Abstract
The pathogenesis of long COVID (LC) still presents many areas of uncertainty. This leads to difficulties in finding an effective specific therapy. We hypothesize that the key to LC pathogenesis lies in the presence of chronic functional damage to the main anti-inflammatory mechanisms of our body: the three reflexes mediated by the vagus nerve, the hypothalamic-pituitary-adrenal (HPA) hormonal axis, and the mitochondrial redox status. We will illustrate that this neuro-endocrine-metabolic axis is closely interconnected and how the SARS-CoV-2 can damage it at all stages through direct, immune-inflammatory, epigenetic damage mechanisms, as well as through the reactivation of neurotropic viruses. According to our theory, the direct mitochondrial damage carried out by the virus, which replicates within these organelles, and the cellular oxidative imbalance, cannot be countered in patients who develop LC. This is because their anti-inflammatory mechanisms are inconsistent due to reduced vagal tone and direct damage to the endocrine glands of the HPA axis. We will illustrate how acetylcholine (ACh) and cortisol, with its cytoplasmatic and cellular receptors respectively, are fundamental players in the LC process. Both Ach and cortisol play multifaceted and synergistic roles in reducing inflammation. They achieve this by modulating the activity of innate and cell-mediated immunity, attenuating endothelial and platelet activation, and modulating mitochondrial function, which is crucial for cellular energy production and anti-inflammatory mechanisms. In our opinion, it is essential to study the sensitivity of the glucocorticoids receptor in people who develop LC and whether SARS-CoV-2 can cause long-term epigenetic variations in its expression and function.
Introduction
Long COVID (LC) was redefined by the US National Academies of Sciences, Engineering, and Medicine as “an infection-associated chronic condition that occurs after severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2) infection, present for at least three months, manifesting as continuous, relapsing, or progressive disease affecting one or more organ systems” (A Long COVID Definition: A Chronic, Systemic Disease State with Profound Consequences, 2024). This consensus classified LC among chronic health issues following infections of any kind, such as myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) and Lyme-associated chronic illness, without needing laboratory confirmation of the initial infection (A Long COVID Definition: A Chronic, Systemic Disease State with Profound Consequences, 2024). LC can worsen preexisting conditions or emerge as new issues, impacting individuals’ ability to work, attend school, care for families, and manage self-care. A recent meta-analysis found that nearly half of Coronavirus disease 2019 (COVID-19) survivors reported lingering symptoms around 120 days post-recovery, with about almost 7.3% developing high-morbidity LC, predominantly affecting young, productive individuals (Mastrorosa et al., 2023). In minors, LC is linked to a more severe acute illness requiring hospitalization, whereas in adults, it often occurs in those with mild to moderate COVID (Su et al., 2022). Risk factors for developing LC include female sex (Bai et al., 2022), multimorbidity, unvaccinated status (Robertson et al., 2023), EBV reactivation, autoimmunity disorders, asthma, type 2 diabetes (Su et al., 2022), attention deficit hyperactivity disorder (ADHD), chronic urticaria and allergic rhinitis (Merzon et al., 2022), although a third of people with LC have no identified pre-existing conditions (Davis et al., 2023). Unfortunately, no biological marker for LC has been found, and no cures exist, with spontaneous recovery being rare (Ely et al., 2024). Actually, various pathogenetic mechanisms of LC have been proposed, including persistent infection, autoimmunity, antigenic mimicry, mitochondrial damage, vagus nerve (VN) injury (Woo et al., 2023), hypercoagulability, microbiota alterations (Alvarez-Santacruz et al., 2024), neurotropic virus reactivation, hypothalamic-pituitary-adrenal (HPA) gland axis dysfunction, and epigenetic modification in gene cell expression (Cheong et al., 2023). Different clinical form of LC have been identified, based on symptoms, but it remains unclear if they reflect distinct pathogenetic pathways. SARS-CoV-2 infection likely has a pleiotropic effect, with multiple causative pathways present concurrently. Notably, LC symptoms fluctuate unpredictably, and the hierarchy and relationships between these mechanisms are still not fully understood, nor is it clear if a common thread connects them.
Vagus nerve damage may play a central role in long COVID pathogenesis, potentially reducing the body’s anti-inflammatory response and mitochondrial cell function
Our hypothesis proposes that VN damage is a primary contributor to LC development, leading to dysautonomia and disruption of key anti-inflammatory pathways, and mitochondrial function (VanElzakker, 2013). SARS-CoV-2 has been shown to induce both direct and indirect VN damage (Andersson and Tracey, 2024). Woo et al. conducted a post-mortem analysis demonstrating direct SARS-CoV-2 infection of the VN, accompanied by significant neuroinflammation (Woo et al., 2023). Specifically, by studying cells gene expression, they observed increased interferon (IFN) signaling in activated monocytes, glial cells, endothelial cells, and Schwann cells within brain tissue, irrespective of viral load (Woo et al., 2023). Conversely, SARS-CoV-2 RNA load in vulnerable neurons showed a positive correlation with upregulation of stress response mechanisms (e.g., autophagy, proteasomal breakdown). However, these higher levels of intracellular SARS-CoV-2 RNA were also associated with reduced activity in genes responsible for neurotransmitter signaling and neuronal transport, showing a dose-dependent direct VN damage and a dose independent neuroinflammatory response (Woo et al., 2023). Moreover, autoantibodies targeting receptors involved in the vagal anti-inflammatory reflex have been found in convalescent COVID-19 patients, indicating potential functional impairment beyond nerve damage (Dobrowolska et al., 2023). In addition to the VN impairment mechanisms already mentioned, alterations in coagulation and the reactivation of herpes viruses can collaborate to neurotoxicity (VanElzakker, 2013). Interestingly, VN dysfunction triggered by SARS-CoV-2 infection occurs early on during the infection and contributes to the virus’s virulence. In the acute phase, the impairment of the anti-inflammatory reflex may sustain the development of severe cytokine storms, leading to conditions like Acute Respiratory Distress Syndrome (ARDS) and micro embolisms, which increase organ damage and worsen prognosis severity (Johnston and Webster, 2009; Song et al., 2020). Accordingly, Woo et al. found that COVID-19 patients who died exhibited a lower respiratory rate compared to survivors, even in the presence of elevated blood carbon dioxide and severe respiratory insufficiency. This finding suggests that damage to the VN, resulting in autonomic dysfunction, may contribute to respiratory failure in severe COVID-19 cases (Woo et al., 2023).
In the post-COVID phase, impaired vagal signaling could have a pivotal role in preventing the body from restoring inflammatory balance, ultimately perpetrating the LC syndrome (Llados et al., 2024). To strengthen our position, this mechanism was already postulated in a cohort of patients affected by irritable bowel syndrome (IBS) and Crohn’s disease (CD) (Pellissier et al., 2014). In fact, this relevant research found that healthy individuals with higher vagal tone in the morning, defined as an high heart rate variability (HRV), tended to have lower cortisol levels in the evening (Pellissier et al., 2014). This inverse relationship proves the clear neuroendocrinal correlation between VN and axis and suggests that greater vagal tone is associated with better regulation of the stress hormone cortisol (Pellissier et al., 2014). Interestingly, this correlation was not observed in patients with IBS, suggesting an impairment in the vagal anti-inflammatory mechanisms. Further supporting this finding, lower vagal tone in CD patients correlated with higher levels of the inflammatory marker tumor necrosis factor-alpha (TNF-α) (Pellissier et al., 2014). The same study underlined that IBS patients with low vagal tone display high epinephrine levels, indicating heightened and potentially maladaptive sympathetic nervous system activity (Pellissier et al., 2014). Accordingly, HRV was found to be reduced in LC patients compared to a control group, thus reflecting a reduced VN tone (da Silva et al., 2023)...
Conclusions
In conclusion, the model of a dysfunctional VN nerve-HPA-mitochondrial axis can provide a comprehensive explanation for the various alterations observed in LC. Disruptions to more components of this intricate system, whether through direct damage, immune dysregulation, autoantibody interference or epigenetic gene modification in predisposed patients may determine the development and the persistence of this complex syndrome. As a consequence, individual susceptibility to LC likely can depends on baseline vagal tone, cortisol sensitivity, basal mitochondrial cell functioning and immunity factors (Dedoncker et al., 2021). According to this theory, individuals with mild pre-existing VN impairment may be more vulnerable to developing LC, particularly when faced with cortisol reductions and epigenetic modifications in GR signaling pathway and mitochondrial genes. Specifically, investigations into the long-term effects of chronically low morning cortisol levels and their impact on the epigenetic regulation of mtDNA will be instrumental, in our opinion, in unraveling the contributions of mitochondria to LC pathogenesis. For example, it has been hypothesized that dysregulation of cellular microRNAs (miRNAs), which are small non-coding RNAs that modulate gene expression by binding to messenger RNA, may contribute to the upregulation of the IL-6/signal transducers and activators of transcription 3 (STAT3) proinflammatory axis, leading to pain in LC patients (Reyes-Long et al., 2023).
With this paper, we aim to highlight the potential fundamental connection between VN dysfunction, the resulting deficit of Ach and cortisol, which directly leads to mitochondrial dysfunction, perpetuating the process and contributing to the exacerbation of LC symptoms. This theory could also partly explain why SARS-CoV infection accelerates aging (Strong, 2023) and facilitates the development of autoimmune diseases (Sharma and Bayry, 2023).
Given this complexity, an integrated and multidimensional approach to patient evaluation is paramount. This evaluation should encompass assessments of VN basal tone, comprehensive autoantibody panels, mitochondrial function, and potentially even genetic typing of relevant receptors and their epigenetic regulation. A thorough understanding of how stress affects mitochondrial physiology requires a multifaceted evaluation that includes measuring mitochondrial copy number, function, and methylation status. An integrated, multidimensional evaluation could enable personalized treatments by targeting simultaneously specific pathways, offering the best hope for addressing LC challenges.
Statements
Data availability statement
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.
Acknowledgments
The authors gratefully acknowledge Dr. Enrica Cipriano for creating the image.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.