Plasmodium parasites invade red blood cells and induce the expression of parasite-derived proteins on the cell surface of the infected erythrocyte (IEs) (Chan et al., 2014). These parasite-derived proteins, known as variant surface antigens (VSAs), enable the IEs to adhere to the endothelium and subsequently infect different organs, including the placenta (Autino et al., 2012). In addition, VSAs bind to serum proteins forming red blood cell aggregates (rosetting), allowing the evasion of the host immune response, and establish chronic infection (Wahlgren et al., 2017). Among, VSAs P. falciparum erythrocyte membrane protein 1 (PfEMP1) is a major surface antigen involved in vascular adhesion and sequestration of IEs (Chan et al., 2014). Notably, fEMP1 proteins are encoded by a polymorphic var multigene family (approximately 60 copies per genome), characterized by expression through allelic exclusion, allowing switching between PfEMP1 proteins and subsequent modification of the antigenic and binding properties of IEs (Wahlgren et al., 2017).

PfEMP1 proteins mediate the adhesion of IEs to different receptors and surface molecules including integrins (Chesnokov et al., 2018), endothelial protein C receptor (EPCR) (Gillrie et al., 2015), CD36 (Hsieh et al., 2016), intercellular adhesion molecule-1 and 2 (ICAM-1/2) (Lennartz et al., 2019), proteoglycans, such as chondroitin sulphate A (CSA) (Pehrson et al., 2016), glycosaminoglycans (GAGs), such as hyaluronic acid (HA) (Beeson and Brown, 2004), P-selectin, E-Selectin (Metwally et al., 2017) and Platelet/endothelial cell adhesion molecule-1 (PECAM-1) (Berger et al., 2013). In the placenta, the PfEMP1 variant VAR2CSA is predominantly expressed in IEs (Tuikue Ndam et al., 2005) and is the main VSA responsible for parasite-binding tropism (Nunes and Scherf, 2007; Smith, 2014) in this organ (Benavente et al., 2018). Notably, a selective accumulation of mature asexual stages of P. falciparum- IEs occurs in the IVS and chorionic villi surface (Beeson et al., 2002; Muthusamy et al., 2004; Figure 1A). The accumulation of IEs is mediated by receptors and molecules on the ST surface, macrophages (MΦ) monocytes (Mo), the fibrinoid deposits, and blood vessels in the VS (Beeson et al., 1999; Matejevic et al., 2001; Sugiyama et al., 2001).

Chondroitin sulphate A is the principal placental IEs receptor (Fried et al., 2006; Chishti, 2015). However, other molecules such as HA, non-immune immunoglobulins (IgG/IgM), and ICAM-1, present on the ST, can act as adhesion receptors (Maubert et al., 1997; Sartelet et al., 2000; Flick et al., 2001; Beeson and Brown, 2004; Barfod et al., 2011; Figure 1A). CD38 is only localized in the cytoplasm of stromal cells, fibroblasts, and MΦ, where the presence of IEs might influence its expression without being directly implicated in the sequestration of these (Sartelet et al., 2000). On the other side, HA is associated mainly with the aggregation or clumping of IEs (Beeson and Brown, 2004). Importantly, the expression of the different receptors is influenced by P. falciparum-induced proinflammatory cytokines (Vásquez et al., 2013).

In addition, other factors determine the accumulation and the sequestration of IEs in the placenta, including the low placental blood flow associated with trophoblast conformational changes, for the formation of cytotrophoblastic prolongations, known as “Coan-Burton bridges” (Moraes et al., 2013), IEs cell deformability (Safeukui et al., 2018), and the presence of Mo (Sugiyama et al., 2001). The adhesion and invasion of IEs also depend on the type of Plasmodium species. Thus, P. vivax and P. falciparum can bind to placental CSA and HA. However, P. vivax cannot bind to CD36 and ICAM-1 (Chotivanich et al., 2012), ligands widely used by P. falciparum.

So far, the Plasmodium mechanisms of adhesion to the placenta are the best known among apicomplexan parasites. However, as we will see in the following sections, these mechanisms are preserved in related apicomplexan parasites such Babesia, Toxoplasma, and Neospora. In these organisms, the parasite proteins expressed in the infected cells, or the proteins present on the parasite’s surface play fundamental roles in the adhesion and infection processes in the placenta.

The development of severe malaria during pregnancy depends on parasite and host factors, including the immune response (Clark, 2019). During pregnancy, an increase of hormones, including cortisol, progesterone, estradiol, and testosterone, polarize the immune response toward a Th2-type one, characterized by an increase of regulatory T cells (Tregs) and anti-inflammatory cytokines (Robinson and Klein, 2012). The presence of the parasite alters this response causing infiltration of immune cells and a strong inflammatory response (Suguitan et al., 2003; Chêne et al., 2014).

Toll-like receptors, particularly TLRs-2 -4 -7-and -9, expressed on the trophoblast and cells of the VS, play a key role in the immune response against Plasmodium (Lucchi et al., 2011; Zhu et al., 2011; Barboza et al., 2014, 2017; Reis et al., 2020). TLR- 2 and -4 recognize glycosylphosphatidylinositol (GPI) anchors (Krishnegowda et al., 2005), while nucleic acids and hemozoin are recognized by TLR7 and TLR9 (Coban et al., 2005; Parroche et al., 2007; Figure 1B). The expression of TLR-4 and -9 is regulated by Plasmodium (Barboza et al., 2017) and polymorphisms of the TLRs have been associated with the susceptibility to infection (Hamann et al., 2010).

The binding of IEs and parasite antigens to TLR expressed on the trophoblast promotes the activation of MAPK pathways leading to the secretion of pro-inflammatory cytokines, including TNF-α, IFN-γ, TGF-β, and IL-8, and consequent activation of immune cell (Lucchi et al., 2008; Reis et al., 2020; Figure 1B). Concomitantly, the chemokines interferon γ–induced protein-10 (IP-10), monocyte chemoattractant protein-1 (MCP-1), and macrophage inflammatory protein (MIP)-1a/b expressed in maternal white blood cells (WBCs) contribute to the accumulation of Mo and MΦ in the IVS (Suguitan et al., 2003). Then, the presence of the immune cells and cyto/chemokines leads to a Th1-type immune response, where in addition to the cytokines mentioned above, IL1β, IL-2, IL-6, (Dobaño et al., 2018) and IL-8 (Fried et al., 1998) are secreted (Figure 1B). Thus, T cells proliferation is induced, and MΦ phagocytosis is enhanced, aiming to limit the parasite’s replication (Ozarslan et al., 2019).

Overproduction of cytokine IL-10, an immunoregulating cytokine, is associated with response to placental malaria infections (Suguitan et al., 2003; Chêne et al., 2014) attenuating the detrimental effects on the placental barrier exerted by the exacerbated inflammatory response (Okamgba et al., 2018). Alternatively, the increase in IL-10 could modulate lymphoid dendritic cell (DC) subpopulations shifting the Th1/Th2 balance toward a TH1 response and consequently promote infection (Diallo et al., 2008).

Mo-secreted IL-1β in response to IEs alters the amino acid transport system in the trophoblast impairing their transfer to the fetus and contributes to PM-associated FGR’s pathogenesis. Moreover, the accumulation of Mo in the malarial placenta promotes the decrease of insulin-like growth factor-1, one of the most influential factors in fetal growth and life (Umbers et al., 2011).

Different MΦ populations are related to functional modification of the placenta and maternal-fetal tolerance (Ordi et al., 1998; Gaw et al., 2019). Thus, maternal-intervillous monocytes (MIM) are associated with massive chronic villositis and placental damage (Ordi et al., 1998). Likewise, a decrease in the anti-inflammatory M2-type Hofbauer cells (HBC), located in the VS, is associated with LBW in placental malaria (Gaw et al., 2019).

Plasmodium also activates the complement cascade (C) and subsequently modulates the release of the anti-angiogenic factors (Conroy et al., 2011; Chau et al., 2017; Figure 1B). Together, an excessive mononuclear cells activation, such as MΦ, and an increase of soluble vascular endothelial growth factor (VEGF) receptor-1 (sFlt-1), which reduce the bioavailability of the pro-angiogenic VEGF and placental growth factor (PGF), impair normal angiogenesis (Azimi-Nezhad, 2014; Chau et al., 2017) and trigger functional placental insufficiency, and ultimately, FGR (Conroy et al., 2011).

The presence of Hemozoin (Hz) is another determining factor in the placental malaria immune environment. Hz can stimulate the trophoblast, MΦ, and DCs to secrete chemokines and cytokines [CCL2, CCL3, and CXCL8 and tumor necrosis factor (TNF)], contributing to the recruitment of peripheral blood mononuclear cells, especially Mo (Lucchi et al., 2011) and inflammatory response (Moormann et al., 1999; Abrams et al., 2003; Figure 1B). In addition, phagocytosis of this pigment increases the expression of matrix metalloproteinase (MMPs), particularly MMP-9, in the trophoblast (Clark, 2019) inducing cleavage and liberation of the extracellular domain of Syndecan-1; therefore allowing the adherence of IEs and promote their accumulation in the placenta (Clark, 2019).

The pathogenesis of Babesia is strikingly similar to that of malaria (Figure 2A). Thus, alterations in the adhesive properties of IEs due to antigenic modifications of their membrane (Cooke et al., 2005; Hutchings et al., 2007), caused by the expression of a variant of the parasite-derived erythrocyte surface antigen protein (VESA), can be observed (Jackson et al., 2014). VESAs are proteins encoded by various families of ves1α and ves1β genes and secreted by the parasite (Allred et al., 2000; Allred, 2019). In addition, VESAs appear to play essential roles in pathogenicity (Pedroni et al., 2013), immune evasion (Jackson et al., 2014), persistence, and survival of the parasites (Allred, 2019). VESAs, variant erythrocyte surface antigenic- 1 (VESA1) protein, is one of the most studied in B. bovis. This protein acts as an endothelial cell ligand and mediator of antigenic variation in B. bovis-IEs (O’Connor and Allred, 2000; Wang et al., 2012). The host cell adhesion receptors for VESA have not yet been identified (Gallego-Lopez et al., 2019). However, structural similarities between fEMP1 and VESA1 suggest that the latter could bind to equivalent host cell receptors of PfEMP1 such as CD36 (Hutchings et al., 2007), ICAM-1, P-selectin, CSA, and CD31 (Gallego-Lopez et al., 2019; Figure 2A). Furthermore, studies performed with B. bigemina and B. ridhaini have demonstrated the adhesion IEs to thrombospondin (PST) (Parrodi et al., 1990) present in endothelial cells (Baruch et al., 1996).

Importantly, the severity of babesiosis is highly dependent on the babesia species (Cooke et al., 2005). For example, babesiosis caused by Babesia microti, B. bovis, B. Lengua, and B. canis is associated with the development of severe syndromes such as brain babesiosis and multi-organ failure due to the sequestration of IEs in the organ’s microvasculature (Wright et al., 1979; Aikawa et al., 1992; Bosman et al., 2013; Sondgeroth et al., 2013; Ripoll et al., 2018; Schetters, 2019).

There are few reports related to the placental immune response against Babesia. However, it has been suggested that pro-inflammatory cytokine activation and erythrocyte adhesion in babesiosis are similar to malaria (Krause et al., 2007; Figure 2B). Some studies have even documented that prior infection with B. microti protects against fatal malaria in mice and primates through a mechanism of cross-protection (van Duivenvoorde et al., 2010; Efstratiou et al., 2020).

Toll-like receptors also recognize Babesia through the presence of its surface of GPI-linked molecules. The main TLRs studied regarding Babesia species are TLR-3 and -4, which can signal downstream through MyD88 independent pathways (Skariah et al., 2017). Similarly, TLR2 could play a role in this inflammatory response, mediating MΦ activation in the placenta. Previous studies have shown that lipid extracts from B. bovis stimulate TLR-2-mediated MΦ activation (Gimenez et al., 2010; Figure 2B) and consequently induces the secretion of pro-inflammatory cytokines, including TNF-α, IL-1, IL-12, and the immunomodulating IL-8 and nitric oxide (NO) (Shoda et al., 2000; Terkawi et al., 2015). In addition, T lymphocytes and natural killer (NK) cells, responsible for INF-γ production, and B lymphocytes are essential elements in the immune response against this parasite (Krause et al., 2007; Vannier and Krause, 2012; Yi et al., 2018; Figure 2B). Notably, the DNA of this parasite also has immunomodulatory effects on both B cells and MΦ, through the induction of IL-12, TNF-α, and NO production (Shoda et al., 2001).

Babesia, like Plasmodium, increases in the expression of MCP-1 and MIP-1a chemokines, contributing to the accumulation of Mo and MΦ in the IVS (Skariah et al., 2017; Zhao et al., 2020). Also, other chemokines such as CCL4 (MIP-1β) and CCL5 (RANTEs) have been associated with an inflammatory process in response to this parasite (Skariah et al., 2017).

Toxoplasma gondii cell invasion is a multi-step process that depends on the parasite’s motility, surface antigens, and the sequential secretion of proteins present in the apical secretory organelles (Carruthers and Boothroyd, 2007; Robert-Gangneux and Dardé, 2012).

The invasion process starts with low-affinity interaction between T. gondii surface molecules, including glycosylphosphatidylinositol (GPI)-attached surface antigens (SAG), SAG-related sequence proteins (SRS), and non-SAG-related surface antigens (SUSA) and the cell membrane of the host cell (Pollard et al., 2008; Blader and Saeij, 2009). The SAGs multigenic family comprises more than 150 genes distributed and tandemly arrayed throughout the genome (Lekutis et al., 2001; Jung et al., 2004). SAG and SRS1 proteins are predominantly expressed on the tachyzoite (rapidly dividing parasites) surface cells (Tomavo, 1996; Manger et al., 1998). Regarding the SUSA molecules, 31 genes have been identified, and the proteins are highly expressed in bradyzoites (slow dividing parasites) (Pollard et al., 2008). Significantly, all these surface antigens are also involved in immune modulation, virulence, and protection of the parasite to survive in the environment (Lekutis et al., 2001; Crawford et al., 2009), for what they are considered redundant systems of adhesion to the host cell (Manger et al., 1998). The host cell surface molecules include GAGs, proteoglycans, and glycosaminoglycan-like carbohydrates (Jacquet et al., 2001; He et al., 2002; Azzouz et al., 2013). In addition, proteins of apical secretory organelles also play a key role in parasite-cell adhesion. For example, rhoptry proteins (ROP 2, ROP 4) and dense granules protein (GRA2) are lectins that bind to sulfated GAGs and then participate in the moving junction complex (MJ complex) (Azzouz et al., 2013).

Additionally, T. gondii is recognized and adheres to ICAM-1 on the host cell surface. Thus, micronemal proteins (MICs), such as MIC2, binds to ICAM-1 and facilitate the transmigration of the parasite through the blood-brain barrier (Lachenmaier et al., 2011). Also, interactions with ECM laminin (Furtado et al., 1992) and sialylated oligosaccharides (Blumenschein et al., 2007) have been reported. In the placenta, T. gondii infects different cell types, including decidual cells, extravillous trophoblasts (EVT), and the trophoblast (Abbasi et al., 2003; Robbins et al., 2012). The parasite’s binding to these cells is mediated by sulfated proteoglycan and ICAM-1 (Abbasi et al., 2003; Barragan et al., 2005; Figure 3A.1). ICAM-1 is abundantly expressed at intercellular junctions and the surface of in human trophoblast cell line (BeWo) (Barragan et al., 2005). Also, T. gondii induces syncytial expression of this adhesion molecule (Juliano et al., 2006) increasing the adhesion of T. gondii-infected immune cells to the trophoblast (Pfaff et al., 2005). Thus, parasite infection of Mo and maternal leukocytes promotes their adhesion to trophoblasts through the upregulation of ICAM-1, contributing to placental and ultimately fetal infection (Xiao et al., 1997; Pfaff et al., 2005). Moreover, decidual DCs could serve as a vehicle for T. gondii to travel across the placenta and infect the fetus (Sanecka and Frickel, 2012; Arranz-Solís et al., 2021; Figure 3A.2). In addition, T. gondii can exploit natural routes for leukocyte extravasation to cross the placental barrier (Figure 3A.3). In vascular endothelium, CD162 is recognized by the vascular adhesion molecule-1 (VCAM-1) and E-selectin of infected cells mediating their attachment (Harker et al., 2013). Interestingly, CD162 has been identified in trophoblast cell lines (Multhaup et al., 2018).

The adhesion of infected immune cells to the endothelium serves as a signal to the parasite, allowing to schedule its leukocyte output at the target organ and minimizing the attack by the immune system (Baba et al., 2017). Thus T. gondii reaches the placenta inside maternal leukocytes and adheres to the trophoblast (Figure 3A.3). In the case of Mo, the presence T. gondii antigens, such as soluble tachyzoite antigen (STAg), induces the overexpression of macrophage migration inhibitory factor (MIF), allowing the adhesion to the trophoblast (Ferro et al., 2008). Moreover, the adhesion of infected immune cells can also induce local destruction of the trophoblast (Juliano et al., 2006) through the induction of pro-inflammatory cytokine-dependent apoptosis (Garcia-Lloret et al., 2000; Juliano et al., 2006). It has also been proposed that the accumulation of leukocytes is responsible for placental villitis (Yavuz et al., 2006).

The ECM degradation is another strategy used by T. gondii to cross and damage the placental barrier (Wang and Lai, 2013; Liempi et al., 2020; Figure 3A.4). Placental infections increase the expression and secretion of MMPs, MMP-2, MMP-9, and MMP-12 in the VS, causing ECM degradation (Wang and Lai, 2013) particularly of fibronectin, collagen I and IV, and laminin (Chen and Aplin, 2003).

Toxoplasma gondii infection in the placenta induces an inflammatory response characterized by a robust production of pro-inflammatory cytokines and chemokines (Sasai et al., 2018; Sasai and Yamamoto, 2019).

Toll-like receptors are critical in initiating defense against T. gondii; they recognize in the parasite glycosylphosphatidylinositol (GPI) anchored proteins and lipid anchors, heat shock protein 70 (TgHSP70), and profilin-like protein (TgPF) (Yarovinsky and Sher, 2006; Denkers, 2010; Yarovinsky, 2014; Sasai et al., 2018). GPI, are recognized primarily by TLR-2, TL-4 and TL2/TL1/6 heterodimers (Debierre-Grockiego et al., 2007) while TgPF is recognized by the TLR-5, 11 and -12 (Andrade et al., 2013; Salazar Gonzalez et al., 2014; Yarovinsky et al., 2005). On the other hand, parasite’s RNA and DNA are recognized by the endosomal TLR-7 and -9 (Yarovinsky, 2014). TLR activation leads to an increase of IL-12, IL-6, IL-8, and TNF-α cytokines, chemokines (CCL5, CCL12, and XCL1), interferons (IFNs), and other effector molecules such as NO, which favors infection control (Yarovinsky and Sher, 2006; Wujcicka et al., 2013).

Haplotype and single nucleotide polymorphisms (SNPs) of tlr genes can influence susceptibility to parasitic diseases (Wujcicka et al., 2013, 2017; Turkey et al., 2019). Thus, SNPs residing in TLR2, TLR4, and TLR9 genes increase susceptibility and development of toxoplasmosis in pregnancy (Wujcicka et al., 2013, 2014, 2017; Turkey et al., 2019). This susceptibility is attributed to the influence of SNPs in the receptor dimerization and the recruitment of adapter proteins involved in TLR signaling and the decreased synthesis of some cytokines (Wujcicka et al., 2017; Turkey et al., 2019), such as INF-γ and TNF-α.

We have shown previously that during ex vivo infection of human placental explants (HPE), T. gondii increases the expression of TLR-9, but not of TLR-2 and -4. However, inhibition of TLR-4 and -9 increases T. gondii DNA load in the explants. Notably, only IL-8 secretion was increased (Muñoz et al., 2016; Castillo et al., 2017a; Figure 3B); even the protective and pro-inflammatory cytokines INF-γ and TNF-α did not show a significant change in response to the parasite (Castillo et al., 2017a). This result could explain, at least partially, the susceptibility of the placenta to T. gondii infection compared to other parasites (Castillo et al., 2017a). In addition, it has been suggested that IL-8 promotes the proliferation and differentiation of the infected host cell, and subsequently, the intracellular multiplication of parasites (Milian et al., 2019), promoting, therefore, congenital transmission (Gómez-Chávez et al., 2019, 2020). Moreover, T. gondii Macrophage Migration Inhibitory Factor (TgMIF) cytokine-like, an analog to the host MIF, promotes IL-8 secretion and subsequent recruitment of immune cells to the site of infection (Sommerville et al., 2013) causing the above-described effects (Lambert et al., 2006).

On the other hand, the low INF-γ expression (Muñoz et al., 2016; Castillo et al., 2017a) could also be a placental mechanism to limit infection. Both INF-γ and TNF-α promote the adhesion of infected immune cells to the trophoblast through surface overexpression of ICAM-1 (Pfaff et al., 2005; Ferro et al., 2008). Therefore, it is possible that the downregulation of these pro-inflammatory cytokines, in addition to benefiting the maintenance of pregnancy, also limits the transfer of the parasite through the placental barrier. Furthermore, this could be related to the production of CCL22 chemoattractant in response to T. gondii (Ander et al., 2018) since CCL22 can replace the effect of IFN-γ and recruit Tregs modulating the inflammatory response. However, this increased T-reg recruitment may also be a parasite strategy to promote its persistence (Rowe et al., 2011). Furthermore, MIF is also overexpressed in responses to T. gondii infection (de Oliveira Gomes et al., 2011; Figure 3B), promoting the adhesion of infected immune cells to the trophoblast, activating the ERK1/2 MAPK pathway and subsequent production of prostaglandin E, favoring parasite proliferation and persistence through inhibition of IL-6, IL17, and TNF-α production (Kelly et al., 2005; Passos et al., 2010; Barbosa et al., 2014; Qiu et al., 2016).

The presence of T. gondii causes an imbalance of the Treg/T-helper type 17 (Th17) cells response inducing deleterious alterations in the placenta that can lead to abortion (Zhang et al., 2012). T. gondii, as mentioned above, promotes the production of IL-12, CCL-2, and CCL-22 activating naïve T cells. In addition, activated T cells secrete IL-2, INF-γ, and TNF-α, developing a pro-inflammatory environment that stimulates MΦ and B cells (Gómez-Chávez et al., 2020) and decreases Tregs and TGF-β levels (Figure 3B). The decrease of Tregs may be due to various factors, including apoptosis (Ge et al., 2008), limitation of proliferation (Oldenhove et al., 2009), and an increase in differentiation into Th17 cells (Zhang et al., 2012).

Importantly, all the factors involved in decreasing Tregs have in common the activation of multiple pathways, including the pro-inflammatory ones (Ge et al., 2008; Oldenhove et al., 2009; Zhang et al., 2012). The increased differentiation of TH17 cells is associated with altered expression of TGF-β, IL-17A, and IL-6 cytokines, transcription factors [i.e., Forkhead box-p3 (Foxp3)], and receptors [i.e., Retinoic acid-related orphan receptor γt (RORγt)], keys in cell differentiation and regulating Treg/Th17 balance (Zhang et al., 2012). Although it is unclear how T. gondii regulates the expression of TFG-B and Foxp3, it influences suppressive functions of Tregs in the placenta (Zhang et al., 2012), resulting in an exacerbated inflammatory reaction, causing placental damage and favors the transmission of the parasite to the fetus.

Neospora caninum can actively invade a large variety of nucleated cells in vitro and in vivo conditions (Naguleswaran et al., 2002; Hemphill et al., 2004; Lei et al., 2005); its mechanisms are very similar to those proposed for T. gondii, where a first stage of low-affinity adhesion and a second stage of firm apical attachment can be observed (Hemphill et al., 2004; Carruthers and Boothroyd, 2007).

The initial adhesion is mediated by tachyzoite surface antigens belonging to the SAG-related sequence proteins (SRS) family and homologous to the T. gondii SAGs family (Howe et al., 1998). SAG1 and SRS2 are two main immuno-dominant antigens of N. caninum tachyzoites (Dong et al., 2012; Sinnott et al., 2017) that binds to host cells (Hemphill et al., 2013). In addition, SRS allows parasite adhesion through binding to GAGs expressed on the host cell’s surface, especially CSA and SA (Dubey et al., 2017). The latter is crucial for self-recognition by complement factor H (Varki and Gagneux, 2012) and fetal defense against maternal complement attack (Abeln et al., 2019).

Then the apical attachment is mediated mainly by MIC (Lovett et al., 2000; Naguleswaran et al., 2002; Reid et al., 2012; Wang J. et al., 2017), MIC2 and MIC19 are unique to N. caninum (Reid et al., 2012). N. caninum MIC proteins have domains similar to those found in vertebrate ECM proteins, including thrombospondin (TPS) -like, integrin-like domain (Pereira et al., 2011) and SA- binding adhesive repeat- (MAR)-like domain (Friedrich et al., 2010). Thus, MICs can bind through GAGs to various receptors present on the surface of target cells (Dubey et al., 2017), and mediate cell adhesion (Keller et al., 2002; Wang J. et al., 2017). MIC1, MIC3, and MIC4 bind to host cells through sulfated proteoglycans (Naguleswaran et al., 2002; Keller et al., 2004; Friedrich et al., 2010). N. caninum proliferation varies between the different cells of the placenta (Gibney et al., 2008). Studies in bovine placenta cell lines have shown that caruncular cells appear to be more resistant to Neospora infection than trophoblasts due to their expression of adhesion molecules (Jiménez-Pelayo et al., 2017, 2019) and phagocytic activity (Machado et al., 2007; Jiménez-Pelayo et al., 2017). However, this resistance also depends on the virulence of the isolate (Jiménez-Pelayo et al., 2017). On the other hand, N. caninum shows a predilection for the fetal chorionic epithelium and placental blood vessels (Buxton et al., 1998).

There are few reports related to the adhesion molecules that participate in the placenta and N. caninum interaction. However, SRS2 participates in the adhesion of N. caninum to the trophoblast (Figure 4A) and the development of an effective immune response against transplacental transmission (Nishikawa et al., 2001; Haldorson et al., 2005, 2006), but the receptor for SRS2 has not yet been identified (Naguleswaran et al., 2002; Figure 4A). Nevertheless, the presence of SA and glycoproteins expressed on the surface of binucleate trophoblast giant cells (BNCs) in the ovine placenta (Jones et al., 1997; Klisch et al., 2006) could be involved in N. caninum adhesion to the placenta (Figure 4A).

Modulation of gene expression in the host cell is also a mechanism for N. caninum to invade and cross the placental barrier (Horcajo et al., 2017; Dorsch et al., 2019). Thus, infection of immortalized bovine trophoblasts with N. caninum induces the expression MMPs involved in ECM degradation (Cui et al., 2017; Horcajo et al., 2017). Additionally, infection by N. caninum alters the glycosylation pattern in the glycocalyx and apical cytoplasm of trophoblast (Dorsch et al., 2019; Figure 4A). The BNCs and the uterine epithelium also shows changes in sugars, including α-D-GalNAc, α-D-Man, β-D-Gal, α-D-Gluc, and NeuNac (Vonlaufen et al., 2004; Dorsch et al., 2019; Figure 4A). The expression of the adhesion molecules E-selectin, P-selectin, VCAM-1, and ICAM-1 is also altered, possibly through paracrine activation, through the pro-inflammatory cascade activation, which involves the release of IL-1b, IL-8, and MCP-1 leading to cell activation and subsequent expression of the adhesion molecules (Taubert et al., 2006).

Neospora caninum can also hijack immune cells to circumvent biological barriers. Thus, infection of DCs by tachyzoites enhances the translocation of parasites across cell monolayers (Collantes-Fernandez et al., 2012; Figure 4A). Furthermore, similar to T. gondii (Lambert et al., 2006; Lambert and Barragan, 2010), infection of DCs by N. caninum induce a migratory phenotype in these cells, enhancing its dissemination. In addition, N. caninum can infect MΦ and NK, cells and take advantage of their basal or inducible mobile properties (Boysen et al., 2006; Dion et al., 2011) (Figures 4A,B).

The immune response at the maternal-fetal interface against N. caninum is characterized by a mixture of Th1 and Th2 responses, implying the presence of pro-and anti-inflammatory cytokines (Cantón et al., 2014b; Hecker et al., 2015; Arranz-Solís et al., 2016; Gutiérrez-Expósito et al., 2020; Figure 4B). Although these responses play a vital role in controlling parasite multiplication, they are responsible for placental damage that might lead to abortion (Maley et al., 2006; Rosbottom et al., 2011; Cantón et al., 2014a; Gutiérrez-Expósito et al., 2020).

The role of the innate immune response in the placenta against N. caninum has been studied mainly in cattle. The parasite is recognized by endosomal TLRs (3, 7, 8, and 9), which are upregulated in response to Neospora infection (Figure 4B; Marin et al., 2017).

Similar to T. gondii infection, TLR 3/7 and 8 is associated with the control of N. caninum intracellular infection. TLR7 activation leads to the development of a protective Th1 response (Andrade et al., 2013; Castillo et al., 2018) through MyD88-dependent signaling (Mineo et al., 2009; Miranda et al., 2019) inducing the secretion of pro-inflammatory cytokines including IFN-γ, IL-12, IL-18, and TNF-α, and promotes recruitment of T lymphocyte (CD3, CD4, CD8, and γδ) subsets, MΦ, NK cells (NKp46 subpopulation) (Maley et al., 2006; Cantón et al., 2014a, b; Hecker et al., 2015; Arranz-Solís et al., 2016; Gutiérrez-Expósito et al., 2020; Figure 4B).

In addition, the Th2 (IL-4) and regulatory (IL-10) cytokines also show alterations during N. caninum infection (López-Pérez et al., 2010; Rosbottom et al., 2011). Thus, the increase of IL-4 attenuates the effects of the pro-inflammatory response and enhances susceptibility to N. caninum favoring congenital transmission (López-Pérez et al., 2010). On the other hand, IL-10 has a regulatory effect on IFN-γ and TNF-α mediated response (Eperon et al., 1999; Quinn et al., 2004) and might also promote the invasion and intracellular replication of N. caninum and consequently its transmission.