Inflammatory Role of CCR1 in the Central Nervous System

Inflammation is a defense response of the body to stimuli. It can be caused by pathogens (such as bacteria and virus) or non-pathogens (such as physical injury, chemical irritation, allergic reactions, or foreign objects). Nevertheless, inflammation is beneficial to the body. It is the automatic defense response of the body because it promotes debris removal and helps tissue repair. However, sometimes inflammation is harmful, and the constant inflammatory signal may override the regulatory ability of the central nervous system (CNS), causing nerve damage [1]. Inflammation is an essential factor in CNS injury and neurodegenerative diseases, and abnormal inflammatory responses in the CNS lead to neuronal dysfunction. Several immune cells, including the central resident microglia and peripheral white blood cells, are involved in many CNS diseases, including stroke [2], multiple sclerosis (MS) [3], and Alzheimer’s disease (AD) [4]. Leukocyte recruitment requires the activation of chemokines and chemokine receptors. Selective targeting of chemokine receptors may allow control of specific immune cell types that enter the brain after CNS injury. CC chemokine receptor 1 (CCR1) is a chemokine receptor that regulates monocyte movement from bone marrow to inflammation sites and has been widely studied in CNS inflammation. In this review, we describe the evidence supporting the involvement of CCR1 in CNS inflammation and the potential advantages of CCR1 inhibition in certain diseases.

The chemokine monomer consists of a central triple-stranded β-fold with a C-terminal α-helix overlying it and an unstructured N-terminal essential in receptor activation [5]. Chemokines are highly homologous; they share approximately 20%–50% of the same gene and amino acid sequence. According to the arrangement of their amino (N-terminal) cysteines, chemokines can be divided into four subgroups: CXC, CC, XC, and CX3C [6]. Among the four subgroups, CC and CXC chemokines are the two major subgroups, with the first two cysteines adjacent to CC motifs and separated by an amino acid residue to form CXC motifs. C-type chemokines lack the first and third cysteines, whereas CX3C chemokines have three amino acids between the first two cysteine residues [7]. Based on their functions, chemokines are classified into homeostatic (responsible for basal leukocyte migration) and proinflammatory chemokines (involved in inflammation). In humans, there are 47 different chemokines: 17 CXC, 27 CC, 2 XC, and 1 CX3C [8, 9]. Chemokines have protective or degenerative effects on CNS cells, including neurons, astrocytes, microglia, and oligodendrocytes [10]. The CC chemokine receptors (CCRs) are an essential member of the G protein-coupled receptor family with seven transmembrane α-helical domains [11]. The transmembrane structure of the receptor molecule divides it into a C-terminus, three inner membrane rings, three outer membrane rings, and an outer membrane N-terminus [12]. The G protein is coupled to one of the inner membrane rings to mediate the intracellular signaling cascade after binding the receptor to the ligand and regulating biological functions. Chemokine receptors comprise 340∼370 amino acids with 25%∼80% amino acid homology [13]. According to the types of chemokine ligands corresponding to binding, chemokine receptors can be divided into four subgroups: CXCR, CCR, XCR, and CX3CR [11].

CC chemokine receptor type 1 (CCR1) was the first CCR discovered in 1993, and human CCR1 (hCCR1), also known as CD191, is located on chromosome 3p21 [14]. CCR1 is activated by several chemokines, including CCL3 (MIP-1α), CCL4 (MIP-1b), CCL5 (RANTES), CCL6 (MIP-RP1), CCL7 (MCP-3), CCL8 (MCP-2), CCL9 (MIP-RP-2), CCL13 (McP-4), CCL14 (HCC-1), CCL15 (LKN-1), CCL16 (HCC-4), and CCL23 (MPIF-1) [8, 15]. CCR1 activation leads to the directed migration of recipient cells. CCL3 (MIP-1α) and CCL5 (RANTES) are two major ligands of CCR1, which mediate inflammatory responses and play an important role in the occurrence and development of autoimmune diseases. Human and mouse CCR1 has a high affinity for CCL3. Furthermore, CCR1 and CCL3 share approximately 80% amino acid identity [8]. The interaction between chemokines and their receptors is cross-related; a chemokine, especially an inflammatory chemokine, can activate multiple chemokine receptors, and conversely, a chemokine receptor can recognize multiple chemokines [5]. After being activated by chemokine and other ligands, chemokine receptors can bind to intracellular β-arrestin and other signaling molecules to transduce downstream signals [8].

CCR1 is primarily expressed on numerous cell membranes of immune cells, including monocytes, immature dendritic cells, T lymphocytes, basophils, eosinophils, neutrophils, NK cells, mast cells, endothelial cells, human umbilical cord blood (HUCB) cells, and other cells, and mainly mediates chemokine signal transduction [16, 17]. The expression of CCR1 on non-hematopoietic cells in the CNS has been partially reported. Previous studies reported that CCR1 is expressed in several cell types in the brain, including microvascular endothelial cells, vascular smooth muscle cells, microglia, astrocytes, and neurons [18, 19]. CCR1 can be found in the cortex, thalamus, striatum, hippocampus, brain endothelium, and ependyma of rats after neonatal hypoxia and ischemia [20]. Furthermore, hypoxia reduces the activity of the histone demethylation-related enzyme JHDMs [21], which leads to histone methylation in the CCR1 gene promoter [22]. This results in decreased CCR1 expression in macrophages and monocytes [22]. However, a study on breast cancer cells reported that chronic hypoxia did not affect CCR1 expression in cancer cells [23]. Therefore, there may be differences in the expression and regulatory mechanisms of CCR1 in different diseases.

CCR1 is essential in leukocyte trafficking, particularly the recruitment of monocytes to inflammation sites and subsequent conversion to macrophages or dendritic cells, and this chemokine receptor has been studied in conditions involving inflammation in the CNS, including MS [24, 25], AD [26, 27], stroke [18, 28, 29], and other CNS diseases.

CCR1 and its ligand CCL5 are known to be involved in the inflammatory response after intracranial hemorrhage (ICH). A previous study reported that the expressions of endogenous CCR1 and CCL5 in the brains of mice after ICH were upregulated and peaked at 24 h. Met-RANTES (Met-R) treatment reduced brain edema and neurobehavioral damage and preserved blood-brain barrier integrity and tight junction protein expression in mice with ICH [28]. Additionally, CCR1 activation may promote a neuroinflammatory response in mice after intracerebral hemorrhage through the CCR1/TPR1/ERK1/2 signaling pathway. Exogenous recombinant CCL5 protein can aggravate neuroinflammation, while the selective CCR1 antagonist Met-R can reduce brain edema and improve neurobehavioral function by reducing neuroinflammation. Consequently, targeting CCR1 activation may provide a promising therapeutic approach for treating patients with ICH [18]. We have recently shown that low shear stress induces macrophage infiltration through the CCL7/CCR1/TAK1/NF-κB axis, aggravating aneurysm wall inflammation. BX471, as a CCR1 inhibitor, can significantly inhibit CCR1 activation, thus reducing the production of inflammatory factors, alleviating the inflammation of the blood vessel wall, and reducing the rupture of intracranial aneurysms [30]. Furthermore, bioinformatics analysis revealed that CCR1 expression was increased after subarachnoid hemorrhage (SAH), and the specific mechanism may be the induction of early brain injury after SAH through the CCR1/JAK2/STAT3 axis. Inhibition of CCR1 reduces neuroinflammation after SAH through the JAK2/STAT3 signaling pathway, which may provide a new target for SAH treatment [31]. CCR1 and its ligand CCL3 are involved in the inflammatory response of ischemic stroke (IS). In middle cerebral artery occlusion (MCAO), an IS model, CCL3 expression was significantly increased in the ischemic hemisphere than in the contralateral side. CCR1 expression can be detected on the surface of HUCB. These results indicated that increased CCL3 could increase the binding of CCR1 on the surface of HUCB and induce the infiltration of HUCB cells into the CNS in vivo. Meanwhile, this cell migration was neutralized by a polyclonal antibody against CCL3 [17]. Another study reported that unstable atherosclerotic plaque (UAP) was associated with IS. CCR1 is a hub gene and can be used as a potential diagnostic and prognostic symptom biomarker for UAP-related IS by analyzing five microarray datasets from the GEO database. CCR1 is a hub gene and can be used as a potential diagnostic and prognostic symptom biomarker for UAP-related IS by analyzing five microarray datasets from the GEO database [32]. Researchers examined the expression of three known CCL3 receptors, CCR1, CCR3, and CCR5, after MCAO. Infarct volumes were reported to be smaller in CCL3-deficient mice than in controls, consistent with the notion that CCL3 is proinflammatory [33]. However, a previous study reported that CCR5-deficient mice have larger infarct sizes than littermate controls [34], suggesting a protective role of CCR5 in IS. Therefore, CCL3 can play a neuroprotective role via CCR5 expression while having a harmful role on CCR1 after MCAO [29]. However, CCR1 activation is deleterious and protective. A recent study has shown that the infiltration of cDC1 (a subtype of dendritic cell) into ischemic brain tissue is at least partially dependent on CCR1, and cDC1s are protective against cerebral ischemia [35]. Additionally, CCL3 is produced by neurons following ischemic attacks and can protect neurons directly or indirectly by producing neurotrophic factors, including BDNF, EGF, and VEGF in the peri-infarct area [29].

AD is the most common chronic progressive neurodegenerative disease. The main pathological changes include amyloid β-protein (Aβ), tau misfolding, neuroinflammation, and neuronal loss [36]. In amyloid precursor protein plus presenilin-1 (APP/PS1) mice, an animal model of AD, APP/PS1 mice exhibited significant inflammatory changes and increased CCR1, CCR3, CCR4, and CCR9 than wild-type mice. Their ligands CCL7, CCL11, CCL17, CCL22, CCL25, and CXCL4 were significantly increased, indicating that chemokines and their receptors were involved in inflammatory changes after AD [26]. The role of physical activity in AD prevention has received much attention in recent years [37, 38]. Chen et al. [27] analyzed the single-cell sequencing results of patients with AD and cognitively normal controls in the GEO database to investigate the effect of exercise on the transcriptional properties of monocytes in patients with AD. The results showed that CCR1, TNF, and APP expressions were upregulated after exercise. Meanwhile, APP, CCR1, TNF, ATF3, KLF4, HES4, and MAFB formed a regulatory network in the ERADMT genome. The ERADMT genome is a potential risk marker for the development of AD and can be used as an indicator of patient compliance with exercise therapy for AD [27]. However, this study is a bioinformatics study; further experimental validation is needed. Clinical studies have shown that blood CCL23 in patients with AD is higher than that in healthy controls, suggesting that CCL23 may be a candidate blood biomarker for progression to AD [39]. CCR1, as one of the receptors of CCL23, is expressed in Aβ 42-positive neuritis plaques and dystrophic neurons, which is positively correlated with the severity of clinical disease and can be used as a unique neuroinflammatory marker of AD [40].

MS is an immune-mediated inflammatory disease of the CNS. In experimental autoimmune encephalomyelitis (EAE), an animal model of MS, CCR1 mRNA was significantly upregulated in the spinal cord during active inflammation and demyelination episodes. Analysis of cerebrospinal fluid (CSF) from patients with MS revealed that CCR1 levels are increased in the early and acute stages of demyelination [41]. Additionally, the CCR1 ligand CCL3 has been reported to be elevated in the CSF of patients with MS [42]. During clinical remission, CCR1 expression was significantly reduced, consistent with diminished spinal cord inflammation and demyelination. However, no CCR1 mRNA expression was detected in the spinal cord of healthy control rats [43]. Treatment with the CCR1 antagonist J-113863 decreased the number of CD4+ GM-CSF+ and CD4+ IL-6+ cells but increased the number of CD4+ IL-27+ and CD4+ IL-10+ cells in the spleen. Additionally, J-113863 inhibited GM-CSF, IL-6, mRNA, and protein expression in rat brain tissue. Similarly, the mRNA and protein expression of IL-10 and IL-27 in brain tissue were increased. These results indicated that J-113863 could inhibit proinflammatory mediator expression and promote anti-inflammatory mediator expression [25]. Another study reported that J-113863 treatment inhibited Th9/Th22 cell expression in the spleen to reduce demyelination and significantly improved the severity of clinical scores in EAE mice [44]. These studies demonstrated the potential role of CCR1 antagonists as new drug candidates for MS treatment.

Parkinson’s disease (PD) is a common motor and neurodegenerative disease associated with inflammatory changes [45]. An RNA sequencing study of peripheral blood mononuclear cells from patients with PD and age-matched healthy controls reported that PD-associated transcriptome signatures were significantly enriched with inflammation. Additionally, the mRNA levels of chemokine signaling proteins CX3CR1, CCR5, and CCR1 were upregulated and confirmed at the protein level [46]. A previous study reported that epilepsy can cause inflammation, which can subsequently affect the epileptic seizure [47]. In status epilepticus (SE), an animal model of epilepsy, CCR1 is significantly upregulated in SE rats after 2, 4, and 8 weeks. Compared with controls, CCR1 was upregulated 5-fold in SE mice after 2 weeks, decreased to approximately 2.5-fold after 8 weeks, and showed no significant difference at 19 weeks. The trend of CCR1 was consistent with that of Iba-1, a specific marker of microglia [48]. This may be linked to decreased levels of inflammation. In another epilepsy study, RT-PCR analysis of hippocampal gene expression revealed that Ccr1 remained upregulated within 72 h, suggesting that CCR1 may be involved in the epilepsy process [49]. Table 1 shows a summary of CCR1 in different diseases of the CNS.

In addition to mediating immune cell migration, activation, and inflammation, the CCR1 signaling pathway may be involved in tissue injury and inflammation by regulating Th1/Th2 cytokine polarization, stimulating microglia function, and secreting protease [50]. The role of CCR1 and its high-affinity ligand (CCL3 and CCL5) in these processes makes CCR1 an attractive therapeutic target to regulate macrophage infiltration and reduce most of the tissue damage associated with inflammation. This concept is supported by animal disease models and data from some clinical trials. These data suggest that inhibition of CCR1 or its ligands partially ameliorates the disease. Basic studies have shown that CCR1 antagonists are partially effective in treating CNS diseases, including stroke and MS.

Met-R, an N-terminal-modified human RANTES, is a selective CCR1 antagonist that competitively inhibits CCR1 activity [51]. In animal studies of ICH, Met-R can reduce the damage to the blood-brain barrier through the CCR1/SRC/Rac1 signaling pathway [28], inhibit neuroinflammation through the CCR1/TPR1/ERK1/2 signaling pathway, improve neurological function and brain edema, and alleviate brain injury [18]. The deleterious effects of CCR1 in the CNS are most evident in MS. Several CCR1 antagonists have been used in MS clinical trials. The BX471 compound is a potent and selective CCR1 antagonist that is effective in many animal disease models, including MS [52], asthma [53], pancreatitis [54], pneumonia, and pulmonary fibrosis [55]. C-6448, a xanthene carboxamide derivative developed by Merck as a CCR1 antagonist, was used in a phase II clinical trial for MS in 2003. However, the program was discontinued [56]. PS-031291 is a pharmacopeia CCR1 receptor antagonist. Nevertheless, the study on PS-031291 was halted during preclinical development [57, 58]. BX-471 was initially used in a phase I study of MS. However, the phase II study was stopped because it failed to prove that inhibiting CCR1 reduced inflammation [59]. Despite the poor efficacy of previous CCR1 antagonists in MS, CCR1 continues to be investigated as a therapeutic target. Two recent animal studies of J-113863, a CCR1 antagonist, demonstrated promising CCR1 inhibition. Similarly, J-113863 can regulate the cytokine balance between anti- and proinflammatory in EAE mice and ameliorate EAE [25, 44]. Whether J-113863 can be applied to human research still needs many experiments. In clinical trials in MS, we found that many CCR1 antagonists failed or exhibited little effect. This may be due to a less rigorous experimental design, including different doses and frequencies of CCR1 antagonists for different clinical stages of MS. New and better CCR1 antagonists should be developed based on the old CCR1 in future studies. Table 2 illustrates CCR1 antagonists in the CNS. Figure 1 depicts the mechanism diagram of CCR1 and CCR1 inhibitors in CNS diseases.

Several inhibitors of CCR1 have demonstrated promising outcomes in animal models of CNS diseases. However, their clinical efficacy has notable limitations. First, many clinical trials have primarily focused on non-CNS ailments, necessitating further investigation into the potential of CCR1 inhibitors specifically for CNS disorders. A limited number of CCR1 inhibitors have gained approval for clinical application, with outcomes from prior randomized controlled trials involving patients with MS proving disappointing [59, 60]. The challenges encountered in translating findings from basic research to clinical settings are anticipated, given the intricate and incompletely understood mechanisms of action between chemokines and their receptors. For example, CCR1 inhibitors, including Met-R, can act on CCR1 and CCR5. Met-R can inhibit CCR1 and CCR5 simultaneously. It is worth exploring whether this will cause other pathophysiological changes, which means we need to synthesize more specific inhibitors of CCR1. Additionally, investigations into the downstream signaling pathways of CCR1 are predominantly focused on stroke, while exploration of other CNS diseases (including AD, PD, and MS) remains limited. Chemokines and chemokine receptors are cross-linked to each other, forming a complex network. Chemokine production and chemokine receptor activation play essential roles in the inflammatory process. Chemokine activation can mediate the activation of a series of signaling pathways, including PI3K/AKT [61, 62], JAK/STAT [63], and MAPK [64, 65]. Inhibition of chemokine receptor activation can partially reduce inflammation and ameliorate brain injury. However, a chemokine may bind to one or more chemokine receptors, and a chemokine receptor may be activated by more than one chemokine. This phenomenon is known as cross-related signaling. For example, CCR2 can be activated by several chemokine ligands, including CCL2, CCL7, CCL8, CCL12 (mouse only), CCL13, and CCL16 (human only) [11]. Alternatively, CCL2 can interact with several chemokine receptors, including CCR1 (with low-affinity binding), CCR2, CCR3 (as an antagonist), CCR4, and CCR5 [66]. Therefore, the poor efficacy of CCR1 antagonists alone in clinical trials of MS is understandable. Furthermore, chemokines and chemokine receptors are cross-linked. When CCR1 is inhibited, its ligands are likely to bind to other chemokine receptors, which may produce proinflammatory effects and weaken the impact of CCR1 inhibition. Hence, exploring antagonists capable of targeting multiple chemokine receptors can represent a prospective avenue for future research, as exemplified by compounds such as cenicriviroc (a double antagonist of CCR2 and CCR5) [67]. The brain is a complex organ whose homeostasis is maintained by a complex network of multiple physiological and pathological mechanisms. As mentioned above, CNS diseases, including stroke, AD, and MS, all involve a series of molecular changes, especially inflammatory alterations. In addition to chemokine and chemokine receptors, other molecules, such as NLRP3, IL-1β, TNF-α, and IL-6, are important inflammatory factors and play essential roles in CNS inflammatory diseases. As a member of the chemokine receptor family, CCR1 may be one of many proinflammatory factors in CNS inflammatory diseases. Besides, the activation of many chemokine receptors mediates inflammation through several important signaling pathways. Therefore, inhibition of critical proinflammatory pathways is a new idea. Additionally, the tolerance to chemokine receptor antagonists highlighted by Grudzien et al. [68] merits attention.

Recent studies have established that CCR1 promotes CNS inflammatory responses, causing neuronal degeneration and death. This inflammatory mechanism related to the innate immune system causes brain tissue damage and plays a beneficial role in brain tissue recovery. CCR1 is activated by several chemokines (CCL3 and CCL5), leading to the downstream inflammatory cascade of CCR1. Numerous CCR1 antagonists that selectively inhibit CCR1 have been developed to reduce CNS inflammation. As mentioned above, the effectiveness of these drugs has been demonstrated in multiple animal experiments. Despite the small results in clinical trials, this provides a direction for future research and development of CCR1 antagonists. Of course, the expression of CCR1 in immune cells is essential for immune surveillance and host defense, and excessive inhibition of CCR1 may cause further damage to host defense after CNS injury. Therefore, understanding the activation of CCR1 and its downstream inflammatory response may help develop effective treatment strategies. At the same time, we should also note that moderate inhibition of CCR1 in combination with other pathway treatment of CNS inflammation may be the focus of future research.

We would like to thank all participants in the study. In addition, we thank Home for Researchers editorial team (www.home-for-researchers.com) for language editing service.

No competing interests in this manuscript.

This work was supported by the National Natural Science Foundation of China (Grant No. 82172173 to Mingchang Li) and the Open Project of Hubei Key Laboratory to Qi Tian (2023KFZZ017).

Q.T., Z.Y., Y.G., Z.C., and M.L. designed the study. Z.C. and M.L. have made strict revisions to the important knowledge content of the final draft. M.L. agrees to be responsible for all work aspects to ensure that issues relating to the accuracy or completeness of any part of the work are properly investigated and resolved. All authors have read and approved the final draft.

Qi Tian, Ziang Yan, and Yujia Guo contributed equally to this work.