The mammalian female reproductive tract has immune responses to protect against foreign invaders, including the attraction of white blood cells to the area. How sperm cells overcome these defenses as well, and at the same time, play a role in white blood cell attraction, are key questions in reproductive biology. The central hypothesis of this grant seeks to link these two important steps of mammalian reproduction into a unifying concept. The investigators have recently demonstrated that molecules called beta-defensins uniformly coat the entire surface of mammalian sperm during maturation in the male tract. Beta-defensins are key effector peptides of innate immunity found throughout the animal kingdom. These peptides have potent antimicrobial activities and act as chemical attractants. While the expression of beta-defensin peptides by reproductive tissue has been described in several invertebrate species, the presence of beta-defensins on the sperm surface is a novel finding and has only been recently described in mammals. Their presence may reflect adaptations of these peptides for internal fertilization. It is proposed that sperm-associated beta-defensins, which form a uniform surface coat on spermatozoa, promote fertility by first protecting sperm from immunorecognition by the female reproductive tract and then providing enhanced innate immune protection in part by attracting white blood cells. The investigators propose to test these ideas through a combination of in vivo and in vitro experiments using a mouse model. This innovative project will yield answers to longstanding questions in reproductive biology and will provide insights into the sources of variation of reproductive fitness in mammals. The understanding of these mechanisms holds significance for managing reproduction in domestic species, evaluating reproductive fitness in wildlife, and maintaining reproductive health in humans.
The mammalian female reproductive tract has immune responses to protect against foreign invaders which actually include spermatozoa. A long standing question in biology has been how do sperm evade detection by the high level surveilance of the female reproductive tract? How sperm cells overcome these defenses and at the same time, play a role in attracting white blood cells which engulf and destroy non-fertilizing sperm are key questions in reproductive biology. The investigators on this project had previously demonstrated that molecules called beta-defensins uniformly coat the entire surface of mammalian sperm during maturation in the male tract. Beta-defensins are key effector peptides of innate immunity (cellular immunity) found throughout the animal kingdom. These peptides have potent antimicrobial activities and act as chemical attractants. While the expression of beta-defensin peptides by reproductive tissue has been described in several invertebrate species, the presence of beta-defensins on the mammalian sperm surface is a novel finding and has only been recently described. Their presence may reflect adaptations of these peptides for internal fertilization. It is proposed that sperm-associated beta-defensins, which form a uniform surface coat on spermatozoa, promote fertility by first protecting sperm from immunorecognition by the female reproductive tract and then by providing enhanced innate immune protection in part by attracting white blood cells involved in their own demise. The investigators investigated this through a combination of in vivo and in vitro experiments using a mouse model. In addition, comparative studies using non-human primates were also included. We developed a genetically altered mouse model where beta-defensin 22 was not expressed on the sperm surface. Sperm morphology and functions all appeared normal. While slight reductions in litter size occurred, overall, mice were able to compensate in some way for the lack of beta-defensin 22. We did demonstrate that beta-defensin 22 was directly responsible for masking unique proteins on sperm and it may be that other defensins took its place in the genetically altered mice. The phenomenon of immunoprotection of sperm by the beta-defensin coat was further demonstrated in a non-human primate (macaque) system where breeding trials were conducted in females that had high circulating antibodies to sperm proteins, but no contraceptive effect was ever observed because of the defensin coating. This project has demonstrated that beta-defensing coatings on mammalian sperm are responsible for protecting sperm as they migrate through the female reproductive tract on their way to the egg. As non-fertilizing sperm die, they release beta-defensin 22 which in turn attracts white blood cells to where sperm are so they can engulf and remove these extra spermatozoa. At least in mice, genetic alteration of the beta-defensin 22 gene results in a replacement by an unknown molecule, which facilitates normal sperm function. This underscores the critical nature of the fertilization process leading to successful reproduction and continuation of the species.
