The Human Contraceptive Antibody Targets A Semenogelin Galectin-3 N-Linked Glycan Complex on the Sperm Surface
The human contraception antibody (HCA) is a non-hormonal topical contraceptive entering Phase II clinical trials that functions by agglutinating and immobilizing sperm. To date, the HCA epitope has been described as an N-linked polylactosamine glycan motif on CD52, but this assignment has been inferred only by colocalization with anti-CD52 and S19 antibodies in western blots. Here, we report that HCA may instead bind semenogelin-galectin-3 complexes associated with the sperm surface. Semenogelins are gel forming, highly glycosylated proteins, with molecular weights of 50-70 kDa that are cleaved into smaller segments that adhere to sperm during semen liquefaction. In this study, we isolated the HCA antigen from seminal plasma and sperm using an HCA affinity column and identified semenogelin by mass spectrometry. Western blot analysis of seminal plasma showed that HCA binds to a large smear of proteins with a higher apparent molecular mass than CD52, and that these may correspond to semenogelin sequences. Flow cytometric analysis of enzymatically deglycosylated sperm demonstrated that HCA binding requires sperm surface N-linked glycans. However, despite this apparent glycan dependence, HCA failed to bind any glycans represented on a comprehensive glycan microarray. In addition to semenogelin, Galectin-3 was identified in the HCA purified sperm lysates, supporting a model in which HCA recognizes semenogelin-derived glycopeptides that are tethered to the sperm surface via a galectin mediated interaction with an N-linked glycan. We found that O-linked glycan modifications on the semenogelin proteins contain LacNAc motifs that can bind to galectins. Finally, we found that depletion of galectins bound to the sperm surface by competitive inhibitors reduced HCA-mediated agglutination, and this could be rescued by addition of recombinant galectin-3/semenogelin-2 complexes to sperm cultures. Collectively, our findings suggest that HCA's binding site involves a semenogelin-galectin-3 complex anchored to the sperm surface via an N-linked glycan.