Adjuvants in COVID-19 vaccines: innocent bystanders or culpable abettors for stirring up COVID-heart syndrome

The adjuvants used in COVID-19 vaccines can be categorized into five classes namely Aluminum salt-based, Emulsion-based, TLR agonists, Metabolic, Cell death, and Epigenetic.^15,22 Each of these adjuvants instigate different mechanisms that are ultimately responsible for onset of spectrum of cardiovascular diseases seen in COVID-19 patients and those receiving vaccination (Figure 1). This review provides a brief overview of different mechanisms that can be arising out of these adjuvants, and each might be contributing at least partially to instigate cardiomyocyte damage.

Coxsackie virus-induced myocarditis animal models were previously utilized for exploring and understanding the relationship between autoimmunity and myocarditis.²³ It appears that there is an element of molecular mimicry involved which literally means that there is overlapping of sequences between viral epitopes and self-antigens expressed on myocardial cells.²⁴ Due to this cross-sharing of antigen-epitopes, neo-antibodies provoked by coxsackie viral antigens traverse along circulation and make their way toward myocardium.²¹

In autoimmune myocarditis and dilated cardiomyopathy, one of most prominent targets for inception of myocardial injury is cardiac-specific α-myosin isoform.^25,26 Some of the patients with autoimmune dilated cardiomyopathy have been found to have autoantibodies devised against β-1 adrenergic receptor.²⁷ Most specifically, these antibodies that are mass-produced during autoimmune phenomenon principally belong to IgG3 subclass.²⁸ It has been documented that mice hearts which have a propensity to develop autoimmune myocarditis are bound to express myosin in the extracellular matrix over and above its expression within the cardiomyocytes.²⁹ It is logical to assume that this supernumerary extracellular expression of myosin is considered a liability as auto-antibodies floating around in the circulation recognize and interblend with them. Accordingly, prior mice studies reveal that overproduced IgG antibodies are prone to bind to myosin or myosin-like protein which is predominantly expressed in the extracellular matrix of cardiomyocytes resulting in formation of antigen–antibody complexes.²⁹ Accumulation of antigen–antibody complexes within the extracellular matrix of myocardium can be detrimental as it paves the way for incipiency of myocarditis.

Vacccines harboring oil emulsion adjuvants and TLR agonist adjuvants were shown to instigate a spectrum of autoimmune diseases ranging from autoimmune diseases ranging from arthritis, lupus, hypothyroidism, vitiligo, and hepatitis.^30–33 The underlying premise based on which induction of autoimmune disease transpires is via eroding self-tolerance, exuberant synthesis of auto-reactive T-cells and massive production of IL-1 and IL-17.^34–36 It is possible that precedent COVID-19 infection might have engendered subclinical myocardial injury thereby sparking exudation of cardiac self-antigens into the circulation. In such a scenario, autoantibodies brought into existence against these spilled self-antigens along with circulating self-reactive T-cells and cytokine storm can rouse the onset of autoimmune myocarditis in genetically susceptible individuals. Plausibly, some of the adjuvants can induce the generation of autoantibodies even without the exposure of self-antigens. In rodents and salmon fish, administration of oil adjuvants in vaccine were associated with formation of autoantibodies along with Systemic Lupus Erythematosus (SLE)-like syndrome, immune complex glomerulonephritis, arthritis and granulomatous hepatitis.³⁷

An alternative mechanism called bystander effect has been described in the literature where viral infection can potentially unmask, and release hidden self-antigens from the body tissues there launching a chain reaction for subsequent development of autoimmunity.^38–40 In Coxsackie-induced diabetes, it was revealed that that viral infection resulted in local pancreatic tissue damage and subsequent decamping of concealed islet antigens into the systemic circulation.³⁹ Breaking loose of these segregated self-antigens from their habitat results in budding of re-energized auto-reactive T-cells.³⁹ These over-reactive and motivated T-cells can be a driving force for autoimmune-mediated destruction of pancreatic tissue thereby instigating the onset of pancreatitis in the setting of coxsackie viral infections.³⁹ This bystander effect can be a potential underlying mechanism for occurrence of myocarditis in COVID-vaccinated individuals.

Adjuvants that are currently incorporated in the COVID vaccines might conceivably incite subclinical myocardial inflammation and thereby boost the extracellular expression of myosin or instigate extravasation of myosin from within cardiomyocytes. Exposing of self-antigens such as myosin initiates a chain of events that will ultimately culminate in myocarditis and cardiomyopathy. Autoimmune myocarditis is putatively seen more frequently in patients in other existing diseases such as Grave’s disease, Celiac Disease, and SLE.^29,41 As compared to the general population, these high-risk patients are inherently more fated to develop auto-antibodies if there is unmasking of self-antigens in the body tissues. That being the case, this unmasking of self-antigens can enkindle the generation of autoantibodies which eventually resettle in myocardial tissues. Once they transmigrate into the myocardium it will also bring to existence a potential binding domain for these circulating autoimmune antibodies in the blood to interact and couple with these unshielded self-antigens. Once these auto-antibodies transmigrate to the myocardium, it will sow the seeds for creation of a scaffolding where circulating autoimmune antibodies can bind with unmasked myocardial self-antigens. This lead to formation of antigen-autoantibody complexes in the myocardium with the resultant un forrseen consequences. On top of that, shackling of IgG-myosin complexes in the extracellular milieu of cardiomyocytes might plausibly become a launch pad for kickstarting pathogenic signaling pathways that ultimately effectuate and abet myocarditis as well as dilated cardiomyopathy in the susceptible patients.

Over and above that, these exposed self-antigens such as myosin can be the driving force for replication of cardiac-specific auto-reactive T-cells, which can subsequently attack the myocardium thereby causing myocarditis.⁴²

Some of the adjuvants used in the vaccines can be directly toxic to the myocardial tissues. For example, aluminum which is a commonly used adjuvant is known for its nefarious features such as oxidative stress, inflammatory gene expression, apoptotic gene expression, immunotoxicity as well as disruption of physiological cellular pathways.^43–46 Previous reports even unearthed the proposition that aluminum-based adjuvants are blameworthy for inciting NLRP3 inflammasome activation in dendritic cells as well as B&T lymphocytes.⁴⁷ The resulting after-effects from this NLRP3 activation in the immune cells can lay the foundation for kickstarting disastrous consequences in the myocardial tissues. Furthermore, aluminum adjuvants were also notorious to trigger Th2 immune bias which is principally characterized by elevated levels of IgE and eosinophils.^48,49 Vaccination with aluminum adjuvant SARS COVID-19 vaccines in mice resulted in Th2-type lung immunopathology with predominant eosinophilia bringing to light paramount presumption of prevailing hypersensitivity against these adjuvants.^49,50 Pragmatically, substitution of these aluminum-based adjuvants with delta inulin-based adjuvants in the COVID-19 vaccines was associated with decreased risk of developing Th2-based lung immunopathology in mice.⁴⁹ Full-fledged engagement of this phenomenon entails massive production of IL-4, IL-5, and IL-13 cytokines which plausibly have deleterious consequences in lung as well as neighboring heart tissues due to their anatomical proximity.⁵¹ On top of that, there is also a high probability that this Th2-mediated hypersensitivity and inflammation can spill over from lung to neighboring myocardial tissues thereby inciting pathological consequences such as pericarditis, myocarditis, and arrhythmias.

Mice studies have revealed that vaccines containing aluminum adjuvants were previously associated with development of Antiphosphospholipid syndrome (APS) syndrome.^48,52,53 The mice developed fetal resorption and decreased fecundity secondary to superlative T-lymphocyte activation and polyclonal B-cell stimulation.⁵² Aluminum’s inherent cytotoxicity along with amassment of uric acid and release of intracellular DNA can putatively trigger NLRP3 inflammasome in dendritic cell and T-lymphocyte populations.^54–57 These cellular alterations can kickstart revamped adaptive immune responses leading to augmented manufacture of anti-phospholipid antibodies.^58,59 These autoantibodies can subsequently bind to negatively charged phospholipids, platelets and endothelial cells giving rise to APS syndrome.^58–60 Once APS syndrome materializes, secondary inflammation and hyper thrombotic tendency can fast-track the emergence of cardiovascular manifestations such as coronary artery disease, valvular disease, pulmonary hypertension, intra-cardiac thrombi and cardiomyopathy.⁶¹ Some of the vaccines that are currently in clinical and preclinical trials have incorporated TLR agonists (TLR3, TLR4, TLR5, TLR7/8, and TLR9) which have a propensity to instigate APS syndrome by a combination of bizarre immune counterattack, molecular mimicry and bystander effect.^54,62

Anaphylaxis can be one of the important mechanisms that might be plausibly responsible for onset of cardiovascular diseases particularly with incorporation of specific types of adjuvants. In such a scenario, adjuvants such as metal ions tend to act as haptens and tether with MHC class II complex of αβ T cells.⁶³ Once this interaction is consummated, there will be substantial alteration of binding structure of MHC class II molecule on T-cells which ultimately becomes the basis for launching a hypersensitivity response.⁶³ This type of mechanism holds true particularly when metal adjuvants such as aluminum, nickel, gold, and mercury are incorporated in vaccines.^43,63 With the polysaccharide adjuvants, activation of complement system, spilling of anaphylatoxins, mast cell stimulation, and anaphylactoid shock might be the lurking manifestations in all likelihood inciting indirect adverse repercussions upon the myocardium.⁴⁸

Autoimmune syndrome induced by adjuvants (ASIA) is a clinical spectrum characterized by myopathy, arthralgias, neurological impairments, gastrointestinal symptoms, respiratory signs, skin involvement, and auto-antibodies.^64,65 The prevalence of this disorder is specifically seen in the individuals with HLA DRB1 and HLA DQB1 genes.^64,65 Practically speaking, it is not feasible to assess the genetic profile of all the vaccinated individuals to assess their risk of autoimmune syndrome. But these reports bring to light genetic susceptibility as a lurking risk factor and this presumption needs to be taken into account particularly in individuals with prior history of autoimmune diseases.

According to Powell and Black, A.M. epitope spreading literally means immune cells and antibodies develop a proclivity to strike not only foreign antigens but also non-specific and non-reactive self-antigens during immune surveillance.⁶⁶ In other words, reactive T-cells develop broad specificity so that they are free to carry on their crusade against a wide variety of self-antigens that become unmasked secondary to tissue damage. Generally, this epitope spreading is bound to be protective and offered recovery in animal model of acute adjuvant arthritis. Unfortunately in other disease models such as multiple sclerosis and type I diabetes, haphazard and uncontrolled epitope spreading had been instrumental in sparking tissue injury and effectuated in disease progression and tissue injury.⁶⁶ Furthermore, epitope spreading has been implicated in the pathogenesis of autoimmune skin diseases, periodontitis, autoimmune bullous diseases, multiple sclerosis, and autoimmune encephalomyelitis.^67–70 In multiple sclerosis and autoimmune encephalomyelitis, propelling of disease progression is primarily contingent upon transition of autoreactivity from primary antigen epitopes to secondary antigen epitopes.⁶⁷ Epitope spreading is speculated to be occurring in two phases.⁷⁰ In the first phase, primary antigen epitope is processed by APC (antigen presenting cells) and presented to T-helper cells via MHC class II proteins.⁷⁰ Subsequently, activated T-helper cells collude with local macrophages and hereafter trigger tissue damage through secretion of mediators such as IFN -γ (Interferon gamma), TNFβ (tumor necrosis factor alpha) and LT (Leukotrienes).⁷⁰ In the secondary phase this preceding tissue damage instigates the spilling of secondary antigen that is eventually handled by APCs.⁷⁰ Sooner or later, this secondary antigen is presented to naïve T-cells, and this breeds cell- or antibody-mediated autoimmunity against secondary antigen.⁷⁰ This adjuvant mediated epitope spreading can be the underlying premise based on which cardiac damage emanates thereby prompting the onset of autoimmune-mediated myocarditis, pericarditis, arrhythmias, and cardiomyopathy in vaccinated individuals. Although this phenomenon cannot happen alone, it can be presumed to proceed parallelly along with other mechanisms set in motion by entry of toxic adjuvants into the body.

This is one of the predominant and plausible mechanisms through which adjuvants might sooner or later affect myocardial tissues. It is already well established that adjuvants by themselves or via uncovering of self-antigens might be influential in eliciting the generation of autoantibodies. In this hypothesis, these autoantibodies can conceivably act as self-antigens and trigger the generation of anti-idiotypic antibodies that at some point inflict myocardial damage.⁷¹ Previous reports theorize that autoantibodies were witnessed in 50% of the hospitalized COVID-19 patients.⁷² These antibodies in all likelihood can target self-antigens thereby enkindling the onset of autoimmune diseases such as multiple sclerosis, myositis, and others in COVID-19 patients.⁷² In addition to targeting self-antigens, these COVID-19 autoantibodies can restructure themselves to display novel antigen epitopes and operate as self-antigens.^71–73 With that being said, autoantibodies assuming a new role as self-antigens can spawn the breeding of anti-idiotype antibodies which will belatedly target myocardial tissues to trigger inflammation and cardiomyopathy.^71–73 The occurrence of this phenomenon is widely documented in diseases such as type I diabetes, Sjogren’s syndrome, Systemic lupus erythematosus (SLE) and neonatal lupus syndrome.^71,74 We presume that breeding of anti-idiotype antibodies might at least have a fair share of culpability in impelling the pathogenic signaling pathways that ultimately culminate into myocardial inflammation, cardiomyopathy and arrhythmias in COVID-19 vaccinated individuals.

Research suggests that these adjuvants are associated with serious immunological disorders with high propensity to develop autoimmunity, neurological inflammation, and other associated widespread systemic complications including cardiovascular disorders.^82,83 Aluminum is associated with autoimmunity-associated adjuvants syndrome (ASIA) as well as autism due to hyperactive immune system.^84,85 A recent study showed that aluminum adjuvants are associated with impairment of social impairments if administered in early postnatal development⁸⁶. Rare cases of antiphospholipid syndrome (APS) have also been reported with usage of aluminum adjuvants in tetanus vaccines.⁵³

Based on these studies, it would be prudent to use some caution and discourage the incorporation of aluminum adjuvants in COVID-vaccines to reduce the risk of autoimmune and inflammatory conditions including cardiovascular disorders.

Oil emulsion adjuvants are known to trigger the production of pro-inflammatory cytokines (IL-1, IL-17).^35,48,87 These cytokines are implicated in collapse of self-tolerance.^34,48 This will permit the percolation of self-reactive T-cell across the blood-brain barrier leading to increased propensity to develop autoimmune diseases including multiple sclerosis, experimental allergic encephalitis, and narcolepsy.^48,87–89 Due to their inherent nature to disrupt the self-tolerance, these adjuvants might possibly also increase the risk of inciting autoimmune cardiovascular disorders.

TLR9 agonists tagged to unmethylated CpG has been used in a wide variety of vaccines including disease (hepatitis B, influenza, malaria, anthrax, human immunodeficiency virus [HIV]), cancer (melanoma, non-small cell lung cancer) and allergic rhinitis.^90,91 TLR9 agonist used in hepatitis-B vaccination was associated with development of autoimmune Wegener’s granulomatosis, autoimmunity, Vitiligo, and hypothyroidism.^33,48,92