Siglecs in the Porcine Oviduct and Sialylated Ligands on Sperm: Roles in the Formation of the Sperm Reservoir

Abstract During mammalian insemination, most of the deposited sperm are lost by retrograde flow or the female reproductive tract’s immune response. Once semen enters the uterus, seminal fluid and sperm elicit leukocyte infiltration that contributes to the elimination of sperm in the uterus. However, unlike the uterus, invading sperm do not trigger a phagocytic response in the oviduct in the absence of dysfunction or disease states. Thus, the oviduct possesses a distinct immunological microenvironment that tolerates sperm while maintaining the capacity to respond to pathogens. It has been suggested that sperm glycocalyx contributes to innate oviductal tolerance, but the cell and molecular mechanisms are not understood. The current investigation focused on the role of sialic acid-containing glycoconjugates on sperm and their potential to elicit innate tolerance via cognate sialic acid-binding immunoglobulin-type lectins (Siglecs) expressed in the oviduct. In this manuscript, we report our discovery of eight Siglecs (Siglecs-1, -2, -3, -5, -10, -11, -14, -15) expressed in the lower pig oviduct, five of which are known for immune inhibitory functions (Siglecs-2, -3, -5, -10, and -11) and how these may play a role in achieving sperm-induced immune suppression in the oviduct microenvironment. Mass spectrometry profiling of porcine sperm revealed the presence of a mixture of α2,3 and α2,6 linked sialic acids with α2,3-linked sialic acids as the dominant linkage. Of the detected glycans, several sialic acid-containing glycoconjugates were identified as potential ligands for Siglecs (among O-linked glycans: NeuAc 1 GalNAc 1 , NeuGc 1 GalNAc 1 , NeuAc 2 Gal 1 GalNAc 1 ; attached to glycolipids: NeuAc 2 Gal 1 GalNAc 1 Gal 1 Glc 1 , Fuc 1 Gal 1 GalNAc 1 NeuAc 1 Gal 1 Glc 1 ). This is the first report of Siglec expression in the mammalian oviduct and total glycan analysis of porcine sperm. The results of this study reveal the potential for a sperm-sialoglycan and oviductal-Siglec axis that may contribute to the distinct immunophysiology of the oviduct fundamentally required for undisrupted reproduction in mammals.