Immune responses and activity of inflammatory mediators in pulmonary vein arrhythmogenesis

Abstract

Atrial fibrillation (AF) is the most common type of sustained arrhythmia with potentially serious complications and risk of mortality. Inflammation plays an indispensable role in the pathogenesis of AF, evidenced by the elevated levels of various cytokines such as C-reactive protein, macrophage migration inhibitor (MIF), interleukin-6 (Il-6), and tumor necrosis factor-α (TNF-α). Pulmonary veins (PVs) are the main site of AF initiation, where cells acquire arrhythmogenic properties under the effects of pro-inflammatory mediators. Calcium/calmodulin-dependent protein kinase II (CaMKII) signaling also plays a critical role in arrhythmogenesis, which is modulated by aberrant Ca2+ homeostasis and ion currents, as well as the electrophysiological properties of PVs influenced by inflammatory mediators, including MIF. Today, pulmonary vein isolation (PVI) is one of the main treatments for AF. This review aimed to outline our current understanding of the underlying mechanisms of AF in an effort to open a new window for discovering novel therapies for this type of arrhythmia.

1. Introduction

Atrial fibrillation is the most common sustained arrhythmia [1]. The combination theory of “trigger” and “substrate” seems to explain the main mechanisms involved in the initiation and maintenance of AF. This arrhythmia can initially develop in PVs, the Marshall ligament, coronary sinuses, or the atrial tissue. Abnormal automation or early/late depolarization further safeguards the initiation and maintenance of this arrhythmia [2] by facilitating structural, electrical, and neural remodeling of the atrial tissue and increasing the likelihood of aberrant firing or reentry. Structural remodeling due to the activation of fibroblasts and metabolic disbalance favors collagen deposition and atrial fibrosis [3]. Electrical remodeling through prompting changes in the number and distribution of ion channels and gap junction proteins shortens the atrial effective refractory period (AERP) [4]. Autonomic nervous system remodeling is characterized by the formation of heterogeneous nerve sprouts and sympathetic and parasympathetic imbalance. These forms of remodeling are not independent and work together to promote the occurrence of AF [5].

Recent studies have shown that the immune system plays an essential role in the pathophysiological process of AF. Disbalanced immune reactions and the aberrant activation of immune cells, leading to the periodic burst of inflammatory molecules, form a type of remodeling immune network before the development of AF [6,7], as well as during its progression and persistence (Table 1) [8]. In fact, immune remodeling is believed to be the master regulator of structural, electrical, and neural remodeling [9,10]. Unlike atrial remodeling, immune remodeling is not limited to the atria but extends systematically so that its consequences are delectable in peripheral blood, offering minimally invasive diagnostic opportunities and reiterating the fact that AF is a systemic rather than a localized disease. Cause-and-effect relationships underlying AF pathogenesis are perplexing and difficult to understand. A critical issue is to distinguish if immune system remodeling is caused by AF, or it is the other way around. Immunological studies have greatly improved our understanding of the nature of this phenomenon. Several studies have noted higher numbers of granulocytes, neutrophils, and lymphocytes in the atria of AF patients compared to normal sinus rhythm (NSR) subjects [11]. Also, other studies have shown changes in the populations of immune cells (especially macrophages) residing in the cardiac tissue during the initiation and maintenance of AF [12,13].

Table 1.

Selected Bio-factors from immune responses that have main role in AF related inflammation.

Bio-Markers	Role in AF	Reference
IL-1	•Pressure overload-induced sustained AF	130
IL-2	•Low serum IL-2 levels on admission associated with successful CV •Predictor of early post-operative AF	131
IL-6	•Outcome of CV and RF catheter ablation •Incidence of AF post-operation •Predictor of stroke and the composite endpoint of stroke and death in AF	130, 131
IL-8	•Longer AF durations or permanent AF.	130
IL-18	•AF recurrence	130
TNF-α	•Predictor of ischemic stroke	131, 132
CRP	•Exclude presence of TEE markers: LA/LAA SEC or LA/LAA thrombus •Incidence of AF, incidence of LAF or paroxysmal AF and recurrence after CV	131
MCP-1	•Incident AF	131
NLRP3 inflammasome	•Reproducible pacing-induced AF •Structural remodeling to lead to frequent atrial ectopy	130
NF-κB	•Alteration of electrophysiological property of cardiomyocyte •Induction of further inflammation	62
HSP27	•Protective effect via protein quality control in cardiomyocyte	62
cfDNA	•Induction of further inflammation	62

Pulmonary veins are one of the important locations where AF is triggered. From an anatomical and electrophysiological perspective, PVs contain several types of muscles and pacemaker cells, as well as cardiomyocytes, causing atrial tachyarrhythmias through inducing spontaneous activity or high-frequency irregular rhythms [14]. It seems that certain immunological factors contribute to histological remodeling and the augmentation of arrhythmogenic properties in PVs, facilitating the onset of AF. In fact, immunological remodeling favors the structural and electrophysiological remodeling of PVs and atria [15]. The understanding of the underlying immunological mechanisms of this phenomenon can help develop more effective treatments for AF.

2. Atrial fibrillation

Atrial fibrillation is the most common arrhythmia and one of the major global health problems. The prevalence of AF has increased during the last few decades mainly due to population aging and also the improvement of the survival of cardiovascular patients. Atrial fibrillation is associated with considerable discomfort, complications, and mortality [16,17] and causes the loss of atrial contractions, leading to thrombus formation in the heart and subsequently embolism in different organs of the body. Heart failure is another extreme consequence of AF, which is caused by ventricular dysfunction and is associated with a doubled mortality rate [[18], [19], [20]]. Significant improvements have been achieved in the treatment of AF; however, the current medication and ablation therapies offer less than optimal outcomes partly due to our incomplete understanding of the mechanisms underlying the initiation and persistence of arrhythmia. Over the last 50 years, the multiple wavelet theory has been used to explain the main pathological mechanisms of AF. According to this hypothesis, AF is caused by the functional re-entry of multiple wave fronts spreading throughout the excitable tissue of the atrium; nonetheless, this hypothesis has been questioned by many scientists [21]. Haissaguerre et al. believe that the high-frequency extrasystoles that trigger AF originate from the inside or around PVs [22] are fragmented with activation fronts and are maintained in the surrounding tissues as a chaotic rhythm called fibrillatory conduction [14,23]. The latest hypothesis proposed so far states that stimuli produced in PVs create multiple functional reentry circuits in large areas of the atrium [24,25]. In a study on a sheep model, it was shown that the rotor or vortex anchored at the PV-atrial posterior wall junction contributed to the persistence of AF. These rotors are produced by functional micro-reentry circuits and lead to high-frequency fibrillation conduction in the atrial tissue [26]. Although it seems that PVs are the epicenter of impulse generation during AF episodes, research evidence suggests that there are also other players in this event. The results of various studies have shown that the isolation of PVs leads to a gradual increase in the length of the AF cycle, which ultimately results in the termination of arrhythmia; however, other areas of the atrium can also be the sources of extrasystoles, triggering AF episodes [27] via either focal or re-entry mechanisms. In focal cases, several points generate irregular impulses that spread throughout the atrium. Extra-systolic beats can also result from increased automaticity either early or delayed after depolarization (EAD or DAD). Regarding the re-entry mechanism, re-entry circuits can be repeatedly discharged as a single circuit or coexist as multiple circuits that create irregular fibrillation activity. In both scenarios, impulses abnormally propagate between two areas, and the action potential begins following the depolarization of the tissue and the deactivation of Na + channels until the cell potential reaches −60 mV (i.e., the refractory period). The duration of the refractory period and the balance between the onset and termination of the ionic current governs the occurrence and maintenance of reentry. The longer the refractory period, the more likely the impulses will hit the resistant tissue, causing re-entry termination [28]. Fernández-Lozano et al. suggested that reentry in the substrate of PVs and the posterior wall of the left atrium was the main mechanism underlying AF maintenance [29].

Atrial fibrillation attacks, with single or multifocal discharges, have important consequences. Frequent discharges cause progressive pathological changes in the atrial substrate and support the continuation of AF [30]. The discovery of PV ectopy, as a trigger and preserver of AF, led PVI to become the cornerstone of the treatment of acute AF. However, the mechanisms that maintain persistent AF (PsAF) are not well understood, justifying wide variations in treatment for PsAF [[31], [32], [33]]. In the largest study on patients with PsAF, it was found that the complexity of AF drivers was increased with elevating AF diversion, and PVs remained as the dominant driver in PsAF [34]. In prolonged AF, not only PVs but also areas outside PVs are involved, especially the inferior posterior region of the left atrium (LA) and the anterior septum of the right atrium. [34]. Extensive research has been performed to identify the underlying cause of this type of arrhythmia, leading to the proposition of a number of pathological processes such as electrical remodeling of ion channels, abnormal movement of Ca2+ from the sarcoplasmic reticulum, and profound changes in atrial and extra-atrial tissues. Studies also suggest a role for inflammatory processes and autonomic neuronal changes in the development of PV-triggered impulses [35,36], which will be explained in more detail in the following sections.

3. Pulmonary veins

Our detailed knowledge of PVs' anatomy has an undeniable role in the success of PV ablation therapy. Magnetic resonance imaging (MRI) provides accurate images to precisely characterize the three-dimensional structure of individual PVs. However, no standard method has yet been described to analyze PV anatomy. For the first time, Nathan and Eliakim studied PVs in the hearts of 16 people postpartum [36]. Few studies have been conducted on the exact sizes of PVs or their abnormal anatomic frequency and distribution. In one study, there was no significant difference in the diameter of four PVs, with average diameter ranging between 1.002 and 1.202 cm [37]. Lin et al., for the first time, measured the size of PVs in patients with AF via contrast venography and reported that the diameters of right superior PV (RSPV) and left superior PV (LSPV) were greater than that of inferior PVs [38]. These results were confirmed by another study that used MRI to evaluate PVs [39]. Another interesting point mentioned in this study was that the average diameter of superior PVs in patients with AF was larger than that of healthy subjects, but the mean diameter of inferior PVs were comparable between the two groups [40]. Ritsushi et al. conducted a comprehensive study in this field and provided information about PVs’ anatomy and morphology, as well as atrial dimensions and the relationship of these structures with left atrial appendage (LAA) in AF patients [41]. In contrast to previous studies, they reported no difference in the size of superior versus inferior PVs in AF patients or between AF patients and control subjects. This discrepancy may be due to using a different measurement technique (multiple plates) to locate PV ostium in the recent study.

Histological analyses show that the atrial myocardium extends to 89 % of all human PVs. Atrial myocardial expansion in PVs is greater in patients with a history of AF compared to those without a history of AF. Also, there are significant differences in the histological appearance of the atrial myocardium extended to PVs between healthy and arrhythmic individuals. A higher frequency of cellular discontinuity, hypertrophy, and a higher degree of myocardial fibrosis extending to PVs were found in patients with AF compared to those without a history of AF; however, immunohistochemical staining showed similar patterns regarding myocytes' extension into PVs and the atrium between the two groups [42]. Myocardial extension into the left upper PV appears to be the most common type of focal stimuli [43]. Even mild degrees of PV fibrosis reduce myocyte coupling and electrotonic inhibition, thus facilitating automaticity. The degree of PV fibrosis has been noted to gradually increase from proximal to distal areas so that the most distal myocytes have the weakest coupling. A combination of fibrosis, hypertrophy, and poor cell coupling results in slow conduction and thus encourages local re-entry into PVs [42]. Blom et al., who isolated single cardiomyocytes with pacemaker activity from PVs of dogs, suggested that PVs could act as secondary pacemakers [44] and key contributors to AF pathogenesis. EAD and DAD caused by isoproterenol can also play a role in the development of arrhythmogenic events in PVs [45]. In fact, PV cardiomyocytes have ionic properties similar to heart cells, and IK1 in PV cardiomyocytes display pacing activity similar to pacemaker cells in the sinus node. Nevertheless, unlike sinus node cells, only parts of PV cardiomyocytes attain pacemaker activity [46]. Nguyen et al. obtained samples of the PV-left atrial connective tissue from patients with AF and identified cells that were positive for Potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) and participated in the formation of the pacemaker flow [47]. Morel et al. suggested that subjects with AF had a high density of Cajal cells in their PVs, which were HCN4-positive cells in smooth muscle and functioned as pacemakers [48]. Perez-Lugones et al. reported that the human heart contained pallid cells with morphological characteristics of P cells, transitional cells, and Purkinje cells. These findings suggested the presence of a conductive tissue in PVs’ walls [49]. Likewise, Gherghiceanu et al., using electron microscopy, identified interstitial Cajal-like cells in PV myocardium [50]. Today, it has been established that the antral regions of PVs are the most common regions where AF stimuli are generated, a notion that has been confirmed by different mapping approaches in human and animal studies [[51], [52], [53]]. In one study on sheep heart, the highest dominant frequency (DF) was detected around the PV ostium [51]. Also, the PV-LA junction was identified to be a common rotor anchor site [52]. In human studies, clusters of DF sites were found around PVs in patients with paroxysmal AF, but atrial DF sites were more common in individuals with persistent AF [54]. The complex orientation of PV antrum fibers and the electrophysiological characteristics of PVs have turned them into potential arrhythmogenic sites [55]. Proximity to PV stimuli may lead to the electrical remodeling of recurrent focal discharges similar to rapid atrial pacing [56]. Currently, PVI alone is an acceptable treatment for these patients [57], causing AF to convert to sinus or simple tachycardia by preventing stimulated impulses from entering the left atrium or causing functional reentry around the left atrium/pulmonary vein (LA/PV) junction to reentry around a larger circuit [58]. Although PVI seems to be an adequate therapeutic strategy for transient AF, its outcomes in patients with persistent AF are unclear.

4. Role of immunological factors in the pathogenesis of AF and initiation of PVs

AF is a multifactorial disease caused by the contribution of a combination of genetic factors, aging, lifestyle, and other comorbidities [59,60]. AF is mainly caused by electrical stimulation originating from PVs, but to maintain it, an arrhythmogenic substrate in the atrium is needed, and if this arrhythmogenic substrate is not provided, AF would disappear [61]. Today, it is well-known that inflammation is deeply involved in the formation of the AF substrate, and understanding its mechanism leads to the development of new therapeutic strategies. The interesting point is not that only inflammation causes AF, but AF itself also triggers inflammation, leading inflammation and AF to form a vicious spiral [62]. Systemic inflammation appears to increase the risk of AF. Meierhenrich et al. reported that the incidence of AF in intensive care units was 7.8 %, while the incidence of AF in patients with sepsis was 46 %, with higher levels of plasma C-reactive protein (CRP) being observed before the occurrence of AF [63], suggesting that systemic inflammation may be an independent risk factor for the occurrence of AF [64]. In autoimmune diseases such as rheumatoid arthritis (RA), psoriasis, and Crohn's disease, the risk of AF correlates with the severity of the disease and the levels of inflammatory factors [65]. On the other side, postoperative atrial fibrillation (POAF) is a common complication of cardiac surgery in the perioperative phase. The incidence of POAF has been reported as approximately 40–50 % after heart valve surgery and 25–30 % after coronary artery bypass grafting (CABG). Several underlying mechanisms have been proposed for POAF occurrence, the most important of which is atrial inflammation, leading to atrial conduction heterogeneity and subsequently POAF. In general, it can be said that elevated levels of inflammatory biomarkers, either systemic or localized to extracardiac tissues, are associated with the risk of AF. The involvement of inflammatory molecules in the initiation and progression of AF has been reported. High-sensitivity CRP (hs-CRP) is a biomarker suggestive of the presence and correlated with the severity of inflammation. Pro-inflammatory cytokines such as tumor necrosis factor (TNFα) and IL-6 are produced by macrophages and monocytes and induce the production of CRP (Fig. 1). The level of CRP has been noted to be higher in people with AF than healthy subjects [66] and also higher in those with persistent AF than in individuals suffering from paroxysmal AF [67]. It has also been observed that in subjects with paroxysmal AF, CRP levels are higher during AF episodes compared to the sinus rhythm [68]. However, there are studies noting that CRP itself may contribute to the development of AF [69]. The induction of heat shock protein 27 (HSP27), an important regulator of inflammatory responses, by geranyl geranylacetone was reported to prevent the development of AF, suggesting a protective role for HSP27 against AF [70], mediated via suppressing inflammation and regulating the expression of TNF-α and IL-10. It has been noted that in patients with AF, the serum level of HSP27 is inversely correlated with atrial diameter [71]. IL-8, also known as neutrophil chemotactic factor, is produced by macrophages, monocytes, and vascular endothelial cells. After CABG, high levels of IL-8 and IL-6 could predict the risk of postoperative AF [72]. Other inflammatory mediators such as MCP1, IL-1, IL-2, IL-10, and IL-18 may also increase the risk of AF [73]. Since the early diagnosis of acute AF may be difficult, these biomarkers can be used to assist timely identifying patients suspected of having AF.

Fig. 1.

Immune responses and their possible link to AF.

Various electrophysiological mechanisms have been proposed for the occurrence and maintenance of AF, such as the rotor hypothesis and the multiple wave hypothesis [[74], [75], [76]], both of which are based on electrophysiological changes (e.g., action potential shortening, conduction velocity reduction, atrial tissue heterogeneity, atrial electrical and structural remodeling, and atrial enlargement). The presence of inflammation has been shown to be critical for atrial electrical and structural remodeling [77]. In fact, the infiltration of inflammatory cells, especially CD68 positive macrophages, into the atrial tissue underlies the initiation of AF [78]. Yamashita et al. reported an increase in the production of IL-6 and transforming growth factor β (TGFβ) by macrophages in the atrial tissue of patients with AF [79]. Another study proved that in patients with AF, the levels of inflammatory factors such as IL-6, IL-8, IL-10, and TNF-α were significantly elevated in the atrial tissue [80,81]. These cytokines directly affect atrial cardiomyocytes’ electrophysiological properties and facilitate atrial arrhythmogenesis.

Lazzerini et al. reported that P-wave parameters in 12-lead ECG were vulnerable to alterations caused by inflammatory processes such as infections and autoimmune diseases, with these changes being normalized after the resolve of inflammation [82]. IL-6 serum level is specifically related to P-wave's width and dispersion. Also, Cx40 and Cx43 connexins are downregulated in peripheral blood in inflammatory diseases, highlighting an inverse relationship with inflammatory processes and, therefore, the level of IL-6. The direct exposure of HL-1 cells to IL-6 caused a rapid decrease in the expression of Cx40 and Cx43 in peripheral blood mononuclear cells, correlating with atrial remodeling and changes in their electrophysiological properties, such as the overexpression of ICa and decreased expression of Ca2+ Sarco/endoplasmic reticulum-associated ATPase-2 (SERCA2). Moreover, IL-6 regulates the expression of gap junction channels on cardiomyocytes, favoring the occurrence of AF [[82], [83], [84], [85]]. TNF-α also has a direct effect on the heart and its electrophysiological properties (Fig. 2). The overexpression of TNF-α contributes to atrial arrhythmias, especially AF [86]. In PV cardiomyocytes, TNF-α leads to the downregulation of SERCA2 and abnormal calcium trafficking, predisposing to delayed afterdepolarization (DAD) and arrhythmia [87].

Fig. 2.

The role of immune cells and related bio-mediators in cardiac remodeling and AF initiation.

Nuclear factor kappa B (NF-κB) is a transcription factor and a key regulator of inflammatory responses by inducing the expression of inflammatory cytokines such as TNF-α, IL-1, IL-6, and IL-8 [[88], [89], [90], [91], [92], [93], [94], [95], [96], [97], [98]]. NF-κB is expressed in different types of immune cells and is activated under the influence of inflammatory stimuli. Phosphorylated NF-kB has been noted to increase in the atrial tissue of AF patients [99], where it contributes to atrial inflammation. NF-κB not only stimulates and exacerbates inflammation, but also suppresses Scn5a gene expression, further facilitating electrical remodeling in AF [100].

Adrenomedullin, a peptide belonging to the calcitonin-related peptide family, possesses anti-inflammatory, natriuresis, and diuresis activities, preserves the integrity of vessels, and prevents vascular leakage and dilatation [[101], [102], [103]]. Adrenomedullin is expressed in blood vessels and has been proposed as a therapeutic target in heart failure (HF) due to its vasodilator properties and high expression in the cardiovascular system. Adrenomedullin prevents the progression of HF [104,105] partly by binding to endothelial and vascular smooth muscle cells and increasing the intracellular levels of cyclic adenosine monophosphate (cAMP) via the activation of protein kinase A (PKA), finally leading to vasodilation [106]. Heart failure is a major risk factor of AF, and AF is the most common sustained arrhythmia in HF [107]. In fact, HF promotes the arrhythmogenicity of PVs as the main origin of AF [108]. Also, with an increase in atrial pressure, the firing rate of PVs accelerates, further facilitating AF development [109]. Adrenomedullin was noted to suppress ICa-L through the PKA signaling pathway during septic shock [110]. Considering that PKA signaling plays a key role in the pathogenesis of AF, adrenomedullin-mediated suppression of PKA can modulate atrial and PV arrhythmogenesis. The activation of PKA seems indispensable for the development of isoproterenol-induced PV arrhythmogenesis [111]. In the healthy heart, adrenomedullin promotes positive inotropic effects characterized by an increase in contractile force, especially in the atrium. During atrial stretching, adrenomedullin signaling decreases, therefore increasing vulnerability to AF [112]. In fact, stretch-induced mechanical feedback regulates PV arrhythmogenesis. Altogether, adrenomedullin can mitigate PV arrhythmogenesis through inhibitory effects on vascular diastolic tension (mechanical feedback) [113].

Another immunological factor that plays a role in the arrhythmogenicity of PVs is macrophage migration inhibitory factor (MIF). Macrophage migration inhibitory factor increases cytosolic reactive oxygen species and activates calcium/calmodulin-dependent protein kinase II (CaMKII). The CaMKII pathway intensifies the L-type calcium flow and facilitates late sodium currents (INa-Late), ultimately causing arrhythmia [114,115].

This inflammatory mediator, a pleiotropic cytokine involved in many acute and chronic inflammatory conditions, seems to be a key player in the onset and progression of AF. MIF controls the inflammatory “set point” by regulating the release of other pro-inflammatory cytokines. Increased levels of MIF have been observed in patients with AF, suggesting a crucial role for MIF in the pathogenesis of AF. Through induction of atrial arrhythmogenesis via CD74 and augmentation of calcium currents, MIF is involved in both the initiation and maintenance of AF, suggesting that MIF may be a regulator of PV arrhythmogenesis during inflammation, a robust evidence for the contribution of immunological processes to the occurrence of AF. MIF modulates electrophysiological and arrhythmogenic properties of PVs by regulating ion currents and Ca2+ homeostasis through CaMKII activation [[114], [115], [116]]. In a study, the exposure of the PV tissue to MIF (100 ng/mL) increased the tissue's beating rate and intensified PV arrhythmogenesis [104]. MIF-treated PV cardiomyocytes showed higher densities of ICa-L, NCX, and INa-Late than control cells, culminating in calcium overload and arrhythmogenesis and highlighting the pivotal role of MIF-mediated calcium trafficking in cardiomyocytes. Also, increased ROS levels following CaMKII activation may be involved in MIF-induced PV arrhythmogenesis [[117], [118], [119], [120]]. Due to the direct role of MIF in PV arrhythmogenesis, drugs targeting MIF can be used to treat AF.

Immune system regeneration is a multidimensional pathological process. CD45 is universally expressed on all inflammatory cells and has been found to be overexpressed in the atria of AF patients compared to subjects with normal sinus rhythms (SR). This observation suggests the atrial infiltration of CD45⁺ cells in subjects with AF, which was confirmed by multiple studies showing that CD45⁺ inflammatory cells penetrated into both the left and right atria of these individuals [[121], [122], [123], [124], [125]]. However, the infiltration of CD45⁺ and CD68⁺ cells seemed to be higher in endo-myocardium and sub-endo-myocardium atria than in the middle part of the tissue, indicating that immune cells tend to propagate throughout the atrial endocardium during AF [121]. Dendritic and mast cells have also been detected in the atria of AF patients. The number of dendritic cells was higher in AF patients than in patients with SR, while the number of mast cells was similar [126]. Compared to those with SR, CD3 + T cells were enriched in the atrial tissue of patients with AF [126]; however, whether CD3⁺ T cell counts differ significantly among AF subgroups is controversial.

Chunxia Yao and colleagues mentioned in their study that the activity of NOD-like receptor protein 3 (NLRP3) inflammasome was enhanced in atrial cardiomyocytes (CMs) of AF patients. CM-restricted activation of NLRP3 can promote both ectopic-firing and AF-maintaining substrates, which ultimately leads to AF. Furthermore, studies on animal models suggested that NLRP3 inflammasome activation in CMs might be both sufficient and necessary for AF occurrence. The over-activation of NLRP3 signaling in CMs enhances the expression of Ryr2, leading to increased protein expression, RyR2-mediated Ca2+ release from SR, and delayed depolarization-associated ectopic firing. Meanwhile, excessive activity of NLRP3 signaling in CMs enhances the transcription of Kcna5 and subsequently augments IKur, modulating AERP and creating a re-entry substrate. On the other hand, the enhanced activation of NLRP3 inflammasome in CMs can increase Casp1–mediated pyroptosis and Casp1 cleavage, inducing the activation of CFs. Afterwards, primed CMs and cardiac fibroblasts can recruit macrophages and other immune cells, secrete inflammatory cytokines (and collagen), and also induce fibrosis and the development of AF-maintaining substrates. The NLRP3 inflammasome initiates a well-established signaling pathway responsible for the release of IL-18 and IL-1β from innate immune cells [[127], [128], [129]]. Several key bio-factors secreted from immune cells are known to have important roles in inflammatory reactions and AF development (Table 1) [62,[130], [131], [132]]. It is clear that various types of immune cells and inflammatory factors participate in the arrhythmogenesis of PVs and occurrence of AF. Therefore, paying attention to these underlying mechanisms can be useful for producing appropriate drugs for AF.

5. Conclusion

Immune cells are integral components for maintaining the balanced function of cardiovascular cells. The main types of immune cells reside in the heart muscle in quantities 12 times higher compared to the skeletal muscle, indicating that many cardiovascular diseases, such as arrhythmias, can be due to dysregulated immune reactions in the cardiac tissue. The frequent coexistence of AF and inflammation suggests a deep mutual link between these two entities. There are reports noting that inflammation can orchestrate the necessary context for the occurrence of AF. Understanding the subsets of immune cells and their function in the cardiovascular system can help better excavate the underlying mechanisms of AF. PVs have been introduced as the main site for the initiation of AF, and studies propose that immune system reactions influence the excitability of PVs and the onset of AF. Currently, several pieces of evidence suggest an important role for NLRP3 inflammasome signaling and other inflammatory mediators in AF pathogenesis. In fact, NLRP3 inflammasome can be involved in not only the initiation but also persistence of AF, with reports suggesting NLRP3 inflammasome activation in CMs of AF patients. Today, antiarrhythmic drugs or PVIs are used to control AF, but perhaps acquiring a deeper understanding of the mechanisms that initiate this arrhythmia will open the window for discovering newer and more effective treatments. More preclinical and large-scale clinical studies are necessary to unveil pharmacological and also biological entities regulating NLRP3 inflammasome and inflammatory signaling pathways in the context of AF. Although AF is the most common arrhythmia, current treatments lack enough efficacy. Targeting NLRP3 inflammasome and immune system-related bio-factors that play a key role in the occurrence of this arrhythmia can be a promising therapeutic approach.

CRediT authorship contribution statement

Entezar Mehrabi Nasab: Writing – original draft, Visualization, Software, Resources, Project administration, Methodology, Investigation, Formal analysis. Saeed Sadeghian: Writing – review & editing, Software, Resources. Ali Bozorgi: Supervision, Methodology, Investigation. Hamidreza Fallahabadi: Writing – review & editing, Software, Formal analysis. Vahid Toupchi-Khosroshahi: Validation, Software, Resources, Data curation. Seyyed Shamsadin Athari: Supervision, Project administration, Methodology, Investigation.

Ethics approval and consent to participate

Not Applicable.

Consent for publication

Not Applicable.

Declaration of conflicting interests

There is no conflict of interest.

Funding

Not Applicable.

Data availability

Not Applicable.