The role of interleukins in pathogenesis and prognosis of atrial fibrillation

ABSTRACT Introduction Atrial fibrillation (AF) is the most prevalent cardiac arrhythmia. Moreover, it is mentioned as one of the important causes of death due to heart disease, which imposes an undeniable financial burden on public health systems. Almost 1 out of 4 individuals aged 40 to 55 will experience AF at some point during their life. Increasing the pressure or the volume of the atria over time leads to the activation of fibroblasts, resulting in the accumulation of extracellular matrix and fibrosis. By disturbing electrical conduction, fibrosis creates microreentrant circuits, which can develop into AF. Areas covered In this article, we evaluated the vital role of interleukins and inflammatory mediators in the pathogenesis, prognosis, and treatment of AF. We also discussed the importance of the broader use of interleukins in the clinical management of AF patients. Expert opinion Interleukins and inflammatory markers can be used as markers of diagnosis, cardiovascular events, and mortality in AF patients. Finally, the utilization of substances upregulating IL-10, such as resolvin D1 (RvD1), or applying IL-6 down-regulators and inhibitors, including anti-IL-6 antibodies, colchicine, and C1q/tumor necrosis factor-related protein-9 (CTRP9), are effective in the reduction of atrial interstitial fibrosis and treating AF patients. Summary Interleukins are mediators of the immune system, which play vital roles, directly or indirectly, in many immune responses and have a wide range of effects on the systems in the body. Similarly, these interleukins can have versatile roles in igniting or attenuating cardiovascular disorders. However, due to the complexity and variability of their effects on the cardiovascular system, research is still ongoing to capture a complete and realistic image of the role of interleukins in the development of cardiovascular disorders. Atrial fibrillation occurs following the irregular rhythm of the beat in the atria. Subsequently, the normal blood flow from the atria to the ventricles will be disturbed. Atrial fibrillation is the most common heart arrhythmia that might need medical care. However, the probability of severe atrial fibrillation outcomes or symptoms is higher in people with underlying comorbidities. This review investigates the role of interleukins in the occurrence of atrial fibrillation and its long-term complications. Atrial fibrillation usually begins as occasional brief episodes that start and stop spontaneously, but it can progress to more persistent forms that are associated with an increased risk of stroke and heart failure.. Interleukins and inflammatory mediators can enhance atrial fibrillation by worsening the conductance of the electrical signal and by boosting the fibrogenesis and structural remodeling of the atria. Currently, based on the role of interleukins in atrial fibrillation, therapeutic methods are being developed to prevent and attenuate its serious consequences. These therapeutic methods can impressively affect and improve the lives of millions of people suffering from this disorder.


Introduction
Atrial fibrillation (AF) occurs when the atrium is excited at a high frequency which leads to the loss of synchronicity of atrial contraction and disturbance of ventricular rhythm [1,2]. It is widely believed that this rapid stimulation causes the creation and dissemination of reentrant waves in the vulnerable atrial layer [1,2]. AF is globally the most prevalent cardiac arrhythmia that has played a crucial role in the incidence of cardiovascular and cerebrovascular accidents and imposes an undeniable financial and human burden on public health systems [2,3]. According to the predictions, at least 6 million Americans and 17.9 million Europeans will experience AF by 2060 [3]. Many conditions can induce AF, including age, arterial hypertension, cardiomyopathies, obstructive sleep apnea, or valvular dysfunction [2]. AF is a substantial underlying cause of heart failure, dementia, and stroke, which can double the rate of mortality due to stroke in both sexes [4].
Considering the high prevalence of AF among cardiac arrhythmias and the increased susceptibility to other cardiovascular events, it is necessary to have diagnostic and therapeutic methods to improve the prognosis of the disease. Despite the acceptable precision of photoplethysmography, the gold standard for AF confirmation is electrocardiography (ECG) [5,6]. ECG changes in AF include the replacement of the normal P waves with oscillatory or fibrillatory waves and irregular R-R intervals [7,8]. Nowadays, the use of artificial intelligence along with ECG or photoplethysmography has created a favorable diagnostic accuracy [9]. Furthermore, the latest findings have proven the remarkable ability of echocardiography to anticipate the occurrence of atrial fibrillation as well [10].
Pharmacotherapy, left atrial appendage occlusion and catheter ablation are the available treatments of choice for AF [8]. The main goal of AF treatment is to prevent stroke and thromboembolic events in high-risk patients. In this case, vitamin K antagonists and direct oral anticoagulants are the first lines of preventive treatment, and left atrial appendage closure would be the alternative therapy [11,12]. Antiarrhythmic drugs and AF ablation as rhythm control treatments are progressively being noted to improve the prognosis of cardiovascular events caused by AF [13].
During the last few years, it has been realized that immune infiltration of atrial tissue, along with inflammatory markers, including cytokines and C-reactive protein (CRP), has played an important role in indicating the occurrence and prognosis of cardiac arrhythmias [14][15][16][17]. On the other hand, the importance of interleukin-6 and PD-1/PD-L1 signaling pathways in the pathogenesis of AF and also in predicting mortality because of AF has been discovered [18][19][20]. Unlike immunothrombosis markers, humoral immunity and some autoantibodies are also claimed to be associated with AF [21,22].
Therefore, according to the countless effects of interleukins and convincing evidence of the role of immunological factors in the development and progression of AF, we intend to investigate the impact of different interleukins in the pathogenesis and prognosis of AF. Subsequently, we aim to assess the therapeutic potential of targeting interleukins for preventing and treating AF.

Epidemiology and importance
AF has the highest prevalence among arrhythmias affecting 1-2% of the world's population and is mentioned as one of the important causes of death due to heart diseases and strokes. This arrhythmia has a morbidity of 0.1% per year in patients over 40 years of age, and its prevalence relates to advancing age [23]. Almost 1 out of 4 people 40 to 55 years of age will experience AF at some point during their entire life [24]. Based on previous research, the worldwide prevalence of AF in adults is computed between 2% and 4% presently, representing a 3-fold increase over the last 50 years [25,26].
In addition to its importance due to the increased all-cause mortality and its close association with morbidity, including thromboembolic events and extensive bleeding, AF causes frequent hospitalizations and thus inflicts heavy blows on the economic structure of the health systems [27]. Even without known risk factors in the occurrence of cardiovascular and cerebrovascular events, AF can independently lead to mortality and morbidity caused by stroke and heart diseases [28,29]. The mortality rate in patients with AF is twice as high compared with the normal population [30]. AF patients are also at a high risk for brain infarcts and stroke [31]. Some exclusive scoring systems are utilized to evaluate the stroke risk (CHA 2 DS 2 -VASc score; Congestive heart failure, Hypertension, Age≥75 years (2 points), Diabetes mellitus, prior Stroke or transient ischemic attack (TIA) or thromboembolism (2 points), Vascular disease, Age 65 to 74 years, Sex category), and bleeding risk (HAS-BLED score; Hypertension, Abnormal renal/liver function, Stroke, Bleeding history, Labile INR, Elderly (>65), Drugs/alcohol) and to determine whether to continue or discontinue oral anticoagulants in patients with AF [32]. However, the implementation of further biomarkers, such as inflammatory mediators which are related to the underlying pathways of AF development and progression might enhance the risk stratification in these patients.

Article highlights
• Inflammatory mediators influencing AF development and prognosis include CRP, IL-6, NLRP3 inflammasome, and IL-17 which can determine the risk of thromboembolism, stroke, the outcome of cardioversion, and mortality in AF patients. • Targeting inflammatory pathways, for instance by the administration of anti-IL-6 antibodies, IL-10, and colchicine can lead to a reduction in the risk of atrial interstitial fibrosis based on preclinical and clinical studies. • Targeting inflammatory pathways, for instance by the administration of anti-IL-6 antibodies, IL-10, and colchicine can lead to a reduction in the risk of atrial interstitial fibrosis based on preclinical and clinical studies.

Diagnosis
Paroxysmal AF that ends automatically, persistent AF that is not self-limiting and lasts more than seven days, and permanent AF with prolonged episodes are the three main groups of AF in clinical medicine [33]. The rate of death and complications caused by AF are higher in the permanent and persistent types than in the paroxysmal type [27]. According to previous studies, continuous ECG, electronic blood pressure monitoring, and palpation of the radial pulse are appropriate and accurate diagnostic methods for AF [34,35]. In addition, the anatomy and physiology of the atria and ventricles should be investigated. Therefore, echocardiography, physical examination, chest radiography, and blood tests should also be applied [30]. AF is generally difficult to detect because patients are often asymptomatic or have nonspecific symptoms [35]. Symptoms of AF are created by its influence on the heart or thromboembolic events and include dyspnea, fatigue, syncope, palpitations, and chest pain [36].

Pathophysiology
Despite many advances in our knowledge of AF, its pathophysiology is still not fully captured [37]. AF appears to result from aberrant reentrant activity originating mainly from pulmonary veins occurring in a structurally or electrically susceptible atrial substrate [24,33,37]. Increasing the pressure or the volume of the atria over time by creating cell tension directly causes the activation of fibroblasts, resulting in the accumulation of extracellular matrix and fibrosis [38]. By slowing or blocking electrical conduction, fibrosis creates microreentrant circuits, which can develop into AF [37]. Genome-wide association studies have revealed the polygenic nature of AF [39][40][41]. More than 100 genes with a possible relationship with AF have been identified, some of which include IGFBP2, C1orf105, IGFBP3, CXCR4, HTR2B, and IL6R [42,43]. Based on recent studies, the role of inflammatory markers such as interleukin 6 (IL-6), interleukin 1 (IL-1), tumor necrosis factor-α (TNF-α), C-reactive protein (CRP), interleukin 8 (IL-8), and interleukin 10 (IL-10) in the structural and electrical changes of the atria resulting in the occurrence and progression of AF have been considered [16,17,37,44].

Management
AF treatment is aimed to prevent stroke and thromboembolic events in high-risk patients. In this case, vitamin K antagonists and direct oral anticoagulants are the first lines of preventive treatments [11,12]. Antiarrhythmic drugs and AF ablation as rhythm control treatments are progressively being noted to improve the prognosis of cardiovascular events caused by AF [13]. In general, the treatment and management of AF have four components [8]: 1-Rhythm control is particularly indicated for individuals under 65 years with severe symptoms or first-diagnosed AF. Commonly, class Ic (flecainide or propafenone) and IIIc (sotalol, amiodarone, ibutilide, or dofetilide) anti-arrhythmic drugs are utilized for rhythm control.
2-The desired resting heart rate in AF individuals is less than 110 beats per minute (bpm) in asymptomatic patients and lower than 80 to 100 bpm in symptomatic individuals based on the latest European Society of Cardiology (ESC) guidelines for AF management. β-adrenergic receptor blockers (β-blockers), non-dihydropyridine calcium channel blockers (ND-CCBs), digitalis, and amiodarone are the first choices to achieve the target.
3-Anticoagulant therapy is strongly nominated to decrease the probability of stroke in AF patients. The CHA2DS2-VASc risk scores of two or more in men, and three or more in women, are highly indicated for the administration of oral anticoagulants (OACs). Anticoagulant treatment is not advised for men with a CHA2DS2-VASc score of zero and women with a score of one.
4-The last pillar in the management of AF is to ameliorate underlying diseases and risk factors. Losing weight, quitting smoking and alcohol, controlling hypertension, diabetes, and hyperthyroidism, along with the treatment of valvular diseases and heart failure, are included in this category.

Diagnostic and prognostic value of interleukins in cardiovascular diseases
The role of different interleukins in the incidence and mortality of cardiovascular diseases has been investigated and proven in various studies. Inflammatory cytokines, namely IL-17, IL-23, IL-1, IL-6, and C-reactive protein (CRP) have been demonstrated to have diagnostic value or reported as biomarkers of worse outcomes in cardiovascular diseases such as myocardial infarction (MI) [45][46][47][48][49][50]. Some inflammatory mediators such as IL-6 have also been associated with arrhythmia with the mechanism of conduction disorders, increased QT interval, and inhibition of ion channels [51,52]. Therefore, due to the extensive roles of different types of interleukins and inflammatory markers in cardiovascular diseases, we will investigate the relationship between interleukins and the pathogenesis, progression, and prognosis of AF as a common arrhythmia.

Interleukin-dependent pathogenesis of AF and their possible diagnostic value
As discussed in the previous sections, IL-6 is one of the most important markers associated with various cardiovascular diseases, including AF. According to a recent study, the level of IL-6 in pericardial drainage (pdIL-6) is a significant predictor for classifying patients undergoing coronary artery bypass graft (CABG) in terms of risk for the occurrence of postoperative AF (POAF) [53]. IL-6 is an important pro-inflammatory cytokine that play a role in developing POAF. In other words, preoperative inflammation can increase the risk of POAF [54]. The increase in the level of IL-8 in CABG patients who suffered AF after surgery indicated the role of this interleukin in the pathogenesis of POAF [55]. Therefore, the measurement of IL-6, IL-8, IL-10, and CRP in hospitalized patients undergoing cardiac surgery can be a good predictor for the diagnosis and management of AF [54]. Figure 1 summarizes the mechanisms involved in the pathogenesis of AF which are initiated by inflammatory cytokines.

IL-6
Many studies have been conducted regarding the mechanism of IL-6 in the development of AF. Some of them have emphasized the role of IL-6, which can be expressed in various cells (cardiac fibroblasts, osteoblasts, lung epithelial cells, and adipocytes), in activating fibroblasts and, ultimately, fibrosis of the atria [56,57]. Atrial fibrosis is an important structural change causing arrhythmias. In addition, some studies report the effect of IL-6 on neutrophil infiltration can lead to the fibrosis of atria [56,58]. Atrial fibrosis can also result from IL-6 activating the Signal Transducer and Activator of the Transcription 3 (STAT3) pathway and microRNA-21 [59]. Some studies have pointed to a causal relationship between IL-6 and POAF. Due to the pro-fibrotic effects of IL-6 on cardiac cells in various mechanisms shortly after cardiac surgery, measuring the level of this interleukin can be a noninvasive useful method for predicting POAF even after the patient undergoes surgery [57].
IL-6 can provoke AF progression with other underlying mechanisms, as well. It can affect the action potential, suppress the expression of connexin, and reduce various types of calcium currents in the cell [56,60,61]. For instance, the abnormal IL-6 levels and its subsequent aberrant calcium currents, especially through ryanodine receptor 2 (RyR2) disturbance, contributed to the occurrence of POAF in sterile pericarditis rats. The result was demonstrated to be independent of the role of IL-6 in stimulating atrial fibrosis [62].

IL-17A
IL-17A, mainly produced by T helper 17 cells (Th17), binds to IL-17 receptor A (IL-RA) and IL-RC on cardiac fibroblasts to initiate cardiac fibrosis [63]. The increase of IL-17A in paroxysmal AF has been observed in previous studies [64]. This interleukin is involved in AF pathogenesis with fibrotic changes of cardiac tissue and left atrial remodeling [65,66]. In confirming the proposition, there was also a positive correlation between the serum level of IL-17A and fibrotic factors, including matrix metalloproteinase 9, procollagen type 1, and procollagen type 3 reported [65,66]. The levels of IL-17A and IL-6 are higher in left atrial spontaneous echo contrast (LASEC) and left atrial thrombus (LAT) patients. Also, the diagnostic accuracy of IL-17A is suitable for detecting the occurrence of LAT and LASEC [66]. Therefore, IL-17A, with its inflammatory effects, indicates cardiac fibrosis and changes in the function of the left side of the heart. It can be used as an independent diagnostic factor to determine the probability of AF recurrence in the first 3 months after catheter ablation [65].

Inflammasomes
Inflammasomes are intracellular receptors of the innate immune system that are stimulated by the molecular patterns of pathogens and initiate immune responses [67]. One of the distinguished of these receptors, which have recently been well described, is nucleotide-binding oligomerization domain or NOD, leucine-rich repeat or LRR, pyrin domaincontaining-3 (NLRP3) inflammasomes. The NLRP3 inflammasome activation has been discovered to play a role in the occurrence and progression of various cardiovascular diseases, including paroxysmal AF, postoperative AF, and chronic AF [67,68]. In addition, the activity of these molecules in atrial cardiomyocytes and macrophages in cardiac tissue has been recorded in the pathogenesis of AF [67,68]. Cardiomyopathy after acute myocardial infarction is one of the NLRP3 inflammasome-involved underlying mechanisms leading to the occurrence of cardiac structural remodeling and, ultimately, AF. This cardiomyopathy is caused by the activation of caspase-1 in cardiomyocytes and the deterioration of cardiac muscle activity [69]. NLRP3 inflammasomes can also activate caspase-1 in cardiac fibroblasts. With caspase-1 activation-induced IL-1β release, cardiac fibroblasts differentiate into myofibroblasts, promoting cardiac structural changes by producing various pro-fibrotic cytokines [69].

Kinase-induced and other immunological pathways
Excessive activation of ryanodine receptor type 2 (RyR2) by serine-threonine kinase and disturbance in calcium equilibrium of cardiac cells have been recognized as a fundamental mechanism in the pathogenesis of AF [70]. Another kinaseinduced mechanism called the mitogen-activated protein kinase pathway (MAPK pathway) is involved in the pathogenesis of AF. The binding of angiotensin II to angiotensin II receptor type 1 (AT1 receptor) activates the MAPK pathway, which also causes the expression of transforming growth factor-β1 (TGF-β1) [71]. The production of TGF-β1 results in the activation of the TNF receptor-associated factor 6/TGF-β1 activated kinase 1 pathway (TRAF6/TAK1 pathway) and, finally the proliferation of atrial fibroblasts and atrial fibrosis leading to AF [71].
Other processes related to the body's inflammatory system also play a role in the occurrence of AF. For example, the peripheral blood levels of IL-18 and microRNA-21 are higher in patients with AF following cryoablation and can indicate the occurrence and severity of AF after ablation [72]. Checkpoint molecules, namely programmed cell death-1 (PD-1) and its ligand (PD-L1) can have immune-modulatory and protective roles in AF development [19]. The cross-talk between PD-1 and PD-L1 restricts the activity of T lymphocytes and by diminishing tissue inflammation can suppress AF progression [19]. The expression of PD-1 on some groups of lymphocytes, and PD-L1 on myeloid dendritic cells were remarkably higher in the control group than in AF patients, implying the downregulation of the PD-1/PD-L1 immunomodulatory axis in patients suffering from AF [19].
CRP, an inflammatory marker, can be utilized to detect AF regarding its association with necrosis and degeneration of cardiac muscle cells [16,19,44,73]. MicroRNAs are also related to the inflammatory system. MicroRNA-222 is independently associated with AF incidence in patients with degenerative valvular heart disease (DVHD). In other words, the level of this molecule, as well as CRP and IL-6, in patients with pure DVHD is lower than in patients with simultaneous DVHD and AF [74]. In addition, the expression of microRNA-21 in atrial muscle cells in people with chronic AF is much higher than in people with sinus rhythm [75]. Table 1 summarizes the underlying mechanisms by which interleukins and other inflammatory molecules play a role in AF pathogenesis.

Obesity and pericardial fat
Increased adipose tissue, for instance in obesity, can lead to increased inflammation and oxidative stress, the underlying initiators of AF at the molecular level. In obese individuals, pericardial fat is related to the increased risk of AF by producing proinflammatory cytokines including IL-1β, IL-6, and IL-8 which impact atrial structure and function. Besides, the Infiltration of adipocytes into the myocardium exacerbates atrial anatomy and physiology. In ovine models, weight loss MAPK pathway Stimulating the expression of TGF-β1 Activation of the TRAF6/TAK1 pathway The proliferation of atrial fibroblasts and atrial fibrosis [71] is associated with reverse atrial remodeling and reduces susceptibility to AF [76,77]. A clinical trial (NCT03478410) has evaluated and compared the cytokines and exosomes released from epicardial fat biopsies from patients with and without AF [78]. The results revealed a higher amounts of released exosomes and higher levels of secreted inflammatory and profibrotic cytokines from the epicardial adipose tissue in AF patients compared to controls. The epicardial fat-derived exosomes could also bring about a reduction in the action potential duration in the stem-cell-derived cardiomyocytes, rendering these exosomes an underlying stimulator of AF and a potential therapeutic target [79].

Immunological indices
Novel inflammatory indices, namely systemic immune inflammation index (SIII) and system inflammation response index (SIRI) have been recently implemented to predict AF development in various settings. They are calculated from the neutrophil, lymphocyte, and monocyte counts. SIII and SIRI have been investigated as markers of new-onset or recurrent AF development after cardiovascular interventions or surgeries such as CABG [80,81], direct current cardioversion [82], cryoablation [83], and PCI [84]. They have also demonstrated promising diagnostic values to predict AF occurrence in some atrisk populations such as patients with STEMI or ischemic stroke [85,86]. As opposed to the use of any single inflammatory marker, these indices might provide a more accurate picture of the body's immune and inflammatory conditions, as they integrate at least three groups of immune cells in peripheral blood with pillar roles in inflammation. However, their administration to predict AF should be more extensively studied. LASEC: Left atrial spontaneous echo contrast; LAT: Left atrial thrombus; POAF: Post-operative atrial fibrillation

Prognostic value of interleukins and inflammatory mediators in AF
AF patients are exposed to complications such as thrombotic events, myocardial infarction, and stroke, which cause mortality and morbidity [31,87,88]. AF risk assessment is currently based on clinical and demographic findings [89]. Nevertheless, inflammatory factors such as CRP, IL-6, and TNF-α can be taken to account to determine the prognosis of AF. Inflammation, along with endothelium damage, triggers the coagulation cascade by stimulating platelet aggregation. In this process, CRP and IL-6 induce the release of tissue factor and von Willebrand factor, which in turn can cause platelet aggregation to prevail over fibrinolytic functions and ultimately thromboembolism creation in AF patients.
The use of biomarkers such as N-terminal fragment B-type natriuretic peptide (NT-proBNP) and high-sensitivity cardiac troponin T (hs-cTnT) to predict the outcome of AF in patients is expanding day by day [90]. For example, biomarkers such as hs-cTnT, IL-6, NT-proBNP, and growth-differentiation factor-15 (GDF-15) are reported to be associated with sudden death, heart failure death, stroke, or thrombotic events [91]. CRP level can also be used to determine the short-or long-term outcomes and sinus rhythm preservation following cardioversion in AF patients [92,93]. Chemokines and their receptors are also effective in determining the outcome of AF and can remarkably increase the predictive value of the CHA 2 DS 2 -VASc score when combined [94]. For instance, CCL18 and CCL23 levels are positively related to the incidence of stroke, and a high level of CXCL16 is associated with mortality [94].

Prognostic role of IL-6
IL-6 not only has a great diagnostic value for AF, but also is useful in determining the outcome of patients with AF. Several studies have conclusively stated that the serum level of IL-6 in AF patients is directly related to the probability of AF recurrence after ablation [95]. By investigating 268 biomarkers in patients with AF taking anticoagulant drugs, cardiorenal factors (NT-proBNP, cTnT-hs), oxidative stress factors (GDF-15), inflammatory factors (IL-6, GDF-15), calcium disorders, renal and vascular disorders (fibroblast growth factor 23 or FGF-23), and apoptosis (NF-related apoptosis-inducing ligand receptor 2 or TRAILR2) have been identified respectively as important markers in determining cardiovascular death [96]. In addition, among anticoagulated AF patients, cardiorenal factors (NT-proBNP, troponin T, cystatin C) and inflammatory biomarkers (GDF-15, IL-6, CRP) are more elevated in patients with heart failure with reduced ejection fraction (HFrEF) than in patients with heart failure with preserved ejection fraction (HFpEF) and are lower in patients without HF than both groups [97]. More interestingly, the abovementioned factors elevate the risk of hospitalization and death due to HF in AF patients independently of the HF status [97]. Therefore, these biomarkers can predict hospitalization and mortality related to HF in patients [97]. IL-6 is independently associated with severe bleeding, thromboembolic events, stroke, cardiovascular mortality, and all-cause mortality in AF patients, and it can even increase the risk of ischemic stroke alone and in the absence of AF through atherosclerosis-induced cerebrovascular disease which is associated with IL6 trans-signaling pathway [20,98,99]. In patients taking anticoagulant drugs, the increase in systemic inflammatory activity, determined by the intermittent measurement of IL-6 levels, alone and independently of the known cardiovascular risk factors, is associated with an increase in mortality [100]. Although the CHA2DS2-VASc risk score is still used as one of the main criteria to determine prognosis in AF patients, its prognostic accuracy can be elevated. This issue is especially important in AF patients with a low to moderate risk of stroke to balance the risk of stroke and the risk of major bleeding by adjusting the dose of anticoagulant medication. Therefore, adding IL-6 or CRP to this criterion can increase its predictive power for cardio-cerebrovascular events in AF patients [20,98].

Other immunological mediators in the prognosis of AF
Other interleukins are also effective in the outcome of AF patients. For example, the level of IL-18 in the blood of the left atrium of AF patients who underwent ablation can be a measure to predict the probability of AF recurrence [72]. As mentioned earlier, LASEC and LAT can directly correlate with the risk of thrombosis in patients [101]; therefore, IL-17, which is associated with the risk of LASEC and LAT, can be a marker of thrombosis in AF [66]. The utilization of a combinatory system based on several biomarkers, including factor VIII, NT-proBNP, IL-6, and cystatin C was reported to have a more accurate prediction of the risk of stroke in AF patients [102]. In another study, the relationship between heat shock protein 70 (sHSP70) and cytokines and the recurrence of AF was investigated in hypertensive patients who have recently developed AF, and it was found that sHSP70 and serum IL-2 play a decisive role in the possibility of AF recurrence following cardioversion [103]. Another possible interleukin that may predict the subsequent complications of AF patients is IL-1, whose inhibition with IL-1 receptor antagonist (IL-1ra) increases the risk of cardiovascular diseases and thrombosisinduced stroke [104]. Table 2 indicates important interleukins that accelerate AF progression and/or are biomarkers of prognosis for AF.

Interleukins as therapeutic targets for atrial fibrillation prevention and treatment: the future perspective
In previous sections, we have comprehensively discussed the substantial role of inflammation and its underlying pathways in the pathogenesis and prognosis of AF. As mentioned earlier, suppressing inflammatory pathways is effective in improving the AF ablation prognosis and reducing the recurrence of AF in the short and long term [109,110]. Besides, some common antiarrhythmic drugs, such as amiodarone and landiolol have been demonstrated to decrease the level of some inflammatory cytokines along with AF management. Amiodarone led to IL-1 reduction in a rat model of epilepsy-induced cardiac dysfunction, and landiolol could decrease the risk of POAF by dropping the level of IL-6 in a group of patients undergoing esophageal surgery [111][112][113]. Hence, it can be deduced that targeting inflammatory responses can ameliorate the progression of AF. Studies, mostly in a preclinical setting, have been mainly administrating either inhibitors of inflammatory or pro-fibrotic interleukins or anti-inflammatory interleukins as therapeutic approaches to AF.

TGF-β1 inhibition
TGF-β1, a pro-fibrotic cytokine, leads to increased fibrillation in cardiac fibroblasts in rats. The administration of a JNK pathway inhibitor, SP600125, along with TGF-β1, could withdraw the process of atrial fibrillation [114]. In another study on rats, the inhibition of the TGF-β1 signaling pathway was conducted through the downregulation of a microRNA, miR-10a, which can further suppress the proliferation of cardiac fibroblasts [115].

IL-10 administration
IL-10, as an anti-inflammatory agent, has also been utilized in preclinical studies, and its therapeutic role for AF has been assessed. Obesity, induced by a high-fat diet (HFD), can stimulate inflammation, fibrosis, and, consequently, AF. It has been demonstrated that in IL-10 knockout mice, fibrosis and AF progression are more augmented than in wild-type mice under HFD. On the other hand, IL-10 administration, whether in the wild type or IL-10 knockout mice, could diminish the fibrotic effects of HFD [116].

Role of colchicine
In some clinical trials, colchicine has been investigated to reduce the risk of POAF. According to these clinical studies, colchicine therapy has been of value in preventing the early recurrence of AF post-pulmonary vein isolation [117] but not reducing the POAF risk [118]. Daily dosage of 0.6 mg colchicine for 30 days after AF ablation is also investigated as a therapeutic agent to reduce the risk of AF after ablation in a phase III clinical trial (NCT05459974) [119]. It should be considered that colchicine can be followed by some major gastrointestinal adverse effects, leading to therapy discontinuation. After colchicine administration, reducing the expression of pro-inflammatory markers, including IL-6, TGF-β, and TNF-α, has been observed in AF rat models. Therefore, it has been suggested that the anti-inflammatory effect of colchicine to reduce IL-6 expression through inhibiting IL-1β is the underlying mechanism of its AF-protecting effect [56].

Anti-IL-6 antibody
It has also been observed in sterile pericarditis rats that the injection of anti-IL-6 antibodies has a significant association with the attenuation of atrial interstitial fibrosis, the probability of AF occurrence, and the prolongation of fibrillation attacks [62]. Renal denervation (RD) reduces the sympathetic nervous system activity in patients with renal impairment (RI) due to the embolization of small branches of the right renal artery. The reason for conducting this procedure is that RI, with the cooperation of the sympathetic nervous system, causes the progression toward AF (RI increases the atrial levels of IL-6, CRP, renin, aldosterone, angiotensin II, and norepinephrine) in these patients. After the application of RD, the suppression of renin-angiotensin-aldosterone (RAAS) system activity and the inflammatory and atrial fibrotic activities mediated by IL-6 and CRP will occur. Therefore, reducing the atrial levels of IL-6 and CRP caused by RD results in the attenuation of RI-induced atrial fibrosis and AF [120].

Ctrp9
C1q/tumor necrosis factor-related protein-9 (CTRP9) with downregulation of IL-1β and IL-6 and upregulation of IL-10 has been shown to reduce the immune response in 3 days after MI. It also attenuates atrial fibrosis by decreasing collagen types I and III, α-SMA (α-smooth muscle actin), and TGF-β1 expression through suppressing the Toll-like receptor 4/ nuclear factor-κB and Smad2/3 signaling pathways at 7 days after MI [121]. Eventually, through the aforementioned mechanisms, CTRP9 ameliorates atrial inflammation, fibrosis, and vulnerability to AF in post-myocardial infarction rats [121]. Furthermore, in rats suffering from rheumatoid arthritis (RA), the administration of resveratrol decreases serum and atrial TNF-α and IL-6, resulting in the reduction of the occurrence of RA-induced atrial fibrosis and AF [122].

Lipid-based AF treatment and the role of NLPR3 inflammasome
Lipid-derived anti-inflammatory molecules such as resolvin D1 (RvD1) have also been evaluated as anti-fibrotic agents. RvD1 administration in rats with AF induced by monocrotaline (MCT) has effectively reduced the fibrosis of both the left and right atrium (MCT causes pulmonary arterial hypertension which itself induces right ventricular hypertrophy and ultimately persistent right atrial inflammation which leads to atrial fibrosis and abnormality of the right atrial conduction system as an AF underlying pathophysiology). RvD1 could also attenuate the duration and inducibility of AF. This anti-inflammatory effect could be conducted through IL-10, as its expression was upregulated due to RvD1 treatment [123]. Studies have been controversial on the impact of omega-3 polyunsaturated fatty acids (PUFA), including eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in preventing POAF. Some earlier studies could not show a significant association between EPA and DHA intake or plasma levels with a reduced risk of POAF [124]. However, it seems that the preventive effect is instead followed by an EPA/DHA ratio of more than 1 than the total level of PUFA [125]. Short-chain fatty acids (SCFA) produced by the gut microbiome were reported to protect against AF, as the fecal SCFA was lower in patients with persistent AF than in patients with paroxysmal AF. The total gut microbiome SCFA level was also higher in healthy individuals than in patients with AF. Moreover, SCFA, either induced by a high-fiber diet or supplementation, has been demonstrated to reduce the pro-fibrotic markers such as TGF-β1, and subsequently, atrial remodeling and AF. On a cell level, this protective effect of SCFA is mediated by impeding the NLPR3 inflammasome activation [126]. NLRP3 seems to be a promising target, as its role in inflammation-induced cardiac diseases such as AF is thoroughly explored. Terpenoids such as oridonin and triptolide and phenols such as salvianolic acid B and ellagic acid are inhibitors for NLPR3 inflammasome, as well. However, their possible therapeutic potentials for AF should be assessed in preclinical and clinical settings [127]. Figure 2 depicts some of the potential therapeutic methods that can be administered for AF patients in the future.

Conclusion
Interleukins have a verified diagnostic, prognostic, and therapeutic value in AF patients (Tables 1, 2). IL-6 is one of the most important inflammatory mediators that causes AF pathogenesis by mechanisms including activation of neutrophil infiltration-induced atrial fibrosis, and aberrant calcium currents. IL-17A is also involved in AF pathogenesis through fibrotic changes of cardiac tissue and left atrial remodeling. In the context of the prognostic role of interleukins, they can be indicators of thromboembolic events, AF recurrence after ablation, mortality, hospitalization, severe bleeding, and stroke in AF patients. Increasing attention has been paid to the role of inflammatory pathways in treating AF. IL-10 administration, inhibition of the TGF-β1, colchicine, and anti-IL-6 antibodies were demonstrated to be effective in diminishing the proliferation of cardiac fibroblasts, atrial interstitial fibrosis, and decrease in the AF occurrence in preclinical or clinical studies. Inflammatory markers such as CRP have also been assessed in clinical trials to monitor the risk of AF occurrence following ablation (NCT04269785) [128], or colchicine administration (NCT01755949) [129]. Considering the extensive diagnostic and prognostic role of interleukins in AF patients, it seems necessary to carry out original complementary studies to create criteria based on inflammatory pathways for evaluating people with AF risk factors.

Expert opinion
According to the importance and prevalence of AF and the significant mortality and morbidity it has caused in various societies, it seems that more research is needed to fully explain the role of interleukins in the diagnosis, prognosis, and treatment of AF patients.

Diagnosis
In the field of diagnosis, most studies have relied on IL-6, IL-17, and IL-18, and this issue calls for future studies on other interleukins. Also, considering the improvement of the prognosis of these patients in case of early diagnosis and treatment, cardiac walls local inflammatory factors such as IL-6 and IL-17 can be used in future studies to find high-risk patients in terms of atrial interstitial fibrosis and follow them more closely. In other words, considering the vital role of heart muscle inflammation in the development of fibrosis and AF, the creation of systems based on the measurement of serum or local levels of inflammatory and anti-inflammatory interleukins can be used in the classification of patients in the future, which may lead to the diagnosis the progression of the disease before the appearance of clinical symptoms and prophylactic treatments based on cardiac fibrosis reduction can be assessed.

Prognosis
Although the CHA2DS2-VASc risk score is still used as one of the main criteria to determine the prognosis in AF patients, its prognostic accuracy can be elevated. Therefore, adding IL-6 or CRP to this criterion can increase its predictive power for cardio-cerebrovascular events in AF patients. Although the utilization of IL-6 in the clinical practice will adjust the anticoagulant dosage in patients with a low risk of stroke and high risk of major bleeding, excessive use of its changes in patients receiving anticoagulants may cause errors in the estimation of AF prognosis. Because IL-6 independently and without connection with other cardiovascular risk factors (including AF) can be associated with an increase in mortality and ischemic stroke in patients undergoing anticoagulant treatment. As a result, it should be determined in future studies how to apply IL-6 hypersensitivity in determining the prognosis of AF patients to prevent overtreatment. Considering the wide role of different interleukins in the development of cardiovascular and cerebrovascular complications caused by AF, it seems that more research is needed to investigate the serum levels of IL-1, IL-2, IL-6, IL-17, and IL-18, as a criterion for determining the prognosis of AF patients in clinical practice. In general, the use of AF prognostic-determining interleukins in practice will have a favorable cost-effectiveness due to early warning of the possibility of complications caused by AF. Therefore, clinical trials exclusively investigating the diagnostic and prognostic effects of interleukins in AF patients can eliminate confounding factors in the future.

Treatment
According to the authors, the utilization of interleukins as therapeutic targets in AF patients will be the area that receives the most studies about the effects of interleukins on AF in the next decade. Despite the proven effects of IL-10 administration and IL-6 and TGF-β1-blocking compounds in reducing atrial interstitial fibrosis in rats, there is a necessity to design clinical trials to investigate these effects in humans. CTRP9, colchicine, and RvD1 are compounds emerging and expanding in this field, and there is a possibility of using them in AF patients in the future. NLRP3 inflammasome-induced cardiac diseases can be a guide for future investigations of the effect of its inhibitory substances in the treatment of AF patients. The limitation that exists in the field of interleukin-based treatment is comparing its effectiveness with existing treatments for AF, which has not been investigated yet. However, it seems that these anti-inflammatory agents can find a place for themselves in the future as a prophylactic treatment in high-risk patients due to their effects on the reduction of cardiac fibrosis.