Project Summary
Sperm have a remarkable and still unexplained capacity to survive for extended periods in the oviduct, contrary to the
strong innate immune response they elicit in the uterus. Following mating, when semen reaches the uterus, sperm interact
with uterine epithelial cells to stimulate an inflammatory reaction. The release of pro-inflammatory cytokines induces a
rapid infiltration of polymorphonuclear neutrophils (PMNs) into the uterus. PMNs then release neutrophil extracellular
traps and phagocytose the majority of sperm. The few sperm that evade phagocytosis in the uterus move to the oviduct
where, in stark contrast, they do not trigger a phagocytic response and can survive for extended periods ranging from
hours to months, depending on the species. Indeed, in the isthmic region of the oviduct (nearest to the uterus), the
presence of macrophages or neutrophils is rare. The oviduct demonstrates unique immunological privilege within the
female reproductive system that enables remaining sperm to avoid elimination by phagocytes. The mechanisms
underlying the different responses between the oviduct and the uterus are unknown. But the ability of sperm to survive
and evade phagocytosis in the oviduct is critical for fertility. Sperm are coated with sialic acid-terminating glycans
(sialoglycans) and changes in sialylation influence the ability of sperm to evade phagocytosis of uterine macrophages.
Sialoglycans can interact with several proteins, including Siglecs (sialic acid-binding immunoglobulin-type lectins), the
most abundant and best-known receptors for sialoglycans. There are many different Siglec proteins, found most
commonly on leukocytes, and they are known for their ability to activate or inhibit the immune system, attract immune
cells, and facilitate cell adhesion. However, Siglecs have also recently been localized to non-immune cells such as kidney
and prostate epithelium and uterine and cervical epithelium. Using endpoint PCR, we discovered for the first time that 8
Siglecs are expressed in oviduct epithelium (porcine) and expression is biased towards Siglecs that inhibit the immune
response. We also found that porcine sperm contain sialylated glycans that are high-preference Siglec ligands. These
results led to our overall hypothesis that sperm sialoglycan binding to oviduct Siglecs alters cytokine production
through Siglec-downstream signaling. This inhibits the innate immune response, allowing sperm to avoid immune
rejection and phagocytosis during the storage period in the oviduct. To test this model using in vivo and in vitro
studies, we will determine the role of Siglecs in the oviduct response to sperm by removing sperm sialic acid and blocking
individual Siglecs to determine if oviduct cell gene expression, including the production of immune mediators, and
chemotaxis is affected. We will determine if sperm interaction with oviduct cells is dependent on direct sialoglycan-Siglec
binding. Finally, we will compare Siglec and intracellular signaling protein abundance and localization in the oviduct and
uterus to investigate if that is responsible for the different innate immune responses of both organs to sperm. Collectively,
these Aims will elucidate the role of oviduct Siglecs and sperm sialoglycans in suppressing the immune response in the
oviduct, despite sperm being foreign cells that are largely phagocytosed in the uterus. The results may be used to enhance
sperm lifespan in the oviduct and perhaps develop therapies for infertile females.