Inhibition of human spermatozoon—oocyte interaction in vitro by monoclonal antibodies to CD46 (membrane cofactor protein)

Abstract

CD46 (membrane cofactor protein) is a cell surface complement regulatory glycoprotein that facilitates enzymatic cleavage of complement component C3b; it is expressed by both human oocytes and acrosome-reacted spermatozoa. Murine anti-CD46 monoclonal antibody (mAb) has been reported to decrease significantly the ability of human spermatozoa to penetrate hamster oocytes. We have investigated the effect of purified anti-CD46 mAbs on spermatozoon-oocyte interaction in an autologous zona-free oocyte penetration test. Oocytes and/or spermatozoa were preincubated with either of two anti-CD46 murine mAbs, TRA.2.10 (a non-blocking mAb) and MH61 (a mAb that functionally blocks C3b-ligand binding activity), or a control isotype-matched mAb, in medium supplemented with human serum albumin. Preincubation of both spermatozoa and zona-free oocytes with TRA.2.10, but not MH61, caused a significant decrease in the number of oocytes showing sperm binding and pronuclear formation (9/23) compared with controls (21/26) in this complement component-depleted medium. This effect was not observed if oocytes or spermatozoa alone were preincubated. These data suggest that CD46 has a role in human spermatozoon-oocyte interaction at the level of the oocyte plasma membrane, and indicate that a novel function other than direct C3b binding could be involved.

Identification and characterization of a silencer regulatory element in the 3′-flanking region of the murine CD46 gene

“…These include its sperm-dominant high expression of protein [14,15], its lack of charged sugars [16,17], its limited distribution to the inner acrosomal membrane until the acrosomal reaction, when it is expressed on the surface of sperm [18], and its deficiency possibly being related to ideopathic infertility [47]. In addition, although monoclonal antibodies against CD46 were reported to block human sperm binding to hamster eggs or to human eggs [18,19], the inhibition of binding was always incomplete. Furthermore, the epitopes recognized by monoclonal antibodies competent to block sperm binding to human and hamster eggs are wholly different [18,19].…”

“…In addition, although monoclonal antibodies against CD46 were reported to block human sperm binding to hamster eggs or to human eggs [18,19], the inhibition of binding was always incomplete. Furthermore, the epitopes recognized by monoclonal antibodies competent to block sperm binding to human and hamster eggs are wholly different [18,19]. Although the role of CD46 in fertilization has not been established definitively, we propose that the novel mechanism of CD46 gene regulation and its tissue-specific expression in the testicular germ cells supports this putative role of CD46.…”

“…One possible interpretation of these findings is that sperm CD46 may act as an egg-binding protein expressed on the inner acrosomal membrane. The fact that monoclonal antibodies against human CD46 partially block binding of human sperm and hamster eggs may support this hypothesis [18,19]. Although conclusive evidence has yet to be discovered, CD46 may be involved in homologous Figure 1 For legend see facing page species recognition by functioning in C-immune effector cell responses (innate immunity) and in sperm-egg adhesion (fertilization).…”

Targeted and restricted complement activation on acrosome-reacted spermatozoa

“…Also, Abs against MCP inhibit binding to and penetration of zona-free hamster eggs (19,29,30) and human zona (31) by human spermatozoa. In particular, Abs against complement control protein repeat 1 (CCP1), the aminoterminal CCP, block binding more efficiently than function-blocking Abs that bind to CCPs 3 and 4 (30). This observation led us to evaluate MCP expression on spermatozoa of New World monkeys (32), since they express a splice alternative of MCP lacking CCP1 on somatic cells (33).…”

“…In these species, another related protein, Crry, is widely expressed and performs MCP's complement-regulatory role (27,28). Also, Abs against MCP inhibit binding to and penetration of zona-free hamster eggs (19,29,30) and human zona (31) by human spermatozoa. In particular, Abs against complement control protein repeat 1 (CCP1), the aminoterminal CCP, block binding more efficiently than function-blocking Abs that bind to CCPs 3 and 4 (30).…”