spermatozoa-derived vaccines that may be utilized as contraceptives are discussed in depth. Antifertility vaccines, as well as treatments for ASA-related infertility, are also highlighted. This review will address many unresolved issues regarding mechanisms involving ASAs in the diagnosis, as well as prognoses, of male infertility. More documented scientific reports are cited to support the mechanisms underlying the potential role of ASA in infertility. The usefulness of sperm antigens or ASAs (recombinant) in human and wild or captive animal contraceptive vaccines has been revealed through research but is yet to be validated via clinical testing.

Immunocontraceptives: new approaches to fertility control

The rapidly increasing global population has bowed the attention of family planning and associated reproductive health programmes in the direction of providing a safe and reliable method which can be used to limit family size. The world population is estimated to exceed a phenomenal 10 billion by the year 2050 A.D., thus presenting a real jeopardy of overpopulation with severe implications for the future. Despite the availability of contraceptive methods, there are over one million elective abortions globally each year due to unintended pregnancies, having devastating impact on reproductive health of women worldwide. This highlights the need for the development of newer and improved contraceptive methods. A novel contraceptive approach that is gaining substantial attention is "immunocontraception" targeting gamete production, gamete outcome, or gamete function. Amongst these, use of sperm antigens (gamete function) seems to be an exciting and feasible approach. However, the variability of immune response and time lag to attain titer among vaccinated individuals after active immunization has highlighted the potential relevance of preformed antibodies in this league. This review is an attempt to analyze the current status and progress of immunocontraceptive approaches with respect to their establishment as a future fertility control agent.

Antisperm antibodies: invaluable tools toward the identification of sperm proteins involved in fertilization

The identification of sperm proteins involved in fertilization has been the subject of numerous investigations. Much interest has been dedicated to naturally occurring antisperm antibodies (ASA) and their impact in fertility. Their presence in men and women has been associated with 2-50% of infertility cases. ASA may impair pre- and post-fertilization steps. Experimental models have been developed using sperm proteins as immunogens to evaluate their involvement in sperm function. Our team has pursued investigations to assess ASA presence in biological fluids from patients consulting for infertility and their effect on fertilization. We found ASA in follicular fluids with ability of inducing the acrosome reaction and blocking sperm-zona pellucida interaction and used them to identify sperm entities involved in these events. We generated and utilized antibodies against proacrosin/acrosin to characterize the sperm protease system. We implemented an ELISA to detect proacrosin/acrosin antibodies in human sera and evaluated their impact upon fertility by developing in vitro assays and a gene immunization model. This review presents a summary of ASA history, etiology, current approaches for detection and effects upon fertility. ASA (naturally occurring, generated by animal immunization and/or of commercial origin) are invaluable tools to understand the molecular basis of fertilization, better diagnose/treat immunoinfertility and develop immunocontraceptive methods.

The identity of zona pellucida receptor on spermatozoa: an unresolved issue in developmental biology

Mammalian oocytes are surrounded by an acellular zona pellucida (ZP). Fertilization begins when a capacitated spermatozoon binds to the ZP. Defective sperm-ZP interaction is a cause of male infertility and reduced fertilization rates in clinical assisted reproduction treatment. Despite the importance of spermatozoa-ZP binding, the mechanisms and regulation of the interaction are unclear partly due to the failure in the identification of ZP receptor on spermatozoa. Most of the previous studies assumed that the sperm ZP receptor is a single molecular species, and a number of potential candidates had been suggested. Yet none of them can be considered as the sole sperm ZP receptor. Accumulated evidence suggested that the sperm ZP receptor is a dynamic multi-molecular structure requiring coordinated action of different proteins that are assembled into a functional complex during post-testicular maturation and capacitation. The complex components may include carbohydrate-binding, protein-binding and acrosomal matrix proteins which work as a suite to mediate spermatozoa-ZP interaction. This article aims to review the latest insights in the identification of the sperm ZP receptor. Continued investigation of the area will provide considerable understanding of the regulation of fertilization that will be useful for practical application in human contraception and reproductive medicine.

Identification of bovine CD52-like molecule by monoclonal antibody IVA-543: distribution of CD52-like molecule in the bull genital tract

The bovine maturation-associated sperm membrane antigen CD52-like molecule has been analysed using a mouse anti-sperm monoclonal antibody developed against bull spermatozoa. The antigen recognised by monoclonal antibody IVA-543 was detected on blood mononuclear cells (including lymphocytes and monocytes) and on a minor population of polymorphonuclear leukocytes. The bovine CD52-like molecule is secreted by the epididymal epithelium and then it is inserted into the sperm membrane during the epididymal transport in the distal part of epididymis. The CD52-like molecule was absent from spermatozoa derived from testes, and the highest proportion of IVA-543-reactive sperm was observed in the cauda epididymis (91.6%). This study has shown that the new molecule identified on bovine cells has properties analogous to those previously described for CD52 molecules in man, mouse, rat, monkey, and dog.

Status of contraceptive vaccines

PROBLEM

This is a review of anti-sperm contraceptive vaccines (CV), and synthesis of human scFv antibodies that can be used as immunocontraceptives.

METHOD OF STUDY

Various methods of proteomics and genomics, peptide synthesis, phage display technology, and antibody engineering were used to obtain multi-epitope vaccines and human scFv antibodies from immunoinfertile and vasectomized men. The present review primarily focuses on the effect of multi-epitope vaccines and Izumo on fertility, and synthesis and characterization of sperm specific human scFv antibodies.

RESULTS

The immunization with Izumo peptides causes a contraceptive effect in female mice. The efficacy is enhanced by combination vaccination, including peptides based on other sperm antigens. Using phage display technology, we were able to synthesize at least four novel scFv antibodies with unique complementarity determining regions (CDRs) that reacted with specific fertility-related sperm antigens. These antibodies inhibited human sperm function in vitro, and their immunocontraceptive effect in vivo by these antibodies is currently being investigated.

CONCLUSION

The multi-epitope vaccines may provide an efficacious and viable approach to contraception. The human scFv antibodies, if they block fertility in vivo, may provide unique and novel immunocontraceptives, the first of its kind for human use. The multi-epitope CV and preformed engineered antibodies of defined specificity may obliterate the concern related to inter-individual variability of the immune response.

Post-translational modifications in glycosylation status during epididymal passage and significance in fertility of a 33 kDa glycoprotein (MEF3) of rhesus monkey (Macaca mulatta)

The present study reports data on post-translational modifications in the glycosylation status during epididymal passage and significance in fertility of a 33 kDa glycoprotein of rhesus monkey (Macaca mulatta), designated as MEF3 (monkey epididymal fluid protein 3). MEF3 exhibited strong affinity for N-linked alpha-D-mannose groups and O-linked N-Ac-galactosamine linkages in epididymal fluids and exhibited moderate affinity for N-Ac-glucosaminylated (wheat germ agglutinin), fucosylated (Tetragonolotus purpurea), and N-Ac-galactosamine (peanut agglutinin) residues on more mature corpus and caudal spermatozoa in a maturation-dependent manner on Western blots probed with specific biotinylated lectins. Polyclonal antiserum raised against affinity-purified MEF3 from caudal epididymal fluid (CEF) cross-reacted specifically with CEF and caudal sperm membrane of macaque and with Triton X-100 extract of ejaculated human spermatozoa, suggesting the existence of antigenically related components in both species. The tangled agglutination caused by anti-33 kDa serum of human spermatozoa, along with localization of MEF3 on entire sperm surface of epididymal and testicular sperm of monkey and human spermatozoa, suggest the significance of MEF3 in sperm function. The 100% inhibition of fertility of immunized female rabbits with this protein in vivo and inhibition of human sperm penetration in zona-free hamster eggs in vitro suggests the functional significance of MEF3 in fertility. Together, these results clearly indicate that MEF3 has potential significance as a target for antibodies that inhibit sperm function and fertility.

Isolation of human single chain variable fragment antibodies against specific sperm antigens for immunocontraceptive development

BACKGROUND

Contraceptive vaccines can provide valuable alternatives to current methods of contraception. We describe here the development of sperm-reactive human single chain variable fragment (scFv) antibodies of defined sperm specificity for immunocontraception.

METHODS

Peripheral blood leukocytes (PBL) from antisperm antibody-positive immunoinfertile and vasectomized men were activated with human sperm antigens in vitro, and the complementary DNA prepared and PCR-amplified using primers based on all the variable regions of heavy and light chains of immunoglobulins. The scFv repertoire was cloned into pCANTAB5E vector to create a human scFv antibody library.

RESULTS

Panning of the library against specific sperm antigens yielded several clones, and the four strongest reactive were selected for further analysis. These clones had novel sequences with unique complementarity-determining regions. ScFv antibodies were expressed, purified and analyzed for human sperm reactivity and effect on human sperm function. AFA-1 and FAB-7 scFv antibodies both reacted with fertilization antigen-1 antigen, but against different epitopes. YLP20 antibody reacted with the expected human sperm protein of 48 +/- 5 kDa. The fourth antibody, AS16, reacted with an 18 kDa sperm protein and seems to be a human homologue of the mouse monoclonal recombinant antisperm antibody that causes sperm agglutination. All these antibodies inhibited human sperm function.

CONCLUSIONS

This is the first study to report the use of phage display technology to obtain antisperm scFv antibodies of defined antigen specificity. These antibodies will find clinical applications in the development of novel immunocontraceptives, and specific diagnostics for immunoinfertility.

The sperm agglutination antigen-1 (SAGA-1) glycoforms of CD52 are O-glycosylated

CD52 is composed of a 12 amino acid peptide with N-linked glycans bound to the single potential glycosylation site at position 3, and a glycosylphosphatidylinositol-anchor attached at the C-terminus. Some glycoforms of this molecule expressed in the male reproductive tract are recognized by complement-dependent sperm-immobilizing antibodies in infertile patients making this antigen an important target for immunocontraception and fertility studies. Although the amount of posttranslational modification is already remarkable for such a small polypeptide, O-glycosylation of CD52 has additionally been implicated by several studies, but never rigorously characterized. In this report, we show clear evidence for the presence of O-glycans in CD52 preparations immunopurified using the murine S19 monoclonal antibody generated against sperm agglutination antigen-1 (SAGA-1), a male reproductive tract specific form of CD52. The O-glycans have been characterized by MALDI-TOF and tandem mass spectrometry after reductive elimination and permethylation. The data indicate that the major SAGA-1 O-glycans are core 1 and 2 mucin-type structures, with and without sialic acid (NeuAc(0-2)Hex(1-3)HexNAc(1-2)HexNAcitol). Minor fucosy- lated O-glycans are also present including some struc- tures with putative Le(y) epitopes (NeuAc(0-1)Fuc(1-3)Hex(1-2) HexNAc(0-1)HexNAcitol). Analysis of O-glycopeptides by tandem mass spectrometry provided an additional level of support for the O-glycosylation of SAGA-1. Elucidation of the O-glycosylation of SAGA-1 adds to the complexity of this molecule and may help to explain its biological activity.

Antisperm antibodies and conception

Sperm have been known to be antigenic for more than a century. There is a strong body of evidence that in humans and in other species at least some antibodies that bind to sperm antigens can cause infertility. Therefore, these antibodies are of interest today for two practical reasons. Firstly, the association of the antibodies with infertility means that they must be detected and then the couples treated appropriately. Secondly, because these antibodies can induce infertility they have the potential to be developed for contraceptive purposes in humans and also for the control of feral animal populations.

Radial spoke protein 44 (human meichroacidin) is an axonemal alloantigen of sperm and cilia

To identify novel sperm alloantigens relevant to immune infertility, sera from infertile men containing antisperm antibodies (ASA) were employed on 2-D immunoblots of human sperm proteins. An immunoreactive protein spot (MW: 44 kDa, pI: 4.5) was microsequenced and the related cDNA was cloned yielding a 309 amino acid sequence corresponding to a gene currently annotated in Genbank as TSGA2 homolog (mouse) to signify 'testis specific gene A2'. In Genbank the protein deduced from this gene is currently named human meichroacidin, an orthologue of meichroacidin previously identified in mouse spermatocytes. Human TSGA2 mapped to chromosome 21q22.3. Human meichroacidin (hMCA) contained a single potential tyrosine phosphorylation site and five casein kinase phosporylation motifs. The N-terminus contained a Membrane Occupation Recognition Nexus (MORN) motif found in the lipid kinase-phosphatidylinositol 4-phosphate 5-kinase (PIP5K) family and junctophilins. However hMCA lacked the characteristic kinase homology domain of PIP5K. Northern blot analysis revealed 1.5 kb hMCA transcripts in testis and trachea with lower levels in thyroid and spinal cord. A semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) analysis demonstrated occurrence of the mRNA messages in all the ciliated tissues tested with highest levels of messages in testis and trachea. Western blot analysis showed the presence of hMCA protein in brain, thyroid and trachea at the identical mass, 44 kDa, as in human testis. However, this immunoreactive pattern differed from that of sperm in which a 38 kDa form was also evident suggesting that hMCA undergoes proteolytic processing. In human testis, hMCA localized to the tails of developing spermatids and did not localize to the nucleus of either spermatocytes or spermatids. EM immunocytochemistry localized hMCA within the radial spokes of the axonemal complex of the sperm flagellum, and immunofluorescence studies revealed h-meichroacidin in the cilia of epithelial cells in the trachea and ependyma. Bioinformatic identification of orthologues of meichroacidin in several lower organisms including ciliates and flagellates suggest the protein plays a role in flagellar motility across phyla. We propose the term radial spoke protein 44 as an accurate designation, preferable to human meichroacidin because it denotes the restricted localization of the protein to the radial spokes of the axonemes of both sperm and cilia. Further, since the human gene is expressed in brain, thyroid, trachea and lung in addition to testis, we suggest that the gene name be changed from TSGA2 [testis specific gene A2] to radial spoke protein 44 [RSP44].

Diagnosis and treatment of immunologically infertile males with antisperm antibodies

The presence of antisperm antibodies (ASA) can reduce fecundity in both males and females. The present review describes a strategy, established by investigations of the diverse inhibitory effects of ASA on fertility, for the appropriate diagnosis and treatment of infertile males with ASA. For infertile males with ASA, diagnosis using the direct-immunobead test (D-IBT), the postcoital test (PCT) and the hemizona assay (HZA) should be carried out as the basis for decision-making. If the patient with ASA has an abnormal hemizona index, it seems reasonable to advise selecting intracytoplasmic sperm injection-embryo transfer (ICSI-ET) as a primary treatment. However, it has been shown that some immunologically infertile males with normal fertilizing ability established pregnancy by timed intercourse (TI) or intrauterine insemination (IUI). In such patients with ASA having normal hemizona index, TI or IUI can be selected based on the PCT result. Therefore, the treatment strategy for males with ASA is similar to that for infertile males with oligozoospermia or asthenozoospermia. In conclusion, it should be emphasized that a diversity of ASA exists and their effects on fertility in infertile males. Although there is an argument that routine testing for ASA in males is not always necessary, one should be aware that in some cases of failed IUI or IVF, ICSI is selected afterward because of the diagnosis of ASA. (Reprod Med Biol 2005; 4: 133-141).

Contraceptive vaccines targeting sperm

Overpopulation is a global problem of significant magnitude, with grave implications for the future. Development of new contraceptives is necessary, as existing forms of birth control are unavailable, impractical and/or too expensive for many individuals due to sociological, financial or educational limitations. Immunocontraception and, in particular, the targeting of antibodies to sperm-specific antigens implicated in sperm-egg binding and fertilisation offers an attractive approach to control fertility. Sperm-specific antibodies may impair fertility by inhibiting sperm motility, by reducing penetration of the cervical mucus by sperm, or by interfering in sperm capacitation or the acrosome reaction; alternatively, antisperm antibodies may invoke the complement cascade, resulting in sperm lysis. The antibodies raised against sperm-specific antigens have proved to be extremely effective at reducing sperm-egg interactions in vitro; fertility trials in subhuman primates will eventually be needed to prove the effectiveness of the sperm antigens in terms of contraceptive efficacy before trials in humans can be justified. In addition, existing and emerging strategies (such as sperm proteomics, the determination of molecular and structural details of sperm proteins, and the modelling of protein-ligand interactions using X-ray and/or NMR structures to name a few) are expected to provide the experimental foundation for the design of small molecule inhibitors with antifertility effects. The technology underpinning vaccine development is constantly being developed and the introduction of DNA/RNA vaccines is certain to impact upon the field of immunocontraception.

Prostasome antigens as targets for sperm agglutinating antibodies demonstrated by 1-D gel electrophoresis and immunoblottings

Many immunoinfertile men have sperm agglutinating antibodies that are directed against prostasome-derived antigens, but these antigens have not been defined so far. We selected serum samples with high ELISA titres against prostasomes from a group of immunoinfertile patients with sperm agglutinating antibodies and used the sera for immunoblottings on 1-D SDS-PAGE of prostasomes and sperm cells. The immunoblottings with individual antiprostasome antisera on 1-D SDS-PAGE of prostasomes, revealed three to 10 bands for each serum. Eighty-five per cent of the serum samples contained bands in the 70-75 kDa region and 80% of the samples contained bands in the 50-55 kDa region. Immunoblottings of extracted sperm cells, revealed one to six bands in the molecular weight range 25-82 kDa and two of the samples recognized two bands with molecular weights (50 and 43 kDa) similar to immunoblottings of prostasomes. The prostasomal antigens recognized by the high titre-antisera of immunoinfertile men were generally different from the sperm antigens recognized by the same sera. This suggests that prostasomes offer a new set of antigens available for research on male immunoinfertility and immunocontraception.

A male genital tract-specific carbohydrate epitope on human CD52: implications for immunocontraception

The identification of unique sperm surface epitopes that are not expressed or exposed in the female reproductive tract is a key element in the development of antibody-based contraceptives. Western blotting and immunohistochemistry were performed to define the tissue distribution of the S19 epitope, which has been proposed as a target for immunocontraception. S19 is an IgG1 murine monoclonal antibody (mAb) directed to an N-linked carbohydrate epitope on a 15-25 kDa glycoprotein, sperm agglutination antigen-1 (SAGA-1), containing a peptide core identical to that of the lymphocytic surface protein CD52. In this study, the S19 epitope was shown to be absent from human lymphocytes, demonstrating a distinction between this epitope and the CAMPATH epitope that is recognized by an antibody against the terminal tripeptide and GPI-anchor of CD52. Further tissue specificity analysis identified the S19 epitope in the epithelium of the human epididymis and vas deferens, as well as on both epididymal and ejaculated spermatozoa. In contrast, the S19 epitope was absent in the five human female reproductive tract and 18 other somatic tissues tested. These results support the use of the S19 epitope as a contraceptive immunogen and the suitability of the S19 mAb as an intravaginal contraceptive. To test the agglutinating activity of the S19 mAb in a formulation designed for vaginal use, S19 mAb were bound to the surface of Novasomes, a multilamellar liposome delivery vehicle. S19-Novasome formulations agglutinated human spermatozoa and were as effective as unbound S19 mAb, demonstrating the feasibility of spermistatic contraceptives targeted to the male reproductive tract specific carbohydrate epitope.

Analysis of a human sperm CD52 glycoform in primates: identification of an animal model for immunocontraceptive vaccine development

Sperm agglutination antigen-1 (SAGA-1) is a human male reproductive tract glycoform of CD52. Unique modification of CD52 N-linked oligosaccharide chains in the epididymis and vas deferens results in the appearance of a carbohydrate epitope that is localized over the entire surface of human spermatozoa. SAGA-1 was characterized by the sperm-inhibitory murine monoclonal antibody (mAb) S19, and it is the target antigen of a human mAb (H6-3C4) associated with antibody-mediated infertility. Collectively, sperm surface localization, antibody inhibition of sperm function, and potential reproductive-tissue specificity identify SAGA-1 as an attractive candidate contraceptive immunogen. To establish an animal model for the study of SAGA-1 in immunologic infertility and immunocontraceptive development, we investigated the appearance of the S19 carbohydrate epitope in nonhuman primates. The S19 mAb demonstrated little to no immunoreactivity by Western blot analysis with protein extracts of spermatozoa from the baboon, marmoset, bonnet, cynomolgus, and pigtailed macaques. Immunohistochemical analysis identified CD52 in the bonnet monkey epididymis; however, the N-linked carbohydrate moiety recognized by the S19 mAb, and unique to SAGA-1, was absent. In contrast, the S19 carbohydrate epitope was identified in chimpanzee sperm extracts by Western blot analysis and in chimpanzee epididymal tissue sections by immunohistochemical analysis, indicating that it is conserved in this close relative of the human. Chimpanzee testis, seminal vesicle, and prostate do not express the S19 epitope. Although anti-CD52 immunoreactivity was identified in the spleen, the carbohydrate moiety recognized by the S19 mAb was absent, corroborating data in the human that demonstrated tissue-specific glycosylation of sperm CD52. Immunofluorescent analysis indicated that the chimpanzee homologue of sperm CD52 was present over the entire spermatozoon. In addition, the S19 mAb agglutinated chimpanzee spermatozoa in a manner similar to the effect observed on human spermatozoa. These data indicate that the distinctive carbohydrate moiety of human sperm CD52 is present in the chimpanzee, and they identify the chimpanzee as the most appropriate primate model to study the potential of this unique CD52 glycoform as a contraceptive immunogen.

Interactions between zona pellucida glycoproteins and sperm proacrosin/acrosin during fertilization

Fertilization is one of the most specific and carefully regulated cell-cell interactions in the animal body and is determined to a large extent by compatibility between ligand and receptor molecules on the surface of each gamete. On the zona pellucida (ZP), sperm receptor activity is associated with glycoproteins ZP3 (primary receptor for acrosome-intact sperm) and ZP2 (secondary receptor for acrosome-reacted sperm) but their complementary binding proteins on sperm are less well defined. In this communication we review the evidence for proacrosin as a secondary ZP binding protein. Proacrosin/acrosin binds non-enzymically to ZP glycoproteins. Binding is a strong ionic interaction between polysulphate groups on ZP glycoproteins (probably on their carbohydrate moieties) and basic residues on the surface of proacrosin. The stereochemistry of the reactants is crucial and determines to a large extent the affinity of binding. Site-directed mutagenesis and a 3D-structural analysis of boar and ram acrosin have identified 2 clusters of basic residues potentially involved in binding. A polysulphonated anticancer drug, suramin, has been shown to bind strongly to proacrosin/acrosin and to inhibit sperm-egg binding in vitro. In the mouse model, 125I-ZP2 and 3H-suramin bind approximately 65% less effectively to acrosin 'null' sperm than to wild-type sperm. Neither ZP2 nor suramin bind to acrosome intact sperm and can, therefore, only exert their effects after exposure of the acrosomal contents. Overall, this combination of biochemical, genetic and functional data supports the hypothesis that proacrosin is a multifunctional protein with a significant role in retaining acrosome-reacted sperm on the ZP surface long enough to enable ZP penetration to begin.

RASA, a recombinant single-chain variable fragment (scFv) antibody directed against the human sperm surface: implications for novel contraceptives

BACKGROUND

A recombinant single-chain variable fragment (scFv) antibody was engineered to a tissue-specific carbohydrate epitope located on human sperm agglutination antigen-1 (SAGA-1), a sperm glycoform of CD52.

METHODS AND RESULTS

cDNAs encoding the variable regions of the S19 [IgG(1)kappa] monoclonal antibody (mAb) were identified, linked, and cloned into the pCANTAB 5E vector. The recombinant anti-sperm antibody (RASA) was expressed in E. coli HB2151 cells as a 29 kDa monomer and, remarkably, also formed multimers of approximately 60 and 90 kDa. RASA reacted with the endogenous SAGA-1 antigen by Western blot analysis, labelled the entire human sperm surface by indirect immunofluorescence, and aggregated human spermatozoa in a tangled (head-to-head, head-to-tail, tail-to-tail) pattern of agglutination, as was also observed with the native S19 mAb.

CONCLUSIONS

These results demonstrate that active recombinant antibodies can be produced to a tissue-specific carbohydrate epitope on the human sperm surface, thereby opening opportunities for novel contraceptive agents.

Differential extraction and enrichment of human sperm surface proteins in a proteome: identification of immunocontraceptive candidates

The objective of this study was to discover previously unknown human sperm surface proteins that may be candidate contraceptive vaccinogens. To this end, methods of concentrating human sperm proteins for microsequencing by mass spectrometry were used, which increased the likelihood of identifying surface proteins. Vectorial labeling, differential extraction and two-dimensional (2-D) gel electrophoresis were employed to identify and isolate proteins accessible at the cell surface. Percoll harvested or swim-up sperm were either solubilized directly or solubilized after surface labeling with sulfo-succinimidyl-6-(biotinamido)hexanoate (sulfo-NHS-LC-biotin). Comparisons were made of proteins extracted with four lysis buffers: (i) Celis buffer containing 9.8 M urea and 2% Igepal CA-630; (ii) 1% Triton X (TX)-100; (iii) 1.7% TX-114 followed by phase partitioning; or (iv) 1 M NaCl. Blots of proteins separated by high-resolution 2-D electrophoresis were probed with avidin and antibodies to known proteins specific for three domains: the sperm surface (SAGA-1), the acrosome (SP-10), and the cytoskeleton (alpha-tubulin). Celis buffer (45 min) extracted proteins from all three major compartments. However, a 20-s extraction in Celis buffer enriched for several proteins and enabled the identification of several novel peptides by mass spectrometry. Mild extraction with TX-100 or 1 M NaCl solubilized mainly membrane and acrosomal proteins, but not cytoskeletal proteins. Comparison of biotinylated proteins extracted by each method showed that the major vectorially labeled proteins solubilized by Celis buffer were also solubilized by TX-100, TX-114, and 1 M NaCl. Extraction with TX-114 followed by phase-partitioning significantly enriched hydrophobic surface proteins and aided resolution and isolation. Eight protein spots microsequenced following all these extraction methods proved to be novel sperm molecules.

Changes of the major sperm maturation-associated epididymal protein HE5 (CD52) on human ejaculated spermatozoa during incubation

HE5 (CD52) is a glycoprotein which is secreted by the epididymis and which becomes inserted onto maturing spermatozoa. We have previously shown that, in cynomolgus monkey spermatozoa, changes occur upon maturation rendering cryptic the epitope to the monoclonal antibody CAMPATH-1G; the recognition site is then re-exposed during incubation under capacitation conditions. The present study investigated human ejaculated spermatozoa during incubation under similar conditions, using monoclonal antibodies that recognize different epitopes of the HE5 molecule comprising parts of the N-glycan (2E5) or peptide segments, including (CAMPATH-1G) or excluding (097) the glycosylphosphatidylinositol (GPI) anchor, to reveal modifications of sperm surface HE5. Flow cytometric analysis showed equally high percentages (approximately 90%) of viable spermatozoa cross-reacting with the antibodies before and after 6 h incubation. However, during incubation, the staining intensity increased 57% with CAMPATH-1G, 31% with 097, but remained unchanged with 2E5. The lymphocyte CD52 antibody CF1D12 stained only approximately 10% of spermatozoa either before or after incubation. Western blotting of sperm protein extracts using lectins indicated an increase in the exposure of sialic acid residues of HE5 after incubation. These results suggest that during incubation in capacitating conditions, there is an opening up of the HE5 glycoprotein molecule, increasing accessibility of some sialic acid residues and of the core peptide, particularly the GPI anchor.

DNA recombination as a possible mechanism in declarative memory: a hypothesis

The durability of declarative memory suggests that it has either a chemical or a structural basis. Current models of long-term memory are based on the general assumption that traces of memory are stored by structural modifications of synaptic connections, resulting in alterations in the patterns of neural activity. Changes in gene expression, regulated at both the transcriptional and the translational levels, are considered essential for structural synaptic modifications. Here we present an alternative hypothesis stating that permanent memory has a chemical rather than a structural basis. We suggest that the mechanism of memory coding in the brain is similar to that in the immune system so that the permanence of memories in the nervous system is ensured at the genomic level by a somatic recombination mechanism. Thus, we hypothesize that traces of permanent declarative memory might present within cerebral neurons in the form of novel proteins coded by the modified genes. This discussion is intended to provide evidence in support of a DNA recombination mechanism for memory storage in the brain and to stimulate further research working toward the evaluation of this hypothesis.

Cloning of a glucose phosphate isomerase/neuroleukin-like sperm antigen involved in sperm agglutination

The mouse monoclonal antibody (mAb) A36 produced by us and shown to induce extensive, "tangled" sperm agglutination was used to isolate cDNAs encoding its cognate antigen. Three overlapping cDNA clones specifically recognized by the mAb were isolated from a human testis cDNA expression library in lambdagt11. Sequencing of these cDNAs yielded the complete nucleotide sequence of a 3-kilobase cDNA that encodes the mAb-related polypeptide, designated sperm antigen-36 (SA-36), composed of 558 deduced amino acids. SA-36 cDNA contained a 5' untranslated region of 234 nucleotides (nt), an open reading frame of 1674 nt, and a 3' untranslated region of 1138 nt. SA-36 cDNA displayed > 99% homology to glucose phosphate isomerase (GPI)/neuroleukin (NLK) mRNA. This surprising homology was confirmed in Western blots demonstrating that mAb A36 reacted specifically with GPI obtained from rabbit muscle and from baker's yeast. Moreover, polyclonal, monospecific antibodies produced against beta-galactosidase/SA-36-3 fusion protein stained human spermatozoa and caused intensive agglutination of these cells in a manner similar to that with the mAb. Taken together, the data presented here demonstrated that mAb A36 cognate sperm surface antigen, encoded by SA-36 cDNA, is a GPI/NLK-like protein involved in sperm agglutination.

FSP95, a testis-specific 95-kilodalton fibrous sheath antigen that undergoes tyrosine phosphorylation in capacitated human spermatozoa

Protein tyrosine phosphorylation has been associated with both capacitation and motility of mammalian sperm. During capacitation, human spermatozoa undergo tyrosine phosphorylation of a characteristic set of proteins, only one of which has thus far been cloned and localized. We report here the sequence of a fibrous sheath protein of 95 kDa (FSP95) that undergoes tyrosine phosphorylation during capacitation of human spermatozoa and has similarity to sperm A-kinase anchor proteins (AKAPs). FSP95 is both auto- and iso-antigenic in humans as it is recognized by sera containing antisperm antibodies from infertile men and women. The 853-residue protein has a calculated molecular weight of 94.6 kDa and an isoelectric point (pI) of 6.0, and it contains multiple potential phosphorylation sites for protein kinase C and casein kinase II as well as one potential tyrosine kinase phosphorylation site at amino acid 435. The sequence has amino acid homology to mouse sperm fibrous sheath AKAP82 (pro-mAKAP82, 34% identity) and to human sperm fibrous sheath AKAP82 (pro-hAKAP82, 32% identity). The gene encoding FSP95 has 5 exons separated by 4 introns and is located on chromosome 12 at locus p13.3. Northern analysis detected a single transcript of approximately 3.0 kilobases, and Northern dot blot analysis of 50 human tissues revealed FSP95 mRNA expression only in testis. By employing sperm immobilization, indirect immunofluorescence, and immunoelectron microscopy with antisera to purified recombinant FSP95, the protein was localized to the ribs of the fibrous sheath in the principal piece of the sperm tail. FSP95 is the second fibrous sheath protein to be cloned, sequenced and localized in human spermatozoa.

Male-specific modification of human CD52

CD52 is an unusually short, bipolar glycopeptide bearing a highly charged N-linked carbohydrate moiety and a glycosylphosphatidylinositol membrane anchor. It is exclusively expressed on lymphocytes and in the male genital tract where it is shed into the seminal plasma and inserts into the sperm membrane. The sperm surface molecule has potential significance as a target for antibodies that inhibit sperm function and gamete interaction. Western blot analyses suggested cell type-specific modifications of the antigen. It was purified from seminal plasma and a detailed structural analysis performed. The majority of anchor structures in male genital tract CD52 showed 2-inositol palmitoylation, rendering molecules insensitive toward phospholipase C, and a sn-1-alkyl-2-lyso-glycerol structure in place of the diacylated anchor described by Treumann et al. (Treumann, A., Lifely, M. R., Schneider, P., and Ferguson, M. A. (1995) J. Biol. Chem. 270, 6088-6099). N-Glycans of the male genital tract product were based on bi-, tri-, and tetraantennary structures of highly charged (up to -7), terminally sialylated complex-type sugars. A substantial proportion carried varying numbers of lactosamine repeats of which nearly 30% were branched. Different from lymphocytes, 10-15% of all N-glycans of the male genital tract antigen also contained peripheral fucose. These data confirm that male genital tract CD52 is distinct from the lymphocyte form by both N-linked glycans and COOH-terminal attached lipid anchor.

Evidence for a unique N-linked glycan associated with human infertility on sperm CD52: a candidate contraceptive vaccinogen

A major objective in developing a sperm antigen-based contraceptive vaccine for humans is the discovery of sperm surface immunogens that are functionally relevant and sperm specific. The latter criterion is deemed essential to avoid the possibility of inducing autoimmune disease upon vaccination. This review presents evidence that a unique carbohydrate epitope is synthesized in the human epididymis, is attached to the core peptide of CD52, a lymphocyte differentiation marker, and is subsequently inserted into the sperm membrane via a glycosylphosphatidylinositol anchor. This unique CD52 glycoform is localized to the entire sperm surface, functions as a potent target for agglutinating and cytotoxic antibodies, and is one of the few well-defined sperm surface glycoproteins indicated in human antibody-mediated infertility.

N-linked glycan of a sperm CD52 glycoform associated with human infertility

In a benchmark study, Isojima and colleagues established H6-3C4, the first successful heterohybridoma immortalized from the peripheral blood lymphocytes of an infertile woman who exhibited high sperm-immobilizing antibody titers. The present report demonstrates the identity between the glycoprotein antigens recognized by the human H6-3C4 monoclonal antibody (mAb) and the murine S19 mAb, generated in our laboratory to sperm agglutination antigen-1 (SAGA-1). Both mAb's recognize N-linked carbohydrate epitopes on the 15-25 kDa, polymorphic SAGA-1 glycoprotein that is localized to all domains of the human sperm surface. Treatment with phosphatidylinositol-specific phospholipase C demonstrated that SAGA-1 is anchored in the sperm plasmalemma via a GPI-lipid linkage. Immunoaffinity purification and microsequencing indicated that the core peptide of the SAGA-1 glycoprotein is identical to the sequence of CD52, a GPI-anchored lymphocyte differentiation marker implicated in signal transduction. Comparison of anti-SAGA-1 and anti-CD52 immunoreactivities revealed that the sperm form of CD52 exhibits N-linked glycan epitopes, including the epitope recognized by the infertility-associated H6-3C4 mAb, which are not detected on lymphocyte CD52. Thus, the two populations of the CD52 glycoprotein on lymphocytes and spermatozoa represent glycoforms, glycoprotein isoforms with the same core amino acid sequence but different carbohydrate structures. Furthermore, mAb's to the unique carbohydrate epitopes on sperm CD52 have multiple inhibitory effects on sperm function, including a cytotoxic effect on spermatozoa in the presence of complement. These results are the first to implicate unique carbohydrate moieties of a sperm CD52 glycoform as target epitopes in the anti-sperm immune response of an infertile woman. Furthermore, localization of CD52 on all domains of the sperm surface coupled with the multiple sperm-inhibitory effects of antibodies to its unique carbohydrate moieties suggest opportunities for immunocontraceptive development.

The potential use of sperm antigens as targets for immunocontraception; past, present and future

Immunocontraception, and in particular the targeting of antibodies to gamete-specific antigens implicated in sperm egg binding and fertilisation, offers an attractive approach to the growing global problem of overpopulation. Such an idea is not new; indeed several immunocontraception trials, using animal model systems, have been reported in recent years and a number are reviewed here. However, the results of these studies have been largely disappointing. We believe that two fundamental flaws attribute to the poor success of most of these preliminary immunocontraceptive trials. Firstly, loss of fertility has invariably been used as the assay. This presupposes that immuno-neutralisation of a single, gamete-specific antigen will be sufficient to cause a significant reduction in fertility; however, recent data suggests that such a premise may not be well-founded for a number of reasons. Secondly, and arguably the most important flaw, is the almost universal, but largely inappropriate, use of systemic immunisation as the sole route of antigen delivery. Whilst systemic immunisation regimes may lead to high serum IgG levels, these levels do not correlate with specific antibody levels in the reproductive tract or with contraceptive efficacy. Hence, an alternative antigen delivery approach is required which will induce an effective local immune response in the reproductive tract. Here we discuss the ways in which this might be achieved.

Characterization of a potent sperm-agglutinating monoclonal antibody and its cognate antigens

OBJECTIVE

To identify sperm antigens that are capable of eliciting infertility-related sperm-agglutinating antibodies.

DESIGN

In vitro laboratory experiments.

SETTING

University research laboratory.

PATIENT(S)

Fertile semen donors.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

Sperm agglutination, immunofluorescence localization, and flow cytometric analysis of surface expression of A36 antigens. Antigen analysis by Western immunoblotting.

RESULT(S)

Monoclonal antibody A36 induced intensive head-to-head, tail-to-tail, and head-to-tail agglutination of motile human spermatozoa. Antigens recognized by A36 were localized on the acrosomal cap and in the principal tail regions of motile, noncapacitated human sperm. Changes in subcellular levels and localization of the A36-recognized epitope occurred after capacitation and acrosomal loss. A36 reacted with a polymorphic series of proteins in Western blots of sperm extracts from humans and various other animal species, including mouse testis extracts. A common 53-kd antigen was recognized by the antibody in the different antigenic preparations.

CONCLUSION(S)

A mouse antibody to human sperm, monoclonal antibody A36, caused intensive agglutination of noncapacitated human spermatozoa and reacted with antigens on the acrosomal cap and in the principal tail regions. Of the multiple polypeptides that were reactive with the monoclonal antibody in sperm extracts from humans and other animal species, a common 53-kd antigen was recognized.

Expression of the human antigen SPAG2 in the testis and localization to the outer dense fibers in spermatozoa

ABSTRACT Antisperm antibodies (ASAs) have been implicated in some instances of infertility. To characterize sperm antigens relevant to immunologic and immunocontraceptive development, SPAG2 (sperm-associated antigen 2) was identified by screening a human testis cDNA library with human sera positive for ASAs. Subsequently, two isoforms, SPAG2-1 and SPAG2-2, were identified in testis and placenta libraries, respectively. In the current study, Southern analysis of human genomic DNA with a probe common to the two SPAG2 isoforms indicated a single SPAG2 gene; therefore, alternative splicing is a likely mechanism for production of variant mRNAs. In situ hybridization of human testis sections demonstrated the expression of SPAG2 in primary spermatocytes, with decreased or arrested expression in postmeiotic cells. Immunofluorescence of Triton X-100-extracted spermatozoa with an anti-SPAG2 peptide antiserum indicate that SPAG2 is an intracellular component of the sperm flagellum. Electron microscopy refined this localization to the outer dense fibers (ODFs), structural filaments associated with the mammalian sperm axoneme. The ODFs have been reported to be composed of keratin-like intermediate filament proteins. However, SPAG2 does not exhibit the molecular characteristics of such proteins, nor does SPAG2 demonstrate sequence homology with previously characterized ODF proteins. Therefore, SPAG2 represents a novel protein of human sperm ODFs. Characterization of SPAG2 will further our understanding of ODF function in normal sperm motility and of flagellar abnormalities that lead to male infertility.