Comparative immunoreactivity of mouse and hamster sperm proteins recognized by an anti-P26h hamster sperm protein

The binding of the spermatozoon to the zona pellucida is a species-specific phenomenon. We have previously shown that the binding of hamster sperm to the homologous zona pellucida involves a sperm 26-kDa glycoprotein, the P26h, originating in the epididymis. In order to establish to what extent this sperm protein is involved in the species-specific recognition of the egg's extracellular coat, we have compared the inhibitory properties of anti-P26h antibodies in a sperm-zona pellucida assay using hamster and mouse gametes. Anti-P26h IgGs inhibit, in a dose-dependent manner, gamete interactions in both species, although in a less efficient manner in the mouse than in the hamster. While anti-26kDa Fab fragments are as efficient as the intact IgG to inhibit hamster sperm-zona pellucida binding, they have no effect on mouse gamete interaction. ELISA, Western blot, and immunohistochemical experiments have been performed in order to characterize the mouse antigen(s) recognized by the anti-P26h antiserum. ELISA and Western blots showed that this antiserum recognized two proteins on mouse spermatozoa that are less reactive than the hamster P26h. These antigens are localized in the acrosomal region of epididymal spermatozoa of both species. These results indicate that the hamster P26H involved in zona pellucida interaction has certain unique epitopes, while others are common to the sperm of both species.

Further characterization of a secreted epididymal glycoprotein in mice that binds to sperm tails

Sperm maturation antigen 4 (SMA-4) is a surface component of the mouse sperm tail. Previously, immunofluorescence studies indicated that SMA-4 may be secreted by principal cells of the distal caput epididymidis and bound to spermatozoa as they pass through that region of the duct. In the present study, detergent extracts of spermatozoa from the cauda epididymidis were subjected to polyacrylamide gel electrophoresis under reducing and denaturing conditions, transferred to nitrocellulose, and immunostained with a monoclonal antibody against SMA-4. A band of approximately 54,000 molecular weight was revealed. The band was also stained by the periodic acid-Schiff (PAS) procedure. This glycoprotein was not detected in extracts of spermatozoa from the proximal caput epididymidis or of spermatozoa from the cauda epididymidis that were preincubated for 4 hours in an in vitro fertilization environment. Blots of sperm-free fluid from the corpus and cauda epididymidis displayed an immunoreactive and PAS-positive band of about 85,000 molecular weight that was not observed in fluid from the caput epididymidis. The difference in the molecular weights of the antigen in the fluid and that in extracts of cauda spermatozoa suggests that SMA-4 may be modified chemically upon association with the sperm surface.

Inhibition of in vitro fertilization by mouse anti-mouse sperm sera and preliminary antigen identification

Antisperm antibodies are implicated as one causative factor of infertility, but the target antigens have not been identified. Immune responses to sperm antigens are qualitatively variable even within a single mouse strain. We took advantage of this variability and immunized individual female mice to allogeneic sperm to reflect their natural exposure during mating. We determined the ability of the individual sera to inhibit in vitro fertilization and to bind to sperm antigens separated by electrophoresis. Compared to preimmune sera, four of five immune sera significantly inhibited in vitro fertilization. The serum from individual mice bound variable panels of sperm antigens. By comparing the panels, we identified two polypeptides with molecular weights of 40,000 and 44,000 that were bound by all sera. We propose that these molecules may be good candidates for further investigation of the immunoprophylaxis of pregnancy.

Monoclonal antibodies to human trophoblast and sperm antigens: report of two WHO-sponsored workshops, June 30, 1986--Toronto, Canada

Two WHO-sponsored workshops were recently held to obtain a consensus view from researchers active in the field of reproductive immunology on the current status of the application of monoclonal antibodies to studies of molecular events underlying reproduction and to determine the feasibility of using this approach to identify trophoblast- or sperm-specific antigens that might represent suitable candidates for the development of antifertility vaccines. A total of 66 mouse monoclonal antibodies reacting with human sperm and 45 monoclonal antibodies reacting with human trophoblast membrane components were submitted by 29 laboratories. These were evaluated in coded form by 42 laboratories with the appropriate expertise in biochemistry, immunohistology and tests of reproductive cell function. The majority of both anti-sperm and anti-trophoblast monoclonal antibodies cross-reacted with cellular elements in non-reproductive tissues. However, at least five monoclonal antibodies (two anti-trophoblast and three anti-sperm) appeared to demonstrate sufficient specificity to warrant further investigation as reagents for the identification of antifertility vaccine candidates.

Immunological identification of sperm antigens that participate in fertilization

Using monoclonal antibodies (mAbs) as probes for specific components of the spermatozoon, we have initiated an investigation aimed at identifying those sperm components that participate in the events of gamete interaction. We intend to exploit these mAbs not only for identifying functional sperm components, but also for defining the constituents of discrete domains that comprise the structure of this highly differentiated cell. Among the large group of anti-sperm mAbs that we have generated, we have focused to date upon two categories. The first category consists of mAbs that localize to the acrosomal crescent, a restricted region of plasma membrane overlying the acrosome. Within this category, the mAbs share many similarities with regard to subclass, species and tissue cross-reactivity, and antigen solubility, in addition to cellular distribution. Nevertheless, despite these similarities, some mAbs in this category (e.g., M42) inhibit fertilization, whereas others (e.g., M41) are non-inhibitory. The block to fertilization observed in the presence of M42 is dependent upon the zona pellucida surrounding the egg. The specific event prevented by M42 appears to be the induction of the acrosome reaction at its physiological site, the surface of the zona pellucida. The sperm components recognized specifically by M42 are a cluster of high molecular weight moieties, ranging from approximately 220,000 to 240,000. The mAbs described in the second category display common localization at the equatorial segment of the sperm head. The pair of mAbs discussed from this category, M2 and M29, again bear considerable similarity to each other, yet differ significantly in their ability to inhibit fertilization. M2 does not inhibit, whereas M29 causes a marked inhibition of fertilization. With M29, however, the block to fertilization is independent of the zona pellucida. The M29 mAb interferes with sperm interaction with the egg plasma membrane subsequent to sperm attachment; since M29 does not prevent sperm binding to the egg plasma membrane, the specific event affacted, in all likelihood, is gamete membrane fusion. M29 recognizes a single sperm component, with subunit molecular weight of approx. 40,000. A variety of experiments are underway currently, both to characterize the antigens recognized by these mAbs further as well as to identify additional sperm components that participate in the fertilization process.

Control of fertilization by immunization with peptide fragments of sperm specific LDH-C4

The use of a well-defined, synthetic antigen is essential to progress in developing a vaccine for fertility control and to establish with certainty the utility of such technology. The studies with LDH-C4 are consistent with the feasibility of using a synthetic antigen in a vaccine to control fertility. LDH-C4 remains the best-developed candidate for future studies. Immunization with LDH-C4 does suppress fertility, and the complete biochemical characterization of this isozyme is well underway. These studies will provide a wealth of synthetic antigens to substitute for the natural product in experiments to determine whether immunologic contraception is appropriate and desirable for wide-scale application in human beings.