Introduction
Within the large family of secreted small proteins known as cytokines, chemokines are a subclass that interact with G protein-bound heptahelical chemokine receptors on the cell surface. These cytokines have a molecular weight of 7–12 kDa, and also known as chemotactic cytokines, because they promote cell migration, particularly that of lymphocytes. Chemokines therefore have roles in the formation and maintenance of immune system homeostasis, as well as in all immunological responses, both benign and harmful [1, 2].
Inflammation, immunity, damage, and repair are directly associated with cardiovascular disease. Directed chemotaxis in responding cells is stimulated by the four chemokine subfamilies: CXC, CC, CX3C, and XC. The cytokines, known as CXC chemokines, influence several cardiovascular diseases, including atrial fibrillation (AF), myocardial infarction, cardiac ischemia-reperfusion injury, hypertension, aortic aneurysm, cardiac fibrosis, postcardiac rejection, and inflammation [3].
AF, the most prevalent clinical heart rhythm, affects approximately one in every three to five people older than 45 years. Worldwide, the number of patients with AF dramatically increased from 33.5 million to 59 million between 2010 and 2019 [4]. Atrial fibrillation is associated with elevated risk of heart failure, ischemic stroke, mortality, vascular dementia, and cognitive impairment [5]. Individuals with hereditary heart disorders and those with no genetic causes for a high degree of fibrosis have an elevated risk of arrhythmias when inflammatory chemokine mediators are present. Moreover, individuals with heart conditions with documented cardiac arrhythmias have much higher levels of inflammatory chemicals in their bodies than those without arrhythmias. Among individuals predisposed to arrhythmia development, inflammation is the likely cause of arrhythmias [6].
Arrhythmogenic ventricular cardiomyopathy, another cardiac muscle disorder, is categorized by fibrofatty replacement of the left and, less commonly, the right ventricles, as well as a high degree of fibrosis. Patients with this disorder, compared with controls, have been reported to have elevated levels of serum inflammatory mediators, such as C-X-C motif chemokine ligand 8 (CXCL8), interleukin 6 receptor (IL-6R), C-C motif ligand 2 (CCL2), and C-C motif ligand 4 (CCL4), as well as a different balance of circulating pro- and anti-inflammatory components. Some inflammatory molecules, either systemically distributed or originating from the heart, have been suggested to cause myocyte damage, arrhythmogenesis, and desmosome structural disruption by leading to plakoglobin’s dislodging from desmosomes and entering the intracellular space [7]. This article describes the pathophysiological aspects of chemokines in patients with AF, as summarized in Table 1.
Overview of the Pathophysiological Aspects and Therapeutic uses of Chemokines in Atrial Fibrillation.
Chemokines Involved in AF Pathophysiology
The involvement of chemokine mediators is becoming significant in AF. Wu and colleagues, in a meta-analysis of observational data aimed at clarifying the function of circulating inflammatory factors in AF, have found that elevated inflammatory chemical levels strongly correlate with AF risk in the general population [8]. People with AF have elevated levels of C-reactive protein (CRP), CCL2, and CXCL8. However, CXCL8 levels are higher in people with paroxysmal AF rather than persistent AF, thus suggesting a possible link between low-grade inflammatory response and persistent AF [9].
Atrial fibrillation is often associated with the activation of monocytes and macrophages, a hallmark of an enhanced inflammatory response. Whether the chemokine receptor C-X-C motif chemokine receptor 2 (CXCR2) has a role in the onset of hypertensive AF is unknown. However, this receptor is a crucial regulator of monocyte mobilization in hypertension and cardiac remodeling. Male C57BL/6 wild-type mice, mice without CXCR-2, mice with bone marrow-reconstituted chimera animals, and mice administered with the CXCR-2 inhibitor SB225002 were all given an Ang II (angiotensin II; 2000 ng/kg per minute) infusion for three weeks [10].
After 3 weeks of Ang II infusion, microarray analysis revealed substantial upregulation of four CXCR2 chemokine ligands in the atria. Considerable time-dependent increases were observed in the number of CXCR2+ immune cells and in CXCR2 expression in Ang II-infused atria. Blood pressure, AF inducibility, atrial width, fibrosis, macrophage infiltration, and superoxide production were higher in Ang II-infused wild-type mice than in mice administered saline. These effects were considerably diminished in the mice given SB225002, mice with CXCR2-deficient bone marrow cell transplantation, and CXCR2 knockout mice [10].
Moreover, circulating blood C-X-C motif ligand 1 (CXCL1) levels and CXCR2+ monocyte counts have been found to be higher in human patients than in sinus rhythm controls, and to be associated with AF. It revealed a new function for CXCR-2 in promoting monocyte infiltration of the atria, which quickens the process of atrial remodeling and AF following hypertension. Blocking CXCR2 activity may provide a novel AF treatment approach [10].
AF and inflammation are associated with oxidative damage. The chemokine-receptor CXCR2 substantially affects the recruitment of monocytes/macrophages, the development of hypertension, and the remodeling of the heart. Nevertheless, the functions of CXCR2 in the etiology of hypertensive AF remain unknown. With administration of the CXCR2 inhibitor SB225002, AF occurs in both spontaneously hypertensive rats (SHRs) and Wistar-Kyoto rats. Studies examining electrophysiology, pathological alterations, and atrial remodeling have shown greater expression of the chemokine CXCL1 and its receptor CXCR2 in the atrial tissue of SHRs than Wistar-Kyoto rats [11].
Compared with vehicle treatment, injection of SB225002 into SHRs significantly decreases blood pressure elevation, AF inducibility and duration, atrial remodeling, macrophage recruitment, superoxide generation, and conduction abnormalities. In addition, SB225002 treatment in SHRs decreases oxidative stress, inflammation, atrial remodeling, and the onset of pre-existing AF. These findings indicate that inhibiting CXCR2 halts and reverses the onset of AF in SHRs. Therefore, CXCR2 might be a useful target for the treatment of hypertensive AF [11].
Elevated expression of C-X-C motif chemokine ligand 12 (CXCL12) and C-X-C motif chemokine receptor 4 (CXCR4) in the plasma or atria in patients with AF is associated with accelerated atrial remodeling, prolonged hospital stays, and an elevated risk of death [12–14].
Liu et al. have used bioinformatics analysis to investigate the roles and underlying processes of hub genes implicated in AF. To identify differentially expressed genes (DEGs), the researchers applied robust rank aggregation to five microarray datasets available in the GEO database. The chemokine signaling pathway, leukocyte transendothelial migration, and extracellular matrix formation were associated with the 35 most significant DEGs. Three hub genes linked to AF were discovered among these DEGs. The CXCL12/CXCR4 axis was among the most promising targets for preventing AF, because of its significant elevation in patients with AF. The impact of this axis on AF pathogenesis and its underlying mechanisms were then examined in vivo using the particular CXCR4 antagonist AMD3100. It showed that anabatic atrial inflammation and fibrosis, together with increased transcription and translation of the CXCL12/CXCR4 axis, were present in AF patients and mice. This provided the basis for the maintenance of AF [15].
Administration of AMD3100 to AF model mice to prevent CXCR4 signaling has been found to decrease AF’s inducibility and duration. This decrease might be partly explained by diminished atrial inflammation and structural remodeling. Comparable mechanistic results have been observed after inhibition of hyperactivation of extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) and protein kinase B (AKT/mammalian target of rapamycin, or mTOR) in the atria of AF model mice, as well as by decreasing the recruitment of CD3+ T cells and F4/80+ macrophages. Inhibiting CXCR4 thus prevents AF onset. Consequently, future AF therapies may focus on the CXCL12/CXCR4 axis [15].
AF ablation is associated with an increase in circulating markers of inflammation. The risk of recurrent arrhythmia may increase with the upregulation of mononuclear cell responses by innate immunological or inflammatory pathways. Innate immune responses are triggered by both serine protease coagulation pathways and chemokines. Inflammatory markers have been examined, and the ability to suppress serine protease and chemokine pathways to prevent cell activation has been evaluated. Inflammatory indicators are markedly elevated in patients with AF ablation. One study has demonstrated that chemokine signaling inhibition, but not serine proteases, decreases the activation of patient-isolated monocytes in vitro; thus the post-ablation stimulation of circulating leukocytes can be decreased by targeting chemokines [16].
Investigations of the expression of CXCR4 have been conducted in individuals with persistent AF and mitral valve dysfunction. A total of 48 patients with persistent AF were divided into two groups according to whether their treatment included renin-angiotensin system (RAS) blockers(AF + RAS group; n = 25; AF - RAS group; n = 23). Seventeen patients with sinus rhythm and mitral valve disease (SR group) served as controls. The left atrium (LA) in the AF + RAS and AF - RAS groups had far greater amounts of CXCR4 mRNA and protein than observed in the SR group. CXCR4 expression was considerably lower in the AF + RAS group than the AF-RAS group. Moreover, the AF + RAS and AF - RAS groups showed higher CXCR4 expression in CD34+ cells than observed in the SR group. In the AF-RAS group, CXCR4 expression was substantially positively correlated with Ang II, collagen I, and LA diameter. Thus, CXCR4 expression is elevated in individuals with atrial remodeling-related mitral valve dysfunction and chronic AF, and RAS blockers mitigate these effects [14].
AF and valvular heart disease (VHD) frequently coexist. To investigate the probable molecular mechanism and create novel therapeutic targets for AF, including AF-VHD, Li et al. conducted a thorough analysis of immune-related genes (IRGs) and examined the function of immune cell infiltration in AF-VHD. The significance of differentially expressed IRGs in immunological and inflammatory responses has been demonstrated by their enrichment. Pro-platelet basic protein (PPBP), CXCL12, CCL4, and CXCL1 were the main differentially expressed IRGs that were evaluated in the protein-protein interaction network and associated with both inflammatory and immunological responses [17].
According to immune infiltration data, left atrium (LA) tissues from patients with AF-VHD, compared with patients with sinus rhythm, have a higher proportion of gamma delta T cells, but a lower percentage of CD8 and regulatory T cells. Correlation analysis has indicated that CXCL1 has a substantial negative correlation with resting mast cells and a positive correlation with active mast cells. The infiltration of various immune cells, including resting dendritic cells, plasma cells, and neutrophils, is positively correlated with CXCL12, CCL4, and PPBP (C-C motif ligand 4). The initiation and course of AF-VHD are influenced by notable variations in immune infiltration in LA tissues and IRG polymorphisms [17].
Although the exact pathophysiological involvement of monocytes in AF is unclear, these cells are crucial in structural remodeling of the LA. No statistically significant difference was observed in the proportions of monocyte subsets, according to CD14 and CD16 expression, between the extended LA group and the normal group. The monocytes from the enlarged LA group, in comparison to the normal LA group, had higher levels of total protein and C-C chemokine receptor type 2 (CCR2) transcripts. Monocytes in the enlarged LA group showed greater increases in migration activity than those in the normal LA group.
Furthermore, the enlarged LA group had a notably greater quantity of CCR2-positive monocytes/macrophages in the LAA. The increased amount of monocytes/macrophages in the atrial wall and the increased migratory activity of circulating monocytes may be associated with the pathophysiology of LA remodeling and AF [18].
CCL4 levels have been observed to be elevated in individuals who are obese or have had cardiac surgery in the past; this finding may be related to new-onset AF [19, 20]. Chemokines are involved in atherosclerosis and inflammation. The prognosis of AF and chemokines, however, are not well understood. A “real-world” cohort study has examined the predictive utility of plasma C-C motif chemokine ligand 23 (CCL23), C-C motif chemokine ligand 28 (CCL28), CXC motif chemokine ligand 14 (CXCL14), and CXC motif chemokine ligand 16 (CXCL16) in patients newly diagnosed with AF. To assess the effects of chemokines on improvements in the CHA2DS2-VASc score, the net reclassification improvement and integrated discrimination improvement were computed. In patients with AF, elevated levels of CCL18 (hazard ratio [HR] 2.65, 95% confidence interval [CI] 1.18–5.98, P = 0.019) and CCL23 (HR 2.78, 95% CI 1.07–7.22, P = 0.036) were associated with stroke. All-cause mortality was elevated in patients with low CXCL14 (HR 0.39, 95% CI 0.15–0.97, P = 0.042) and high CXCL16 (HR 3.02, 95% CI 1.39–6.58, P = 0.005) levels [21].
Cardiovascular death is associated with high CCL16 levels (HR 5.41, 95%CI 2.32–12.63, P < 0.001) but not CCL28 levels. Chemokines enhance CHA2DS2-VASc score reclassification and are associated with net clinical benefits. AF is independently associated with the plasma levels of CCL18, CCL23, CXCL14, and CXCL16. When added to the CHA2DS2-VASc score, chemokine measurement significantly enhances outcome risk assessment. Therefore, incorporating chemokines into clinical decisions may facilitate the prescription of AF treatment [21].
A search for sensitive biomarkers indicating the risk of bleeding and thrombosis has been undertaken through identifying inflammatory chemokines in older patients with non-valvular AF. In that study, interleukin-6, interleukin-4, and E-selectin baseline levels were significantly elevated in older patients with AF who experienced thrombosis and hemorrhage events during the 2-year follow-up, although these patients were already at high risk of thromboembolism and bleeding. Consequently, these chemokines may be useful markers of bleeding and thrombosis risk [22].
An association between lower risk of major cardiovascular events and lower plasma levels of C-X3-C motif chemokine ligand 1 (CX3CL1) has been independently demonstrated by the West Birmingham Atrial Fibrillation Project in a large cohort of patients with AF. However, the studies have been observational and have not clarified the molecular processes linking AF risk to inflammation. Measurement of chemokine plasma concentrations has been suggested to potentially enhance risk categorization for patients with AF [23].