Targeting foam cell formation to improve recovery from ischemic stroke

Abstract

Inflammation is a crucial part of the healing process after an ischemic stroke and is required to restore tissue homeostasis. However, the inflammatory response to stroke also worsens neurodegeneration and creates a tissue environment that is unfavorable to regeneration for several months, thereby postponing recovery. In animal models, inflammation can also contribute to the development of delayed cognitive deficits. Myeloid cells that take on a foamy appearance are one of the most prominent immune cell types within chronic stroke infarcts. Emerging evidence indicates that they form as a result of mechanisms of myelin lipid clearance becoming overwhelmed, and that they are a key driver of the chronic inflammatory response to stroke. Therefore, targeting lipid accumulation in foam cells may be a promising strategy for improving recovery. The aim of this review is to provide an overview of current knowledge regarding inflammation and foam cell formation in the brain in the weeks and months following ischemic stroke and identify targets that may be amenable to therapeutic intervention.

1. Introduction

Every year in the United States, nearly 800,000 people have new or recurrent strokes, costing approximately 34 billion dollars annually (Waldman et al., 2021). Up to one-third of stroke survivors never regain their independence and rely on continued care for the rest of their lives (Walcott et al., 2014). 87% of all strokes are ischemic, and the remaining 13% are hemorrhagic (Benjamin et al., 2017). An ischemic stroke occurs when there is a blockage in a blood vessel that provides oxygen, glucose, and other essential factors to a region of the brain (About Stroke | American Stroke Association, 2022).

Currently, there is only one U.S. Food and Drug Administration (FDA)-approved drug that can be administered to patients that experience ischemic strokes, tissue plasminogen activator (tPA) (Waldman et al., 2021). However, if it is not given to the patient within 4.5 h of the initial formation of the blockage to restore cerebral blood flow, increased injury occurs, further damaging the patient’s brain tissue (Kim, 2019). As a result of this narrow therapeutic time window, only 4% of ischemic stroke patients received tPA treatment between the years 2005 and 2011 (Many stroke patients do not receive life-saving therapy – ScienceDaily, 2022). Mechanical thrombectomy can also be used to restore cerebral blood flow. However, less than 20% of patients are eligible for this kind of reperfusion therapy. It also requires access to a well-resourced health facility, and it must be initiated within 24 h of stroke onset (Rabinstein, 2020).

In contrast to acute stroke, there are currently no pharmacological interventions approved for use in the days and weeks following stroke to speed up recovery and enhance healing. Motor deficits occur in approximately 80% of stroke patients and cause dependency and chronic disability in at least 30% of cases (Warlow et al., 2008; Ingwersen et al., 2021; Langhorne et al., 2009; Dworzynski et al., 2015). Additionally, stroke survivors have a 200% increased risk of developing dementia, with nearly one-third experiencing subsequent cognitive decline (Tatemichi et al., 1992; Pendlebury and Rothwell, 2019; Prencipe et al., 1997). The greatest risk occurs in the first six months following stroke, and risk is associated with stroke severity (Pendlebury and Rothwell, 2019). One way by which stroke recovery and rehabilitation could be improved is by focusing on the chronic inflammatory response to stroke. In animal models, it has been shown that the inflammatory response to stroke worsens neurodegeneration and creates a tissue environment that is unfavorable to regeneration for at least several months. When the infarct is adjacent to the hippocampus, it can also cause the onset of delayed memory deficits (Doyle et al., 2015), (Sanchez-Bezanilla et al., 2021). Evidence that the chronic inflammatory response to stroke causes further neurodegeneration in humans is provided by the observation that the area surrounding chronic stroke infarcts undergoes encephalomalacia (Nguyen et al., 2016). This softening of the tissue surrounding stroke infarcts suggests that the tissue is being damaged by a chronic lytic process occurring within the infarct. When the infarct in a human is adjacent to a brain region important for memory, the chronic inflammatory response in the infarct may be a cause of post-stroke dementia.

2. Myelin breakdown triggers the formation of foam cells in the ischemic stroke infarct

At the forefront of inflammation in the infarct core are lipid-phagocytosing myeloid cells (Beuker et al., 2022). Following an ischemic stroke, these cells must clear a substantial volume of lipid due to the abundance of myelin within the brain. To complete this task, microglia from the peri-infarct region and monocytes from the periphery infiltrate the infarcted tissue. Microglia are the first to arrive, migrating to the site of injury within minutes to hours of stroke onset. This is followed by the recruitment of monocytes and macrophages within 1–3 days. Depending upon the severity of the stroke, these myeloid cells remain within the infarct for a few weeks to several months (Nguyen et al., 2016), (ElAli and LeBlanc, 2016), (Chung et al., 2018). Microglia comprise 10–15% of all cells within the brain and are specialized for CNS-specific functions, such as synaptic pruning and plasticity (Bar and Barak, 2019). However, microglia also share many functions with macrophages from other tissues, including scavenging, phagocytosis, and antigen presentation (Aloisi, 2001). Their receptor expression is dynamic, and the markers they express overlap with the markers expressed by hematogenous macrophages. Although Beuker et al. have shown that the lipid-phagocytosing myeloid cells within the infarct are predominantly microglia-derived, they have also shown that hematogenous macrophages within the infarct take on an equivalent phenotype, suggesting that microglia and infiltrating macrophages take on the same phenotype independent of developmental origin (Beuker et al., 2022).

Following their recruitment to the infarct, microglia and macrophages are activated by the release of damage-associated molecular patterns (DAMPs) that originate from dead and dying cells (Zhao et al., 2017). Corr et al. have shown that one of these DAMPs is damaged myelin (Corr et al., 2016). These DAMPs skew microglia and macrophages toward pro-inflammatory phenotypes, as evidenced by their secretion of TNFα, IL-6, IL-1β, IFN-γ, and other mediators that increase cellular damage and activate pro-inflammatory immune cells (Shichita et al., 2014), (Zbesko et al., 2018). These myeloid cells also secrete reactive oxygen species (ROS) and matrix metalloproteinases (MMPs) (Jayaraj et al., 2023). ROS are directly toxic to cells and can cause lipid peroxidation. MMPs contribute to the conversion of the solid tissue into a liquefactive slurry, as well as prolonged blood brain barrier (BBB) breakdown through the degradation of tight junctions (Lakhan et al., 2013), (Auten and Davis, 2009).

Myelin breakdown after ischemic stroke leads to the generation of a pool of polyunsaturated fatty acids, free cholesterol (FC), cholesterol esters (CEs), and phospholipids that can be loaded onto low-density lipoprotein (LDL) particles. The average LDL particle, which is recognized by the LDL receptor (LDLR), consists of 600 molecules of FC, 1600 molecules of CE, 700 molecules of phospholipid, 180 molecules of triglyceride, and 1 molecule of apolipoprotein B (ApoB) (Levitan et al., 2010). However, the inflammatory milieu of stroke infarcts harbors multiple routes to enzymatic and non-enzymatic lipid peroxidation (Zeiger et al., 2009), leading to an increased likelihood of LDL oxidation. The oxidizable components of LDL consist primarily of polyunsaturated fatty acids, including arachidonic acid and linoleic acid. When these, as well as other oxidized lipid species, are incorporated, the LDLR can no longer recognize the LDL particle. Instead, oxidized LDL (oxLDL) is recognized by scavenger receptors and TLR4 (Stewart et al., 2010). The scavenger receptors for oxLDL include lectin-like oxidized low-density lipoprotein 1 (LOX-1, also known as SR-E1), class A scavenger receptor (SR-A1), class B scavenger receptors (SR-B1 and SR-B2, also known as CD36), and class D scavenger receptor (SR-D1, also known as CD68). Unlike the uptake of LDL by LDLR, the uptake of oxLDL by scavenger receptors generates a feed-forward loop, resulting in lipid accumulation in phagolysosomes (Stephen et al., 2010). The uninhibited accumulation of lipids results in the transformation of macrophages and microglia into foam cells.

Foam cells are defined as cells that have engulfed and accumulated large amounts of lipids, particularly cholesterol and CEs. This accumulation gives them a foamy appearance when viewed under a microscope. Foam cells are typically derived from macrophages and microglia, but other cell types, such as smooth muscle cells and endothelial cells, can also transform into foam cells. Foam cells cause chronic inflammation in atherosclerosis, several cancers, and multiple metabolic, infectious, and autoimmune diseases (Guerrini and Gennaro, 2019). As a result of dysregulated lipid metabolism, foam cells lose immune functions and induce tissue damage. Therefore, their emergence in the brain after stroke is an important therapeutic target.

The transformation of microglia and macrophages into foam cells may not only be caused by scavenger receptor uptake of oxLDL. Natural antibodies produced by a type of B cell, called B1 cells, can recognize similar epitopes on oxLDL, apoptotic cells, and some pathogens (Levitan et al., 2010), (Binder and Silverman, 2005). These antibodies bind to oxidized phosphocholine-containing phospholipids but do not recognize unmodified LDL. In atherosclerosis, immune complexes formed by antibody-bound oxLDL are taken up by macrophages via Fcγ receptors, further contributing to the development of foam cells (Levitan et al., 2010). The production of natural antibodies by a T cell-independent process has been observed within the infarct after experimental stroke, which indicates that these antibodies may also play a role in foam cell formation after stroke (Zbesko et al., 2020).

The high lipid content of myelin strongly supports that the lipid that accumulates within foam cells is predominantly derived from myelin rather than circulating lipids or lipids from other plasma membranes. The dry weight of human myelin is comprised of approximately 30% protein and 70% lipid. Of the lipid content, 19.4% is cholesterol, 31.1% is phosphoglycerides, 5.5% is sphingomyelins, and 18.6% is glycolsphingosides (15.9% cerebrosides and 2.7% sulfatides) (Kiernan, 2007). The high lipid content of myelin means that even though the human brain typically only comprises 2% of an individual’s body weight, it contains 20% of the body’s total cholesterol (Kiernan, 2007). Therefore, in the aftermath of a stroke, the immune system must clear away substantially more lipid than after an ischemic injury to an alternative area of the body, such as the heart following a myocardial infarction.

In fact, the inflammatory response to ischemia in the protein-rich heart was recently compared to the inflammatory response to ischemia in the lipid-rich brain. It was demonstrated that the inflammatory response to ischemic injury in the mouse heart takes 4 weeks to resolve, while in the brain, it takes between 12 and 24 weeks. Further, in the brain, there was a biphasic cytokine response, with the second phase initiated between weeks 4 and 8 after ischemic insult. The second phase coincided with the accumulation of intracellular and extracellular cholesterol crystals, the activation of NLRP3 inflammasome signaling, and the recruitment of large numbers of T and B lymphocytes to the infarct (Zbesko et al., 2020), (Chung et al., 2018). Foam cells, cholesterol crystals, and lymphocytes did not accumulate in mouse hearts following the experimental model of myocardial infarction, and in the heart, the inflammatory response was not biphasic. An overview of foam cell formation in response to myelin breakdown is provided in Fig. 1.

Fig. 1.

Model for myelin breakdown and foam cell formation as the trigger for chronic inflammation after ischemic stroke.

(1) Following stroke, oxidative stress and the release of DAMPs results in acute inflammation. Microglia and peripheral monocytes respond to MCP-1 and CX3CL1, as well as other chemokines, including MIP-1α/β, CCL5/RANTES, and OPN/SPP1, to survey the area of damage and clear the cellular debris that has accumulated due to cell death. (2) However, myelin breakdown leads to a pool of lipids that have the potential to be oxidized by enzymatic and non-enzymatic processes in the infarct before or after loading onto lipoprotein particles, such as oxLDL. Oxidized lipids are recognized by TLRs, as well as scavenger receptors that lack feedback inhibition. Excessive scavenger receptor uptake transforms microglia and macrophages into foam cells, which secrete degradative enzymes, such as MMP12, and chemokines that attract T and B cells and activate cell death pathways. This results in a second wave of inflammation that may last for several months and cause further neurodegeneration. Created with BioRender.com

3. Therapeutic approaches for targeting foam cells in the ischemic stroke infarct

While the inflammatory responses to ischemia within the heart and brain are quite different, the inflammatory response within chronic stroke infarcts bears significant similarities to the inflammatory response in atherosclerosis. In atherosclerosis, excessive cholesterol leads to the oxidation of LDL and the formation of cholesterol crystals within foamy macrophages in the intima of arteries. Both oxLDL and cholesterol crystals then contribute to a chronic inflammatory process that features the activation of NLRP3 inflammasomes and the production of high levels of MMPs. Regarding immune cell infiltrates, atherosclerotic plaques are predominantly comprised of monocytes, foamy macrophages, and T lymphocytes (Tse et al., 2013). In atherosclerosis, foam cell formation is dictated by the balance between intracellular lipid uptake and reverse lipid transport to extracellular lipid acceptors, such as high-density lipoprotein (HDL) (Yu et al., 2013), (Kzhyshkowska et al., 2012). The similarities between foam cell formation in atherosclerosis and ischemic stroke are consequential from a therapeutic standpoint, as it alludes to comparable mechanisms and common targets. Furthermore, in ischemic stroke patients with atherosclerosis caused by hyperlipidemia, the elevated levels of LDL cholesterol in their blood may exacerbate the formation of foam cells in their brains after stroke. Therefore, controlling hyperlipidemia after stroke may be an important strategy for both reducing the risk of foam cell formation in the infarct as well as reducing the risk of recurrent ischemic stroke. Controlling hyperlipidemia is already an important component of the current standard of care after a stroke. Therefore, in the next section, we will explore new pharmacological approaches and targets for improving stroke recovery by modulating microglia and macrophage function, drawing on knowledge from both atherosclerosis and stroke research.

3.1. Lipid chelation

One method for reducing lipid accumulation within microglia and infiltrating monocytes is to administer compounds that can entrap and solubilize the lipid debris generated by the breakdown of myelin. In support of this, it was recently demonstrated that the recruitment of T lymphocytes, B lymphocytes, and plasma cells in chronic stroke infarcts is attenuated by repeated administration of 2-hydroxypropyl-β-cyclodextrin (HPβCD). HPβCD is a macrocyclic oligosaccharide comprised of glucose units. It is commonly utilized as a nanocarrier for drugs that have poor aqueous solubility. Due to its hydrophobic cavity and hydrophilic exterior, it can transport hydrophobic drugs throughout the blood stream. Importantly, the hydrophobic cavity also has a high affinity for cholesterol and other lipids and is able to chelate excessive cholesterol in mouse models of atherosclerosis and Niemann-Pick type C (NPC) (Zimmer et al., 2016), (Camargo et al., 2001). HPβCD can also promote liver X receptor (LXR)-mediated transcriptional reprogramming of macrophages (Zimmer et al., 2016). HPβCD treatment in a mouse model of distal middle cerebral artery occlusion (DMCAO) reduced lipid droplet accumulation in the infarct, enhanced cognitive function, and preserved NeuN immunoreactivity in the striatum and thalamus (Becktel et al., 2022). It did not alter immune cell populations in the spleen, indicating that it is not immunotoxic. These results provide proof of principle that increasing the solubility of cholesterol and other lipids by pharmacological means can have beneficial effects on stroke recovery. An advantage of this approach is that it is less likely to have immunosuppressive effects than directly targeting specific cytokines or immune cell populations, which provide protection against infection and other diseases. Fig. 2 illustrates the therapeutic strategy of reducing lipid accumulation in microglia and infiltrating monocytes by administering HPβCD.

Fig. 2.

The therapeutic strategy of reducing lipid accumulation in microglia and infiltrating monocytes by administering HPβCD. The left panel shows that excessive uptake of oxidized lipids derived from myelin debris can turn microglia and infiltrating monocytes into foam cells, resulting in increased oxidative stress, the release of degradative enzymes, and the release of inflammatory cytokines and chemokines that attract lymphocytes. In contrast, the right panel shows that the administration of HPβCD can alleviate the burden of phagocytosing myelin debris from microglia and monocytes by directly encapsulating and removing cholesterol and other lipids. Created with BioRender.com

3.2. Myeloid cell recruitment

Another therapeutic approach for preventing foam cell formation is to target monocyte and microglial recruitment to the stroke infarct. However, while blocking recruitment would reduce the number of cells that can turn into foam cells, it would also reduce the number of cells available to clear, repair, and remodel the damaged part of the brain. Therefore, the ideal blockade would prevent the recruitment of deleterious myeloid cell populations, while preserving or increasing the recruitment of beneficial myeloid cell populations. Although many monocyte-derived macrophages and microglia within the stroke infarct are classically activated to pro-inflammatory phenotypes in the acute and early subacute phases following stroke, there is also a population of activated macrophages and microglia that are anti-inflammatory. These macrophages and microglia produce a variety of anti-inflammatory cytokines and chemokines, including IL-4, IL-10, and TGF-β (Viola et al., 2019). An ideal therapeutic approach would preserve this population.

Chemokines that attract monocytes and microglia to the stroke infarct include monocyte chemoattractant protein-1 (MCP-1), also known as C-C motif chemokine ligand 2 (CCL2), chemokine (C-X3-C motif) ligand 1 (CX3CL1), also known as fractalkine, macrophage inflammatory proteins α and β (MIP-1α/β), CCL5, also known as RANTES, and osteopontin (OPN), also known as secreted phosphoprotein 1 (SPP1) (Beuker et al., 2022), (Cowell et al., 2002), (Mirabelli-Badenier et al., 2011). One method of controlling monocyte recruitment could be to target MCP-1, which binds to C-C motif chemokine receptor 2 (CCR2). CCR2 is an integral chemokine responsible for pro-inflammatory monocyte entry into the stroke infarct (Fang et al., 2018; Dimitrijevic et al., 2007; Willenborg et al., 2012). Previous studies have revealed that an elevated level of MCP-1 within the blood is associated with worse stroke outcome in both mice and humans (Fang et al., 2018; Dimitrijevic et al., 2007; Willenborg et al., 2012) For example, the overexpression of MCP-1 in mice exacerbates injury and increases immune cell infiltration in the first 48 h post-stroke (Chen et al., 2003). Conversely, the genetic ablation of Ccr2 results in decreased infarct size, immune cell infiltration, edema, and pro-inflammatory cytokine levels and is, therefore, protective during ischemic injury (Dimitrijevic et al., 2007). These findings show that the MCP-1/CCR2 axis is a key signaling mechanism for the recruitment of a deleterious population of monocytes to the infarct early after stroke. MCP-1 has also been shown to be important in recruiting monocytes to atherosclerosis lesions. In atherosclerosis models, mice lacking MCP-1/CCL2 or its receptor CCR2 have a considerable reduction in lesion development.

Before entering the infarct, circulating monocytes have to extravasate from blood vessels. Activated endothelial cells express cell adhesion molecules and chemokines that attract monocytes to the stroke infarct. The initial adhesion process involves selectins, which facilitate rolling interactions, followed by integrins, which allow firmer attachment. Under the influence of chemoattractant molecules, adherent monocytes can then migrate into the infarct. Intercellular adhesion molecule 1 (ICAM-1) is an important integrin ligand for the initial recruitment of pro-inflammatory monocytes to the infarct (Yang et al., 2019), (Supanc et al., 2011). The administration of a murine anti-ICAM-1 antibody after transient MCAO results in reduced infarct volume in rats (Zhang et al., 1995), and mice deficient in Icam-1 have decreased infarct size following focal cerebral ischemia compared with wild-type (WT) mice (Kitagawa et al., 1998). These data suggest that ICAM-1 is a pivotal effector in the regulation of circulating monocyte infiltration into the infarct after stroke (Yang et al., 2019). Additional adhesion molecules, including P-selectin and vascular cell adhesion molecule 1 (VCAM-1), are alternative targets for reducing monocyte recruitment to the infarct (Li and Glass, 2002).

Microglia have a more direct route to the infarct after stroke than monocytes. Rather than arriving via the circulation, they follow the gradient of chemoattractant molecules radiating from the infarct into the parenchyma. One of the most important of these chemokines is CX3CL1. Under homeostatic conditions, microglia express large quantities of the CX3CL1 receptor, CX3CR1, while CX3CL1 itself is produced by neurons. When membrane-bound, neuronal CX3CL1 binds to microglial CX3CR1 and maintains microglia in a quiescent state. However, cleaved CX3CL1 acts as a potent chemokine that draws microglia to the infarct. CX3CL1 expression is upregulated within 48 h after ischemic injury and decreases by 7 days (Taylor and Sansing, 2013). Several studies have shown that interruption of the CX3CL1/CX3CR1 signaling pathway, either by deletion of Cx3cl1 or deficiency in Cx3cr1, reduces stroke injury (Soriano et al., 2002), (Pawelec et al., 2020). However, there is also evidence that the intracerebroventricular administration of exogenous CX3CL1 results in a long-lasting neuroprotective effect against cerebral ischemia in rodents (Cipriani et al., 2011). Therefore, the outcome of modulating CX3CL1/CX3CR1 is equivocal and may depend on the timing and method of therapeutic intervention (Cardona et al., 2006).

In summary, modulating MCP-1/CCR2 and ICAM-1 signaling are potent methods of controlling monocyte recruitment to the stroke infarct. An equally powerful method for controlling microglial recruitment appears to be modulating the CX3CL1/CX3CR1 signaling axis. However, microglia and circulating monocytes are highly plastic cells which continuously shape their functional profile in response to environmental cues in healthy and injured tissues. Without them, clearance, remodeling, and repair of the damaged brain tissue is likely to be impeded. Therefore, controlling their functional state within the stroke infarct and preventing their transformation into foam cells may ultimately be more important than blocking or augmenting their recruitment (Gliem et al., 2016).

3.3. Scavenger receptors

Scavenger receptors are cell surface receptors found in various immune cells, including macrophages and microglia. They are responsible for recognizing and removing a wide range of endogenous and exogenous ligands, such as oxidized lipids, modified LDL particles, and apoptotic cells (Stephen et al., 2010). The mechanism by which scavenger receptors work depends on the specific receptor and its ligand. In optimal conditions, once bound, their ligand is internalized by the cell, usually through receptor-mediated endocytosis or phagocytosis, and either degraded or processed for antigen presentation (Greaves and Gordon, 2005).

After ischemic stroke, myelin debris must be removed quickly because its sustained presence prevents oligodendrocyte differentiation, remyelination, and axon regrowth (Cantuti-Castelvetri et al., 2018). Scavenger receptors play a major role in accomplishing this with the limitation that their uptake lacks negative feedback inhibition (Stephen et al., 2010). Macrophages and microglia express multiple scavenger receptors, including SR-A1, SR-B1, CD36, CD68, and the scavenger receptor for phosphatidylserine and oxidized lipoprotein (SR-PSOX) (Stephen et al., 2010), (Kzhyshkowska et al., 2012), (Paresce et al., 1996).

Following internalization via scavenger receptors, oxidized lipid particles are delivered to the lysosome, where CEs are hydrolyzed to FC by lysosomal acid lipase (LAL). To prevent FC-associated cell toxicity, FC is released from the lysosome and re-esterified in the endoplasmic reticulum (ER) by acyl cholesterol acyltransferase (ACAT1) for storage in cytoplasmic lipid droplets (Kzhyshkowska et al., 2012), (Park, 2014), (Moore and Freeman, 2006). In atherosclerosis, the accumulation of excess CEs results in the retention of FC in the lysosome, and when the FC level surpasses the solubility limit of the lipid phase in the lysosome, the FC crystallizes (Tangirala et al., 1994). Concurrent to the formation of cholesterol crystals in the lysosome, the unconstrained engulfment of CE-containing particles by scavenger receptors generates the large number of cytoplasmic lipid droplets that give foam cells their distinctive appearance. The nucleation of cholesterol crystals in foamy macrophages leads to apoptosis and their expulsion into the extracellular environment (Tangirala et al., 1994). The scavenger receptor mediated propagation of cholesterol crystals increases inflammation through activation of the NLRP3 inflammasome and subsequent production of pro-inflammatory cytokines (Sheedy et al., 2013), (Rajamaki et al., 2010).

In atherosclerosis, intracellular lipid uptake by macrophages is primarily mediated by CD36, a class B scavenger receptor known to bind and internalize ligands, including oxLDL, long-chain fatty acids, and oxidized phospholipids (oxPL) (Park, 2014). These modified phospholipids located on the surfaces of lipoproteins and apoptotic cell membranes interact with CD36 to trigger pro-inflammatory signaling cascades and inhibit macrophage migration. Higher CD36 expression after stroke is associated with increased phagocytic activity, while pharmacologically inhibiting CD36 decreases phagocytosis (Woo et al., 2016). These findings show that CD36 is a key phagocytosis mediator during the post-stroke recovery phase. However, the extent to which CD36 plays a reparative or harmful role during the resolution of inflammation likely depends on how effectively it removes myelin debris without leading to the propagation of cholesterol crystals and formation of foam cells.

SR-A1 is another scavenger receptor that plays a crucial role in the pathophysiology of ischemic injury. It has the ability to recognize pathogen-associated molecular patterns (PAMPs), such as lipopolysaccharide (LPS), lipoteichoic acid, bacterial CpG DNA, double-stranded RNA, and yeast zymosan/β-glucans, and, therefore, plays an important part in inducing innate immune responses (Lu et al., 2010). In addition, SR-A1 acts as a coreceptor with TLR4 in modulating the inflammatory response to TLR agonists. Toll-like receptor ligands induce SR-A1 expression and promote macrophages to bind and internalize TLR4 ligands via SR-A1. SR-A1 may subsequently act as a negative regulator of TLR4-mediated immune responses, as internalized SR-A1 ligands can trigger apoptosis in ER-stressed macrophages. SR-A1 may contribute to early cerebral ischemic injury by enhancing inflammation. This is supported by the observation that mice lacking SR-A1 have significantly smaller infarcts in response to acute ischemic injury (Lu et al., 2010). The mechanism behind this seems to involve the prevention of increased activation of apoptotic signaling and NF-κB binding activity. The role of SR-A1 in mediating foam cell formation after stroke is currently unknown, though SR-A1 has a crucial role in driving atherosclerotic lesion formation in diet-induced atherosclerosis in C57BL/6J mice (Kamada et al., 2001).

While inhibiting macrophage scavenger receptors could be a potential approach to mitigating foam cell formation, redundancy in their function means that targeting multiple scavenger receptors simultaneously may be necessary. Additionally, some scavenger receptors, such as CD36 and SR-B1, have additional functions beyond lipid uptake. CD36 serves as a fatty acid transport protein in adipose tissue and muscle, and SR-B1 mediates selective uptake of HDL CEs in liver and steroidogenic tissues. Also, while studies suggest that SR-A1 and CD36 play significant roles in oxLDL uptake and atherosclerosis development, the gene encoding SR-B1 appears to have an anti-atherogenic function due to its role in mediating reverse cholesterol transport by HDL (Stephen et al., 2010), (Kzhyshkowska et al., 2012), (Moore and Freeman, 2006). This indicates that targeting these receptors could have negative effects on other processes.

Given the critical role that scavenger receptors play in foam cell development, therapeutic approaches that limit their expression have been explored as prospective treatments. For example, statins have been used to modulate scavenger receptors. Although statins lower total and LDL cholesterol by blocking HMG-CoA reductase in the liver, they also act by other pleiotropic effects, including downregulation of the scavenger receptors SR-A1, CD36, and LOX-1 (Li and Glass, 2002). CD68-Fc immunoadhesin, a fusion protein consisting of the extracellular domain of the human CD68 and a human Fc domain, is another promising tool that has been tested for preventing foam cell formation (Daub et al., 2010). CD68 is a member of the lysosomal-associated membrane protein (LAMP) family and is a class D scavenger receptor. Dendritic cells, Langerhans cells, microglia, and macrophages have all been found to express CD68, which attaches to and internalizes modified lipoproteins. Because of its robust ability to bind to oxLDL, and its widespread intracellular distribution, it plays an important role in atherogenesis. In contrast to class A and class B scavenger receptors, it only binds atherogenic oxLDL and not other lipoprotein types, such as protective HDL. CD68-Fc has been shown to specifically bind to oxLDL and prevent uptake by macrophages, and thereby inhibit foam cell formation (Daub et al., 2010).

3.4. Lipid peroxidation

Several different cell types have been shown to contribute to oxidative stress in stroke infarcts, including macrophages, microglia, endothelial cells, and neutrophils. These cells can produce ROS and reactive nitrogen species (RNS) through the activation of various enzymes, including lipoxygenases (LO), myeloperoxidase (MPO), inducible nitric oxide synthase (iNOS), and NADPH oxidases. ROS and RNS can then react with lipids, proteins, and DNA, leading to oxidative modifications (Ozkul et al., 2007).

Generally, lipoproteins must undergo oxidation in order for microglia and macrophage scavenger receptors to become activated. Although microglia and macrophages may not be the only mediators of lipid peroxidation, they contribute to this process in infarcted tissue. While the precise mechanisms responsible for lipoprotein oxidation are not yet fully defined, LO, MPO, iNOS and NADPH oxidases are likely the key enzymes (Li and Yang, 2016). This is because these enzymes are known to contribute to lipoprotein oxidation in vitro and are expressed within stroke infarcts (Ozkul et al., 2007), (Li and Yang, 2016). Therefore, the inhibition of lipid peroxidation using antioxidants is a potential target for therapeutic intervention.

An alternative to using antioxidants to manage lipid modifications is to focus on regulating the enzymatic activities that contribute to these modifications. For instance, angiotensin II may induce macrophage-dependent LDL oxidation through a LO-dependent mechanism. Additionally, angiotensin-converting enzyme (ACE) is activated in macrophages during chronic inflammation, and ACE inhibitors, irrespective of their blood pressure-lowering effects, increase the resistance of circulating LDL to oxidation and decrease the development of atherosclerosis in Apoe−/− mice (Li and Glass, 2002), (Hayek et al., 1999).

The use of nuclear factor erythroid 2-related factor 2 (Nrf2) modulators to increase antioxidant pathways is another method for reducing lipid peroxidation after stroke. Nrf2 is a transcription factor that regulates the expression of several antioxidant and anti-inflammatory genes. When activated, Nrf2 binds to antioxidant response elements (AREs) in the DNA of cells, leading to the expression of genes that help protect against oxidative stress and inflammation (Farina et al., 2021). Studies have shown that Nrf2 activation can reduce the severity of damage caused by stroke by reducing oxidative stress and inflammation. In animal models of stroke, Nrf2 activation has been shown to improve outcomes, including reducing infarct size, improving neurological function, and reducing mortality (Zhang et al., 2017; Ya et al., 2017; Shi et al., 2015; Li et al., 2013; Tanaka et al., 2011). Overall, the activation of Nrf2 has shown promising results as a therapeutic target for stroke. However, further research is needed to understand the full potential of Nrf2 activation in preventing foam cell formation after stroke (Farina et al., 2021).

3.5. TREM2

TREM2 (triggering receptor expressed on myeloid cells 2) is a transmembrane protein that is primarily expressed on immune cells, particularly microglia, in the brain. TREM2 has been implicated in a range of biological processes, including immune regulation, lipid metabolism, and tissue repair (Deczkowska et al., 2020). The ligands for TREM2 include phospholipids such as phosphatidylserine and phosphatidylcholine, as well as apolipoproteins, such as apolipoprotein E (ApoE) and apolipoprotein J (ApoJ) (Deczkowska et al., 2020). TREM2 has also been reported to bind to LPS, fungal zymosan, and the β-amyloid protein. TREM2 binding to these ligands can activate downstream signaling pathways that modulate the immune response and phagocytic activity of myeloid cells (Deczkowska et al., 2020).

Recent studies have suggested that TREM2 may play a key role in modulating the immune response to ischemic stroke and promoting tissue repair and regeneration in the brain. After stroke, TREM2 has been shown to modulate the activation and function of microglia, influencing the balance between pro-inflammatory and anti-inflammatory processes in the brain (Ma et al., 2022).

Several studies have suggested that TREM2 may play a beneficial role in stroke recovery. For example, one study found that Trem2-deficient mice had increased brain damage and worse functional outcomes following stroke compared to control mice (Wu et al., 2017). Another study found that TREM2 activation in microglia promoted the clearance of cellular debris and the activation of immune cells involved in tissue repair and regeneration (Kawabori et al., 2015). TREM2 has also been implicated in regulating microglial phagocytosis of myelin debris, as Trem2-deficient microglia have been shown to have reduced phagocytic activity and impaired clearance of myelin debris in experimental models of demyelination (Cignarella et al., 2020).

TREM2 has also been shown to play a role in foam cell formation within atherosclerotic plaques. Studies have shown that Trem2 deficiency in mice leads to reduced foam cell formation and atherosclerotic lesion development (Porsch et al., 2020). Overall, these findings suggest that TREM2 plays a crucial role in modulating the immune response to stroke and promoting tissue repair and regeneration in the brain. Further research is needed to fully understand the mechanisms underlying the effects of TREM2 on stroke pathogenesis and the extent to which its beneficial actions are via the modulation of foam cell formation.

3.6. Cholesterol efflux

When intracellular cholesterol levels rise, endogenous cholesterol production and LDLR expression is inhibited. However, this mechanism is insufficient to maintain cholesterol homeostasis in the presence of continuous cholesterol uptake via scavenger receptors. Since mammalian cells lack the ability to degrade cholesterol, macrophages and other cell types must export cholesterol to extracellular acceptors for transport to the liver for biliary excretion, which is primarily carried out by HDL particles. HDL particles transfer CEs to the liver either directly through selective uptake via SR-B1 or indirectly via other lipoproteins that can be broken down by the liver (Li and Glass, 2002).

The cellular efflux of cholesterol is controlled by the binding of oxidized derivatives of cholesterol called oxysterols to LXR-α and LXR-β, which are members of the ligand-dependent transcription factor family of nuclear receptors. Oxysterols are generated via autooxidation by free radicals and ROS or formed enzymically by a variety of enzymes such as mitochondrial and ER cytochrome P450 family members (CYP). LXR-β is ubiquitously expressed, while LXR-α is highly expressed in the liver and intestine and is induced in macrophages in response to cholesterol accumulation. LXRs form heterodimers with retinoid X receptors (RXRs) and bind to specific response elements in target gene promoters or enhancers. The main LXR target genes include ATP-binding cassette transporter A1 (Abca1) and G1 (Abcg1), responsible for cholesterol efflux from cells and involved in cholesterol and phospholipid transport, respectively. LXR also upregulates sterol regulatory element-binding protein 1c (Srebp-1c), fatty acid synthase (Fas), and acetyl-CoA carboxylase (Acc), all of which are involved in fatty acid synthesis. Additionally, LXR regulates the expression of apolipoprotein E (APOE) and lipoprotein lipase (LPL), which are involved in the metabolism of plasma lipoproteins. LXR also regulates cytochrome P450 7A1 (CYP7A1), which plays a role in bile acid synthesis (Li and Glass, 2002). Studies in genetically engineered mice suggest that ABCA1 is crucial for HDL formation, systemic lipoprotein metabolism, and mediating cholesterol efflux from macrophage foam cells in atherosclerotic lesions (Li and Glass, 2002). Humans with common ABCA1 genetic variants have altered lipoprotein levels and an altered risk of coronary artery disease (Clee et al., 2001). It will be interesting to see if these variations also affect stroke recovery by modifying the formation of foam cells in the infarct.

The potential of LXR-RXR heterodimers to regulate Abca1 and other genes associated with cholesterol homeostasis suggests that they could be promising targets for the development of anti-foam cell drugs. Indeed, synthetic LXR and RXR agonists have been shown to decrease atherosclerosis in mice lacking Ldlr and Apoe genes (Joseph et al., 2002), (Claudel et al., 2001). Additionally, findings by Zhou et al. support the utility of targeting cholesterol efflux pathways after injury in the CNS due to their observation that intravenous administration of AdipoRon after spinal cord injury (SCI) reduces inflammation and macrophage accumulation within the lesion, ameliorates post-SCI tissue damage and astrogliosis, and improves motor function (Zhou et al., 2019). AdipoRon is an adiponectin receptor agonist that suppresses myelin lipid accumulation in macrophages through LXRα/ABCA1-mediated lipid efflux.

Targeting macrophage peroxisome proliferator-activated receptors (PPARα, PPARγ, and PPARδ) may be another method of upregulating cholesterol exporters after stroke. Lipoproteins are characterized by a surface coating of phospholipids and a high content of triglycerides. Macrophages secrete various lipases, including LPL and hepatic lipase, which hydrolyze triglycerides in lipoproteins, and endothelial lipase and members of the secretory phospholipase A2 family, which hydrolyze phospholipids. The resulting fatty acids act as ligands for PPARα, PPARγ, and PPARδ, which inhibit inflammatory gene expression and can promote cholesterol efflux via the upregulation of ABCA1 (Rader and Puré, 2005). An overview of foam cell signaling pathways involved in the regulation of cholesterol metabolism and inflammation is provided in Fig. 3.

Fig. 3.

The pathways involved in the regulation of cholesterol metabolism and inflammation in foam cells. This figure highlights the following key points: (1) Oxidized lipoproteins derived from myelin debris bind to TLRs, such as TLR4, and scavenger receptors, such as CD36, resulting in pro-inflammatory NF-κB pathway activation and lipid particle delivery to the lysosome, respectively. (2) Excessive cholesterol accumulation in the lysosome leads to the formation of cholesterol crystals, activating the NLRP3 inflammasome and potentiating cytokine release. (3) Free cholesterol released from the lysosome can be re-esterified by ACAT1 in the endoplasmic reticulum (ER) and stored in cytoplasmic lipid droplets. (4) Excessive cholesterol in the ER can generate oxysterols through enzymes such as CYP. (5) Oxysterols bind to LXRs and activate cholesterol efflux genes, including Abca1, Abcg1, Apoa-1, and Apoe, leading to the restoration of cholesterol homeostasis, provided scavenger receptor uptake is limited. (6) Excessive lipid accumulation in foam cells can cause mitochondrial dysfunction, resulting in oxidative stress and additional CYP activity in the mitochondria. Created with BioRender.com

3.7. T lymphocytes

T lymphocytes, particularly CD4+ T cells, play an important role in the development and progression of atherosclerosis. These cells are capable of differentiating into various subtypes, including T helper type (Th)1, Th2, Th17, regulatory T cells (Tregs), and T follicular helper cells (Tfh), each with unique cytokine profiles and functions (Saigusa et al., 2020), (Tse et al., 2013). In ischemic stroke, Th1 and Th2 cells enter the infarct in the acute stages, as early as 24 h post-stroke, and stay within the infarct for weeks to months (Selvaraj and Stowe, 2017), (Zhang et al., 2018).

In atherosclerosis, Th1 cells are thought to promote disease by secreting pro-inflammatory cytokines, such as IFNγ and TNFα, which activate macrophages to take up oxLDL and generate foam cells. Similarly, in stroke, the negative impact of CD4+ T cells on recovery is mostly attributed to Th1 cells through their production of pro-inflammatory cytokines, including IFNγ (Yilmaz et al., 2006). On the other hand, Th2 cells are typically viewed as anti-inflammatory in atherosclerosis by secreting cytokines like IL-4 and IL-10, which can suppress the pro-inflammatory response of macrophages and promote cholesterol efflux from foam cells (Saigusa et al., 2020), (Tse et al., 2013) (Liu et al., 2016). Th2 cells have been detected acutely within human stroke infarcts, and Theodorou et al. have identified an increase in the Th2 to Th1 ratio in the peripheral blood of patients in the post-acute phase of stroke recovery (Theodorou et al., 2008). Although not yet known, by producing IL-4 and IL-10, it is possible that Th2 cells contribute positively to stroke recovery by suppressing the pro-inflammatory response of microglia and macrophages and promoting cholesterol efflux from foam cells (Arumugam et al., 2005).

Tregs are a subtype of CD4+ T cells that also produce IL-10. They have immunosuppressive functions and can suppress the activation and proliferation of other T cells. In atherosclerosis, Tregs have been shown to play an anti-atherogenic role. Tregs have also been shown to have a beneficial role in stroke recovery. Importantly, mice lacking IL-10 exhibit increased infarct size and impaired functional recovery (Grilli et al., 2000), (Pérez-De Puig et al., 2013). Further, the depletion of Tregs, through administration of an anti-CD25 antibody, increases the size of the stroke infarct and worsens neurological function in mice (Liesz et al., 2009). By contributing to the IL-10 pool, Tregs may also be able to inhibit the pro-inflammatory response of microglia and macrophages and increase the outflow of cholesterol from foam cells after stroke.

CD8+ T cells are also present in atherosclerotic plaques and can contribute to plaque progression through the release of cytotoxic molecules, such as perforin, which can induce apoptosis in smooth muscle cells and endothelial cells. In stroke, CD8+ T cells are one of the first adaptive immune cell types to enter the infarct, arriving within 3 h following permanent cerebral ischemia (Chu et al., 2014). In the acute phase following stroke, activated CD8+ T cells become neurotoxic and increase the size of the stroke infarct through the release of perforin (Mracsko et al., 2014). Additionally, it has been found that ovalbumin-specific OT-1 CD8+ T cells do not localize to stroke infarcts or cause the neurotoxic effects observed when WT CD8+ T cells are transferred into lymphocyte-deficient mice (Mracsko et al., 2014). This study by Mracsko et al. suggests that the neurotoxic effects of CD8+ T cells following stroke are CNS antigen-specific. Though little is known about the relationship between CD8+ T cells and foam cells in chronic stroke, previous work has demonstrated that CD8+ cells are present within the infarct for at least 7 weeks after stroke in mice (Nguyen et al., 2016).

Gamma delta (γδ) T cells have T cell receptors (TCRs) comprised of γ and δ chains instead of the more common α and β chains, as in CD4+ and CD8+ T cells (Holtmeier and Kabelitz, 2005). γδ T cells may express CD8, and rarely CD4, but most are double negative for CD4 and CD8 (Cron et al., 1989). γδ T cells can respond to classical adaptive immune system signaling or to cytokine signals and TLR signaling without TCR engagement, meaning that γδ T cells may or may not have specificity for the antigen to which they respond (Gelderblom et al., 2014). Significantly, γδ T cells can respond to lipid presented on CD1 molecules (Luoma et al., 2014), indicating that they may react to the high levels of myelin lipid debris present within the stroke infarct. It is known that γδ T cells begin to enter the stroke infarct within the first 24 h and peak in number around day 3 post-stroke. Within the lesion, γδ T cells produce IL-17A in response to stimulation through pattern recognition receptors (PRRs) (Gelderblom et al., 2012), (Martin et al., 2009), which leads to increased neutrophil infiltration (Gelderblom et al., 2012). This suggests that the TCR specificity of these cells is not vital to this aspect of their response to stroke (Chamorro et al., 2012). Shichita et al. discovered that depletion of γδ T cells using γδ T cell-specific antibodies reduced brain damage following cerebral ischemia-reperfusion injury, suggesting that γδ T cells play a predominantly negative role in stroke recovery (Shichita et al., 2009). The extent to which they play this role by responding to the lipid component of myelin debris is unknown and should be explored further.

In summary, in atherosclerosis and stroke, T lymphocytes can have both pro- and anti-inflammatory effects depending on their subtype and the context of the immune response. In atherosclerosis, Th1 cells promote inflammation, and Th2 cells and Tregs have anti-inflammatory and anti-atherogenic roles. In stroke, CD4+ T cells can also have both positive and negative effects. Tregs produce IL-10 and have a beneficial role in stroke recovery. CD4+ Th2 cells also play a positive role by secreting IL-4 and IL-10. CD8+ T cells, on the other hand, contribute to plaque progression in atherosclerosis and increase the size of the stroke infarct through the release of perforin. Although antigen specificity is unknown for both CD4+ and CD8+ T cells, in vitro studies with CD4+ T cells from human atherosclerotic plaques show that many of these cells recognize oxLDL when processed and presented by APCs; therefore, peptides from the ApoB component of oxLDL represent viable target antigens (Saigusa et al., 2020), (Stemme et al., 1995). Although there are no clinically applicable drugs that target T cell responses in atherosclerotic plaques, therapies that prove effective should also be screened for the ability to modulate chronic inflammation and foam cell development in the infarct core after stroke.

3.8. B lymphocytes

In atherosclerosis, oxLDL has been shown to induce B- cell activation and the production of T cell-independent natural antibodies that bind to oxLDL (Boullier et al., 2000; Tsiantoulas et al., 2014; Sage et al., 2019). As mentioned above, immune complexes formed by antibody-bound oxLDL are taken up by macrophages via Fcγ receptors, further contributing to foam cell formation (Levitan et al., 2010). Confoundingly, however, natural antibodies may also limit the uptake of oxLDL by macrophages, thereby preventing their transformation into foam cells (Shaw et al., 2000). We know that IgA natural antibodies are produced following stroke because it was recently discovered that following stroke, a distinct population of B cells matures into IgA+ plasma cells independently of T cells through interaction with a TI-2 antigen (Zbesko et al., 2020). However, we do not yet know if these antibodies bind oxLDL and whether they augment or prevent the transformation of microglia and macrophages into foam cells. This is an important area for future investigation because natural antibodies may be a tool for preventing excessive uptake of oxLDL by macrophages during ischemic stroke recovery.

Migrating IgA+ plasma cells have been shown to enter chronic multiple sclerosis (MS) lesions and secrete IL-10, limiting neuroinflammation (Rojas et al., 2019). Rojas et al. demonstrated that plasma cells producing IgA specific to commensal bacteria migrate from the gut to the CNS following injury in a model of experimental autoimmune encephalomyelitis (EAE). They further showed that EAE was exacerbated using bone marrow chimeras in which plasma cells cannot produce IL-10 but other cells can, indicating that IL-10-producing plasma cells have an anti-inflammatory role in EAE (Rojas et al., 2019). It is possible that the IgA+ plasma cells within the stroke infarct are also IL-10-secreting migratory plasma cells. It is important in future work to evaluate the secretion of IL-10 by these IgA+ plasma cells to determine if they function similarly to those found in chronic MS lesions. If these cells do produce IL-10, it may suggest they also migrate to the CNS from the gut and are helpful for skewing the myeloid response to stroke toward a reparative phenotype, while assisting with the clearance of oxLDL. Promoting the migration of these cells to increase the production of IL-10 could be a potential therapeutic approach to preventing foam cell formation.

B cells may also play several other beneficial roles after stroke. The genetic ablation of B cells increases the size of the infarct at 48 h post-stroke (Ren et al., 2011). Bodhankar et al. and Ren et al. have shown that this is due to the presence of regulatory B cells that produce IL-10 (Ren et al., 2011), (Bodhankar et al., 2013), indicating that B cells are beneficial to stroke recovery acutely. B cells may also provide a long-term protective effect, as post-stroke depletion of B cells has been found to reduce hippocampal neurogenesis (Ortega et al., 2020).

Stroke researchers have also begun to investigate the deleterious roles B cells may play in stroke recovery. Importantly, autoreactive antibodies specific for GFAP, NMDA receptor, MBP, PLP, S100β, and neurofilament have been found in the blood of human stroke patients (Bornstein et al., 2001; Tanne et al., 1998; Zhang and Popovich, 2011; Kamchatnov et al., 2010; Shibata et al., 2012; Becker et al., 2016). Though it is unknown if the production of these autoantibodies is directly linked to the occurrence of a stroke, the development of delayed post-stroke dementia was shown to correlate with the increased presence of autoantibodies against MBP in the blood but did not correlate with levels of autoantibodies against PLP (Becker et al., 2016).

The removal of B cells both genetically and pharmacologically also halts the development of post-stroke cognitive decline in mice (Doyle et al., 2015). This study reported the presence of ectopic lymphoid-like structures within the infarct at 7 weeks following stroke (Doyle et al., 2015), and these observations were also supported by the findings of Weitbrecht and colleagues (Weitbrecht et al., 2021). Ectopic lymphoid structures are organized aggregates of lymphocytes that orchestrate T cell-dependent B-cell activation outside of lymph nodes (Doyle et al., 2015), (Dieu-Nosjean et al., 2014; Ruddle, 2014; Stranford and Ruddle, 2012). In light of these discoveries, a subset of B cells may be activated by self-antigens in a T cell-dependent mechanism. In support of this, following SCI, the development of plasma cells occurs through a T cell-dependent mechanism to produce pathogenic antibodies against GluR2/3 that cause damage through a complement-mediated mechanism (Ankeny et al., 2006), (Ankeny et al., 2009).

It is currently unclear whether the antibodies produced by T cell-dependent or T cell-independent B-cell activation are helpful or harmful to foam cell formation during stroke recovery. Future investigations are needed to evaluate the role of these antibodies in stroke recovery and their potential as a tool for preventing excessive uptake of oxLDL and other lipid particles by microglia and macrophages.

4. Overlap with SCI, TBI, and MS

SCI, traumatic brain injury (TBI), MS, and stroke appear to have similar inflammatory responses to myelin debris. For example, SCI also triggers a chronic inflammatory response with lipid-laden myeloid cells that persist for many months within the injury site. In a study performed by Zhu et al., single sample gene set enrichment analysis (SSGSEA) revealed that these lipid-laden cells closely resemble foam cells from atherosclerotic lesions (Zhu et al., 2017). Furthermore, neuroinflammation in MS is accompanied by the presence of foamy macrophages that contribute to maladaptive immune responses. In MS, foam cells in areas of demyelination develop in three stages. Resident microglia and infiltrating macrophages have a disease-promoting behavior in the first stage, which is accompanied by the release of pro-inflammatory cytokines and toxic mediators. In the second stage, myelin catabolism produces intracellular lipid mediators that drive an anti-inflammatory mechanism, likely through the activation of the LXRs and PPARs. This enables microglia and macrophages to export excess lipids and secrete anti-inflammatory cytokines that facilitate remyelination. However, when microglia and macrophages become overwhelmed by excessive uptake of cholesterol-rich myelin debris, presumably through scavenger receptor uptake, a third phase is triggered, characterized by lipid inclusions, cholesterol crystals, and a long-lasting, disease-promoting phenotype (Guerrini and Gennaro, 2019), (Zierfuss et al., 2020), (Grajchen et al., 2018). These data demonstrate that foam cells are an attractive therapeutic target for a number of CNS diseases and suggest that a therapy approved for one would likely be beneficial for the others.

5. Summary

Foam cells are a strong link in the chain of chronic inflammation after stroke because they exhibit impaired immune functions and prolong inflammation. They may also serve as a central controller of the adaptive immune response to stroke via production of pro-inflammatory chemokines and presentation of lipid antigens. As a common target in cardiovascular disease, SCI, TBI, MS, and other CNS diseases, targeting foam cells holds tremendous promise for helping millions of people. Due to their long-established role in atherosclerosis, there are also numerous in vitro models available to study them (comprehensively reviewed in (Chen et al., 2022)). There are multiple potential methods for preventing foam cell development in the brain after ischemic stroke, including the use of Nrf2 modulators to increase antioxidant pathways and targeting chemokines such as CCL2/MCP-1 that may draw in pro-inflammatory, rather than anti-inflammatory, myeloid populations. Scavenger receptor antagonists could also be used to prevent lipid overload, as well as LXR agonists that activate lipid efflux pathways. The delivery of molecules that can entrap and solubilize lipid debris also appears to be a viable strategy. Many of these approaches have the advantage of being less likely to have immunosuppressive effects than directly targeting specific cytokines or immune cell populations, which, despite playing negative roles in stroke recovery, may still be providing protection against infection and other diseases. These opportunities for further research and therapeutic development, as well as outstanding questions regarding foam cells, are summarized in Box 1. In conclusion, targeting foam cell formation after stroke is a highly promising avenue for developing treatments that help individuals recovering from stroke and other CNS diseases, and, by targeting foam cells, it may be possible to reduce inflammation and improve tissue repair and regeneration following stroke.

Box 1. Summary of future opportunities and outstanding questions.

Future Opportunities:

• Reduce lipid levels with a lipid chelator: This approach involves the use of compounds that can bind to excess lipids and remove them from the body, potentially reducing the amount of lipids available for foam cell formation.

• Target myeloid cell recruitment: Inhibiting the recruitment of myeloid cells to sites of inflammation may reduce the number of cells that can become foam cells.

• Target scavenger receptors: Scavenger receptors are involved in the uptake of lipids by myeloid cells. Targeting these receptors may reduce lipid uptake and subsequent foam cell formation.

• Target lipid peroxidation: Lipid peroxidation is a process that leads to the formation of oxidized lipids, which can contribute to foam cell formation. Targeting this process may reduce the formation of oxidized lipids and subsequent foam cell formation.

• Target cholesterol efflux pathways: Cholesterol efflux pathways are involved in the removal of excess cholesterol from cells. Enhancing these pathways may help to reduce the accumulation of cholesterol in macrophages and prevent foam cell formation.

• Target TREM2: TREM2 is a receptor that is involved in the regulation of inflammation and lipid metabolism in macrophages. Modulating this receptor may help to reduce the formation of foam cells.

• Target lymphocytes: Lymphocytes are involved in the immune response and can contribute to inflammation and foam cell formation. Targeting these cells may help to reduce the accumulation of foam cells q.

Outstanding Questions:

• Several drugs are being developed that target foam cells. These drugs may have potential therapeutic benefits for ischemic stroke. However, it is unclear whether targeting foam cells will be effective as a standalone therapy or if it will need to be combined with other treatments.

• The safety and potential side effects of drugs that target foam cells need to be carefully evaluated before use in clinical practice.

• Further studies are needed to determine the optimal timing, dosing, and duration of treatment with foam cell-targeting drugs.

Data availability

No data was used for the research described in the article.