To enhance the opportunities of fertilization, millions of sperm are deposited into the bovine uterus during artificial insemination (AI). However, only some hundreds to thousands could find their way to the oviduct (site of fertilization) and most of sperm are eliminated (Katila, 2012). The deposited semen is considered foreign to the uterus and so inducing an inflammatory response. This inflammatory response is characterized by a rapid and transient leukocytic infiltration, mostly polymorphonuclear cells (PMNs), which is important not only to remove the excess or dead sperm with associated uterine contaminants (Robertson, 2005), but also to prevent acquired immune response against sperm (Hansen, 2011).
At first glance, immune system and reproductive system seem to be separately working, but the uterus is indeed an immunologically competent organ (Kannaki et al., 2011). The uterus plays pivotal roles in regulation of estrous cycle, implantation, placentation and maintenance of pregnancy till normal parturition (Turner et al., 2012). Concurrently with these reproductive functions, the uterus needs to overtake several immunological challenges for attacking invading pathogens (bacteria, viruses, fungi or parasites) as well as tolerating allogenic sperm and semi-allogenic embryo. The capability of uterus to differentially respond to various antigens to which it is exposed requires a unique local immune system (Robertson, 2000).
The immune system reacts via two main responses namely innate immunity and adaptive immunity. Although the adaptive response is a much more specific, the innate one is considered the first line of defense (Koga and Mor, 2008). Pattern recognition receptors (PPRs) are innate immune cell receptors involved in the stimulation of inflammatory responses towards infection or sterile tissue injury through recognition of pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) respectively (Janeway and Medzhitov, 2002). Among the families of PPRs, Toll-like receptors (TLRs) have been found the most important family for distinguishing “infectious non-self” from “non-infectious self” (Akira and Hemmi, 2003).
TLRs are membrane-spanning proteins with extracellular domains of leucine-rich repeats. To date, eleven mammalian TLRs have been detected with each receptor differs in its ligand specificity (Mogensen, 2009). Like other mucosal surfaces, the bovine endometrium expresses TLRs 1 to 10, whereas purified populations of epithelial cells express TLRs 1 to 7 and 9, and stromal cells express TLRs 1 to 4, 6, 7, 9 and 10. From these 10 receptors, TLR2 and TLR4 have gained a special importance not only due to their fundamental roles in the defense mechanism against invading Gram-positive or Gram-negative bacteria through recognition of peptidoglycan or lipopolysaccharides (LPS) respectively (Davies et al., 2008), but also because of their participation in ovulation (Liu et al., 2008) and fertilization (Shimada et al., 2008) processes. Specifically, TLR2 and TLR4 could regulate the former physiological events via binding with their endogenous ligands like hyaluronan, high mobility group box-1 protein (HMGB1), heat shock protein 60 (HSP60) or 70 (HSP70), etc (Piccinini and Midwood, 2010).
Recently, we demonstrated that live sperm, but not dead ones, could attach to bovine uterine epithelial cells (BUECs) and induced an acute inflammatory response. Additionally, BUECs-conditioned medium triggered by sperm enhanced the phagocytic activity of PMNs towards sperm in vitro (Elweza et al., 2018). Nevertheless, the mechanism responsible for sperm attachment to BUECs with the subsequent inflammatory cascade is still unclear. Therefore, this study aimed to investigate the role of TLR2 and/or TLR4 in the signaling pathway whereby sperm might stimulate inflammation in BUECs in vitro.