Subarachnoid hemorrhage (SAH) is a type of hemorrhagic stroke that accounts approximately for 5% of all strokes. The incidence of SAH depends on the geographical location and varies from 0.5 to 28 per 100,000 person-years (Etminan et al., 2019). SAH is a devastating cerebrovascular disease and is a significant cause of morbidity and mortality all over the world. More than 80% SAH cases are due to the rupture of a cerebral aneurysm (Connolly et al., 2012). Intracranial pressure increases rapidly upon aneurysm rupture and blood fills the subarachnoid space. Along with blood components, this sudden rise in intracranial pressure (ICP) triggers the initial pathophysiological cascades resulting in early brain injury (EBI) and the later complex reactions known as delayed cerebral ischemia (DCI) (van Lieshout et al., 2018). One of the sites where SAH-induced changes occur is the choroid plexus (CP) (Solár et al., 2020b,a,c). The CP is found in the brain ventricles. It forms the blood-cerebrospinal fluid barrier (B-CSF barrier) and is responsible for production of cerebrospinal fluid (CSF). The CP consists of a highly vascularized stroma containing fenestrated capillaries covered with a single layer of cuboidal epithelial cells. The epithelial cells are connected by tight junction (TJ) proteins that form the main component of the B-CSF barrier. The production of CSF by the cuboidal epithelial cells is controlled by the B-CSF barrier that maintains homeostasis of the central nervous system (CNS) through the selective transport of ions and molecules. Epiplexus cells, also called as Kolmer cells (KC), exhibit phagocytic activity, adhere to the ventricular side of the cuboidal epithelial cells and are considered immune cells direct contact with CSF (Wolburg and Paulus, 2010; Solár et al., 2020c). Following SAH, we have previously observed increased numbers of KC with the activated (ED1+) immunophenotype, along with increased numbers of resident (ED2+) macrophages, CCR7+ and CD206+ cells and MHC II+ antigen presenting cells. Interestingly, similar changes were also found (but to a lesser extent) following the application of artificial cerebrospinal fluid (ACSF) indicating that increased intracranial pressure can also contribute to the observed changes (Solár et al., 2020b). Accumulation of immune cells with increased hemoxygenase-1 (HO-1) amounts predominantly in the epiplexus position indicates that they have a role to play in hemoglobin degradation. Increased levels of HO-1 in activated and resident macrophages in the epiplexus position as well as biliverdin-reductase (BVR) in epithelial cells may also contribute to the restoration of CNS homeostasis (Solár et al., 2020a). On the other hand, accumulation of immune cells of different phenotypes in the CP may contribute to a pro-inflammatory response resulting in hypersecretion of CSF and thus in the development of post-hemorrhagic hydrocephalus, which is one of the major complications after SAH (Karimy et al., 2017; Wan et al., 2019). However, the precise source of the trigger and its mechanism underlying immune cell accumulation in the CP following SAH remains a mystery.

Toll like receptors (TLRs), a class of pattern recognition receptors (PRRs), are transmembrane and intracellular molecules containing domains comprised largely of leucine-rich repeats and intracellular domains that activate signaling cascades (Akira et al., 2006). TLRs are one of the main effectors in the innate immune system (Iwasaki and Medzhitov, 2004). Activation of TLRs either by various pathogen-associated molecular patterns (PAMPs) or endogenous molecules from damaged tissue [called damage-associated molecular patterns (DAMPs)] leads to the increased expression of inflammatory molecules involved in chemotaxis and phagocytosis (Wu et al., 2009; Piccinini and Midwood, 2010). TLRs located on the membranes of CP epithelial cells have an important role in CNS immunity. Moreover, their activation may lead to cytoskeleton restructuring and the downregulation of junctional proteins of the B-CSF barrier resulting in hypersecretion of CSF. Alteration of the B-CSF barrier along with the activation of proinflammatory genes may play an important role in attracting circulating monocytes and their transition to the CP inducing various diseases (Stridh et al., 2013; Thompson et al., 2022). The aim of our present work was to assess changes of DAMP receptors in response to experimentally induced SAH or increased intracranial pressure induced by the application of ACSF. We also focused on some of the proinflammatory signaling cascades triggered by SAH or increased intracranial pressure in the CP that could influence or modulate leukocyte chemotaxis and accumulation.

All experimental protocols, including animal use, care protocols and all operation procedures were approved by the Ethical Committee of Masaryk University, Brno, and the Departmental Committee of the Ministry of Education, Youth and Sports, Czechia (Approval No.: MŠMT 21101/2016–3). We used 135 adult male rats (8 weeks old Wistar rats weighing 200–250 gr; Animal Breeding Facility, Masaryk University, Czechia) in our study (Table 1).

All animals that underwent induction of SAH by applying blood into the cisterna magna or injection of ACSF survived the experiments. The presence of blood in the subarachnoid space (SAH group) indicated the successful application of blood in all animals. The most obvious blood clots were found in animals 1 and 3 days following SAH induction. Blood clots were found mainly on the ventral surface of the brain stem, in the basal cisterns. In the later stages of SAH, blood clots had been partially removed at 7 days and completely removed 14 days after blood application (Figure 1). In animals after application of ACSF, no blood clots or brain damage were found in the subarachnoid space.

In the study presented we show elevated levels of the DAMP receptors, TLR4, TLR9 and FPR2 in the CP epithelial cells at various time points following SAH induction or ACSF application of. The increased levels of pro-inflammatory receptors were associated with upregulation of TNFα and IL-1β, both powerful proinflammatory cytokines, as well as the accumulation of macrophages expressing CCR2 and CXCR1 in the CP epiplexus position (Figure 5). Intraventricular hemorrhage or SAH triggers a pro-inflammatory reaction resulting in the disruption of the blood–brain barrier (Solár et al., 2020c, 2022). Inflammatory pathways are initiated in the first few seconds following aneurysm rupture and they continue to remain activated even in later stages. Cerebrovascular inflammation both in the early and the later course of SAH contributes to the development of the principal complications such as vasospasms and hydrocephalus (Provencio, 2013; Solar et al., 2022; Solár et al., 2022). Blood elements and blood-degradation products in the CSF are in direct contact with epithelial and epiplexus cells/KC, of the CP (Liszczak et al., 1984). SAH induced inflammation involves adhesion molecules, cytokines, migration of immune cells and complement activation (Sehba et al., 2011; Miller et al., 2014). These changes in the CP are linked to the abnormal functioning of the B-CSF- barrier (Simard et al., 2011; Xiang et al., 2017). Although several pathways have been proposed, the exact source of these pro-inflammatory molecules and the increased number of immune cells in the CP following SAH is not known. Circulating monocytes could migrate into the CP via fenestrated capillaries and pass through the epithelial cells by “emperipolesis” to the ventricular side of the epithelial cells and become KC (Ling et al., 1998). Proliferation of ED2+ macrophages may be another source of increased macrophage number after SAH (Solár et al., 2020b). Moreover, meningeal vessels may also contribute to the accumulation of immune cells in the CP following SAH (Wilson et al., 2010). Pathophysiological cascades induced by SAH are linked to blood degradation products in the CSF which serve as alarmins called DAMPs (Miyake, 2007; Pan et al., 2020). DAMPs can activate TLRs which play an important role in innate immunity and the development of neuroinflammation (Lin et al., 2010; Kwon et al., 2015). TLR4, a transmembrane protein, and TLR9, located in intracellular vesicles of the endoplasmic reticulum, lysosomes, and endosomes were found to be expressed primarily in microglia, but also on other cells including astrocytes, neurons, endothelial cells as well as CP epithelial cells (Buchanan et al., 2010; Skipor et al., 2015; Benbenishty et al., 2019). The prototypical ligand for TLR4 is bacterial lipopolysaccharide (LPS) (Vallières et al., 2006). Similarly, mitochondrial DNA which is similar to bacterial DNA has unmethylated CpG DNA motifs which act as ligands for TLR9 (Zhang et al., 2010). In addition to their typical ligands, TLRs are known for their capacity to recognize many other DAMPs or PAMPs and activate the immune system leading to inflammation (Paradis et al., 2016; Li et al., 2020; Metzemaekers et al., 2020). Activation of TLRs by DAMPs following hemorrhagic stroke has been reported. An experimental study using the intracerebral hemorrhage model showed that activation of TLR-9 increased Iba-1 and HO-1 positive cells with enlarged cell bodies around the hematoma thus facilitating macrophage/microglial phagocytosis (Wei et al., 2022). Studies focused on TLR4 revealed elevated activation of two inflammatory cell-signaling pathways, MAPK and NF-κB pathways (Khey et al., 2020). Activation of MAPK leads to the phosphorylation of transcription factors resulting in proinflammatory effects including production of proinflammatory molecules and chemotaxis. Intracellular MAPK-activated signaling pathways involve the expression of cytokines and chemokines such as IL-1β, IL-6, IL-8, TNF-α, and MCP-1 (Sasaki et al., 2004). This is in accordance with our finding of the increased amount of IL-1β and TNF-α mainly 3- and 7-days following induction of SAH as well as application of ACSF with a decrease to near-normal values after 14 days. Activation of NF-κB was also increased after induction of SAH in experimental animals (Khey et al., 2020). Following SAH and subsequent activation of TLRS by various DAMPs, NF-κB translocates into the nucleus and upregulates the expression of pro-inflammatory molecules, cell adhesion molecules, TNF-α, COX, and others (Okada and Suzuki, 2017). Moreover, studies focused on TLR4 revealed that DAMPs lead to the activation of myeloid differentiation primary response protein 88 (MyD88)-dependent NF-κB signaling cascade resulting in increased secretion of TNFα (Figueiredo et al., 2007, p. 4; Karimy et al., 2020). TNFα induces DNA fragmentation, chromatin condensation and activation of caspase-3 resulting in apoptosis in the CP. These changes decrease the transepithelial membrane potential and increase paracellular permeability of the B-CSF barrier. In parallel, the cytotoxic effect of TNFα characterized by LDH-release and the detachment of HMGB1 from chromatin in CP epithelial cells indicates post-apoptotic events or secondary necrosis. Therefore, it seems that not only apoptosis but also necrosis plays a role in the TNFα-induced alteration of the B-CSF-barrier (Silva et al., 2008; Schwerk et al., 2010). In addition, up-regulation of TNFα induces CSF secretion mediated by the STE20/SPS1-related proline/alanine-rich kinase (SPAK) – Sodium Potassium Chloride cotransporter-1 (NKCC1) complex on the apical membrane of the CP epithelial cells. TNFα as well as interferon γ (IFN-γ) stimulate SPAK signaling through the NF-κB-dependent pathway, leading to increased ion transport in epithelial cells resulting in CSF hypersecretion (Karimy et al., 2017; Gregoriades et al., 2019). Moreover, activation of TLR4/NF-κB/TNFα signaling cascade upregulates the expression of other strong chemotactic and pro-inflammatory effector molecules including CCL2 and IL-1β (Gram et al., 2014). These proinflammatory molecules are critical for monocyte trafficking into the CP leading to hypersecretion of CSF (Solár et al., 2020c). The CP is a unique structure through which immune cells can pass into the CNS as a response to various inflammatory or non-inflammatory pathologies. The role of chemokines and cytokines in the trafficking of immune cells through the CP is currently attracting considerable research interest. CCL2/CCR2 signaling plays an important role in the migration of peripheral immune cells in the brain during neuro-inflammation but also in the systemic inflammatory reaction (Cazareth et al., 2014; Llovera et al., 2017; Shimada and Hasegawa-Ishii, 2021). The presence of blood and blood degradation products in the CSF is accompanied by increased expression of pro-inflammatory and chemotactic effector molecules including CCL2 and TNFα (Gram et al., 2014). Following the activation of the CCL2/CCR2 signaling cascade, both proinflammatory and anti-inflammatory actions have been observed—and the response depends on the disease (Deshmane et al., 2009). CCL2 ligand expression was found in neurons, astrocytes, perivascular microglia and infiltrating leukocytes. An important role for CCL2 has been confirmed in various diseases including multiple sclerosis, stroke, Alzheimer’s disease, acute brain injury and experimental autoimmune encephalitis (Mildner et al., 2009; Conductier et al., 2010; Saederup et al., 2010). The CP responds to different stimuli by increased CCL2 mediating the transmigration of CCR2+ leukocytes (Thibeault et al., 2001; Reyes et al., 2003). Secretion of CCL2 in the CP epithelium occurs throughout the apical as well as the basolateral membranes and acts as a strong chemoattractant (Mahad et al., 2006; Kaur et al., 2016). However, in our study, the level of CCL2 was slightly decreased following SAH or ACSF application. A possible explanation for this is that CCL2 is consumed by the increasing number of CCR2+ cells (Mahad et al., 2006). Moreover, there is some evidence that constitutively internalized and recycled CCR2 removes CCL2 from the cellular environment (Zhao et al., 2019). Based on this finding, it can be assumed that the increased number of CCR2+ macrophages removeCCL2 from the CP microenvironment. Another possible explanation is that CCL2 is not necessary for macrophage accumulation. Other mechanisms, or other chemokines may be involved, such as resident macrophage proliferation and CCR2-independent monocyte recruitment (Talsma et al., 2022).

CCR2 and CX3CR1 are membrane receptors involved in trafficking along inflamed vessels resulting in invasion and accumulation of circulating monocytes in the CNS. Therefore, accumulation of CCR2 and CX3CR1 positive macrophages in the CP indicates their origin from peripheral monocytes (Combadiere et al., 1998; Auffray et al., 2007; Prinz and Priller, 2010; Meghraoui-Kheddar et al., 2020). Monocytes expressing CCR2 and CX3CR1, the so-called “inflammatory” monocytes, are highly mobile and larger in size compared to “resident” monocytes (Prinz and Priller, 2010). Several hypotheses have been put forward to explain how circulating monocytes can become CP epiplexus cells. The most-widely accepted one proposes that circulating monocytes can enter the CP through the fenestrated capillaries and pass through the epithelial cells by “emperipolesis” to the ventricular side of the epithelial cells to become CP epiplexus cells (Ling et al., 1998). Moreover, meningeal vessels may also contribute to the accumulation of macrophages in the epiplexus position following SAH (Wilson et al., 2010). FPR2, an innate immune receptor, may play a role in the orchestration and polarization of CP macrophages following SAH. Immunohistochemical studies showed the expression of FPRs in a different part of the CNS including CP epithelial cells (Becker et al., 1998). Mitochondrial N-formylated peptides released from injured and disintegrated cells following tissue damage act as key ligands for FPRs (Wei et al., 2020). Activation of FPR2 leads to dual effects, pro-inflammatory or anti-inflammatory activity based on the types of endogenous ligands. Activation of FPR2 by different endogenous ligands such as Serum Amyloid A (SAA), Prion Protein (PrPc), Annexin A1 (ANXA1) or mitochondrial formyl peptides (mFP) derived from damaged mitochondria, exert pro-inflammatory reactions characterized by the elevated expression of pro-inflammatory molecules like TNFα, IL-1β and IL-6. On the other hand, ligands such as Lipoxin A4 (LXA4), Resolvin D1 (RvD1) or Specialized Pro-resolving Mediators (SPMs), induce the inhibition of pro-inflammatory cytokine production, suppression of metalloproteinases, upregulation of anti-inflammatory cytokines including IL-10 and transforming growth factor-β1 (TGF- β1), as well as enhancing the clearing capacity of macrophages (Dufton and Perretti, 2010; Guo et al., 2016; Wenceslau et al., 2016; Tylek et al., 2021). Following SAH, some cellular components such as mFP released from damaged mitochondria act as DAMPs and contribute to the development of neuroinflammation (Zhang et al., 2022). FPR2 mediates a pro-inflammatory response upon binding to mFP. In experimantal models of various diseases, FPR2 mediates extracellular signals into the cell through the ERK1/2 pathway. ERK1/2 activates downstream transcription factors including c-myc, c-jun, and NF-κB leading to the expression of inflammatory cytokines (Cao et al., 2023). On the other hand, the protective and anti-inflammatory role of FPR2 is probably associated with M2 macrophages, which potentiate tissue repair and regeneration (Yuan et al., 2022). In our previous work, we found a higher number of M2 macrophages in the CP following SAH induction mainly in the later stages (Solár et al., 2020b). The ability of CP epithelial cells to increase the FPR2 number may be one of the mechanisms leading to the accumulation of macrophages with an anti-inflammatory M2 phenotype. This may contribute to restoring CNS homeostasis after SAH. However, in the current study, we find that the expression of FPR2 on the surface of CP epithelial cells was only slightly increased. Thus, the role of FPR2 in the development of either a pro-inflammatory or anti-inflammatory reaction in the CP following SAH is probably marginal. As for increased ICP following SAH, we found increased immunofluorescence intensities for TLR4, TLR9 and TNFα in the CP after ACSF application. It is known that ICP rises immediately after aneurysm rupture (Prunell et al., 2002). Here, we mimic increased ICP following SAH by applying ACSF into the cisterna magna. Intrathecal application of ACSF is widely used as a control in experimental models without evidence of immunogenicity (Hylden and Wilcox, 1980; Hernangómez et al., 2016). Despite these findings, we showed that increased ICP induced by ACSF application leads to immune cell accumulation in the CP. The exact mechanism of how elevated ICP contributes to cellular and molecular inflammatory changes in the CP is not known. However, based on our results it can be assumed that TLR4 and TLR9 respond not only to blood degradation products but also to increased ICP following aneurysm rupture and extravasation of blood into the subarachnoid space. Activation of Piezo channels—mechanosensitive ion channels located in the cell membrane – might be potentially involved. Activation of Piezo channels by mechanical stretching leads to the influx of extracellular Ca2+ (Liu et al., 2022). In addition, it was revealed that these channels cooperate with TLR4 to promote their response (Geng et al., 2021). Moreover, Piezo mechanoreceptors are also involved in immune cell response regulation (Liu et al., 2018).

Our study has several limitations. First, we used an experimental model of SAH with the application of blood into the cisterna magna that mimics the bleeding into the subarachnoid space in humans. Due to the deep location of the CP in the brain, using an experimental model of SAH is the only possibility to describe the changes in the B-CSF barrier. However, using an animal experimental model partially limits the application of pathophysiological changes to humans. Furthermore, we must consider the absence of aneurysm rupture, including the fact that the cellular and subcellular pathways may differ in mammals and rodents. An alternative is to use in vitro models; however, these models lack the interaction of surrounding tissues, including blood and CSF. Another limitation is the use of only adult male rats. The main reason for using male rats rather than female ones is that fluctuating female sex hormones may generate less controlled experimental conditions and influence the result. Therefore, to have the experiment as standardized as possible, based on current studies, we used male rats. In our study, to mimic elevated ICP following SAH, we used artificial CSF according to the composition that is widely used in studies. Prunell et al. (2002). referred that increased ICP following the application of ACSF mimics the SAH. However, the absence of other cellular and molecular components from blood may limit the changes caused by the increased ICP following SAH.

In the current study, we observed changes in the amount of different TLRs and proinflammatory molecules also in the animals after ACSF application, but these changes were less pronounced compared to SAH animals. Therefore, it can be assumed that the changes after the ACSF application were mainly caused by elevated ICP.

Our results outline a possible mechanism for the accumulation of immune cells in the CP following SAH. TLRs, mainly TLR9, probably play an important role in the development of inflammatory changes in CP epithelial cells. Activation of TLRs leads to the elevation of chemotactic molecules in the CP and subsequent trafficking of peripheral monocytes. Importantly, increased ICP induced by ACSF application also leads to increased expression of pro-inflammatory cytokines as well as an increased number of monocyte-derived macrophages. These findings provide the basis for further studies to develop potential new therapeutic tools. In particular, the reduction of inflammatory changes may attenuate the development of post-hemorrhagic hydrocephalus and other complications after SAH. Our results suggest that not only blood components but also acute increase in ICP contribute to the development of the complex inflammatory reaction in the CP that eventually leads to the alteration in the B-CSF barrier.