1. Introduction
The recent emergence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus, responsible for the disease caused by the novel coronavirus (COVID19), has already affected nearly seven hundred million individuals worldwide, with 6,919,573 deaths until September 2023 [
1]. As evidenced in recent years, the occurrence of viral epidemics characterized by unpredictable clinical outcomes has been highly frequent on a global scale [
2]. Therefore, the end of the public health emergency of international concern declared on May 5, 2023, by the World Health Organization (WHO) does not mean that COVID-19 has ceased to be a serious public health problem. Like in other viral infections [
3]–[
5], extensive studies have provided strong evidence of the frequent occurrence of neurological manifestations that resemble clinical patterns seen in autoimmune para- or post-infectious diseases triggered by SARS-CoV-2 [
6], [
7]. Moreover, the hypothesis that viral infections can act as triggers for the development of autoimmune diseases is not a new one [
8]. Due to the association between infection with other coronaviruses and autoimmunity, it is reasonable to assume that there is a connection between SARS-CoV-2 infection and certain autoimmune diseases that will be diagnosed later [
9]–[
11].
It is already well discussed in the literature that viral diseases have been identified as potential triggers of inflammatory demyelinating diseases (IDD) and autoimmune encephalitis (AE) [
12], [
13]. Numerous demyelinating disorders such as multiple sclerosis (MS), Neuromyelitis Optica Spectrum Diseases (NMOSD), Acute Disseminated Encephalomyelitis (ADEM), Myelitis and Myelin Oligodendrocyte Glycoprotein Antibody-associated Disease (MOGAD) have been described as post and para-infectious complications of COVID-19 [
14], as well as the most common forms of AE following SARS-CoV-2 infection are limbic encephalitis and NMDAR receptor antibody-mediated (anti-NMDAR) encephalitis[
15]. Important studies have identified the presence of autoantibodies associated with-glutamic acid decarboxylase 65-kilodalton isoform (anti-GAD65) [
16], myelin oligodendrocyte glycoprotein (anti-MOG) and others [
17]–[
20].
Several hypotheses have been proposed to explain the molecular basis of the loss of immune tolerance and induction of autoimmune mechanisms, including hyperinflammation syndrome caused by SARS-CoV-2, molecular mimicry (MM) by viral proteins, immune cell activation through bystander effect, the release of autoantigens from virus-damaged tissues, lymphocyte activation mediated by superantigens, and epitope spreading [
21]–[
24]. Some recent studies on MM have shown similarities between SARS-CoV-2 protein sequences and human proteins found in multiple organs/tissues (neurological, vascular, and cardiac), indicating the potential for cross-reactive immune recognition of these regions by T cells and antibodies produced by B cells [
22], [
25]–[
27]. However, the true spectrum of autoimmune conditions, their pathophysiology, prevalence, and the risk of their development in individuals after SARS-CoV-2 infection remain unknown, representing just the tip of the iceberg.
In this study, we intend to investigate the potential role of MM between SARS-CoV-2 antigens and human autoantigens of CNS autoimmune diseases.
4. Discussion
SARS-CoV-2 is widely studied for the generation of multi-system autoimmune reactions [
33]. In that sense, the triggering of CNS autoimmune diseases seems to be a consequence of an imprecise adaptive immune system response to the presence of viral antigens. It is well known that some viruses demonstrated neurotropic features [
34]–[
37], and replication within the brain tissue, as shown by our team with Zika virus (ZIKV) [
34]. However, even ZIKV nervous system manifestation is not always associated with acute infection and MM seems to justify these events [
38], [
39]. Similarly, in COVID-19, viral load or severe acute infection doesn’t seem to be the only mechanism to justify CNS involvement [
40], [
41]. The occurrence of IDD phenotypes and encephalitis as para or postinfectious event, seems to be an immune-mediated response induced by SARS-CoV-2 [
42].
A large study from various global health organizations found that the incidence of autoimmune diseases was significantly higher in the COVID-19 cohort compared to the non-COVID-19 group after a 6-month follow-up period [
43]. Another similar study identified a 43% higher likelihood of developing an autoimmune disease between 3 to 15 months after infection compared to a non-COVID-19 cohort [
44]. Despite the progress made, cases of CNS autoimmunity after COVID-19 are rare and mainly consist of isolated case reports or case series, which provide limited information regarding clinical outcomes [
45].
Although the target of such supposed autoimmune mechanisms, precisely regarding the CNS manifestations, are still not fully understood, our findings suggest that cross-reaction with selected CNS proteins associated with autoimmune brain diseases is possible to occur secondary to the immune response to SARS CoV-2 infection. However, the risk of developing these diseases or experiencing relapses in the setting of COVID-19 remains relatively low [
46]. In our cohort, 3 patients developed IDD following SARS-CoV-2 infection or vaccination. Since the CNS autoimmune manifestations after COVID-19 are rare, it is expected that genetic predisposition plays an essential role in the disease mechanism [
47]. Despite the low frequency, the identification of IDD in such circumstances is primordial, taking in consideration the high prevalence of SARS-CoV-2 infection and the possible critical state in which the patients may encounter it. For example, our first patient evolved with EDSS 9.5, a near-death experience in a young individual with no previous comorbidity. Moreover, is important to consider SARS-CoV-2 infection as a possible trigger of IDDs, because some patients present demyelinating events as the only symptom of COVID-19, as happened with our second patient.
Molecular mimicry has been described as an essential immune mechanism involved in autoimmune reactions, especially from viruses [
48]. The sharing of a linear amino acid sequence or a three-dimensional conformation fit between an antigen of the virus and a host self-protein can trigger a cross-reaction from the adaptive immune system and, therefore, have a major role in initiating an autoimmune response in genetically susceptible individuals [
49]. Several researchers have recognized molecular mimicry as a component of COVID-19 pathophysiology [
50]–[
52]. As example, Lucchese et al. observed that molecular mimicry between SARS-CoV-2 antigens and respiratory pacemaker neurons may contribute to understanding respiratory failure [
53]. Hence, MM may be a key component of the immune system dysregulated response in the CNS.
In this study, 80 possible arranges of identity among SARS-CoV-2 antigens and self-antigens related to autoimmune CNS diseases were made. Among these arranges, eleven models had a significant linear and three-dimensional overlap of autoimmune CNS proteins and SARS CoV-2 proteins (TM-score ≥ 0.5). NMDAR1, MOG and MPO were the self-antigens with more significant identity with SARS-CoV-2 antigens, followed by GAD65. Notably, dysregulated serum levels of autoantibodies NMDAR, GAD65 and MOG were detected in patients with severe COVID-19 compared with healthy controls and mild COVID-19 patients [
54].
MS is a classic example of an autoimmune CNS disease characterized by chronic inflammation and demyelination [
55]. SARS-CoV-2 must likely act as a precipitating factor rather than being a direct cause of MS, triggering autoimmunity in genetically predisposed individuals. In our cohort, two patients had SARS-CoV-2 related events (vaccination and infection) as trigger for MS. Both patients were female, however their ages, comorbidities, symptoms and MRI lesions were considerably different, highlighting the importance of genetic predisposal and other environmental factors on the course of the disease [
56].
MBP, MOBP, PLP and MAG are myelin proteins known to be critical autoantigens in causing demyelination in CNS leading to MS [
57], [
58]. In our study, the TM-scores among these proteins and SARS-CoV-2 antigens were low, but not considered randomly arranged, unrelated proteins. This can mean that MM among these MS autoantigens is feasible, however, the evidence is not strong. Nevertheless, MPO, a pro-oxidative enzyme associated with immune-inflammatory, oxidative stress pathways, and cortical demyelination [
59], has been MPO gaining acceptance as an important modulator of MS activity [
60]. Higher expressing MPO genotype is overrepresented in early-onset MS in females [
61] and immunohistochemical analysis shows that MPO is present in microglia in and around MS lesions [
61]. This study found a significant overlap of MPO among 3 different SARS-CoV-2 antigens (M, N, nsp3). SARS-CoV-2 ability to mimic MPO seem to provide a greater threat for triggering new onset MS or worsening of MS symptoms in genetically predisposed patients, as seen during the COVID-19 pandemic [
62], [
63].
MOGAD is an emerging subset of CNS demyelinating disease [
64], [
65], and has also been related to COVID-19 [
14]. In our study, one patient evolved with MOGAD during an asymptomatic SARS-CoV-2 infection. Besides, our
in silico analysis showed that MOG shared significant linear and three-dimensional identity with 3 different virus antigens (S, ORF7a and nsp1), being the most prominent overlap with Spike protein. This goes accordingly to recent literature, that identified anti-MOG antibodies in the acute and post-infectious phase of SARS-CoV-2 infection and COVID-19 vaccination [
66], [
67]. Moreover, the literature has shown that diseases associated with anti-MOG almost tripled during the COVID-19 pandemic [
68].
Our study highlights that NMDAR1 has 3 domains with significant linear and three-dimensional identity with SARS-CoV-2 antigens, including Spike protein, and all of them have binding capacity to T-cells that would consider them epitopes. It has been proposed that SARS-CoV-2’s molecular mimicry may induce anti-NMDAR encephalitis after COVID-19 [
69]. This may be a key mechanism beneath CNS manifestations of COVID-19 disease and vaccination associated with anti-NMDAR antibodies [
15], [
70].
Moreover, GAD65 had the strongest binding capacity to HLA in this work, with two different combinations of mimicry. Cases of autoimmune encephalitis associated with GAD65 have been described following SARS-CoV-2 infection [
71], [
72]. This finding reinforces the association of SARS-CoV-2 MM and the clinical findings related to anti-GAD65 antibody.
A growing body of evidence has demonstrated the relationship between ADEM and SARS-CoV-2 infection [
73]. ADEM following SARS-CoV-2 infection and vaccination have been associated to MOG [
74]–[
77] and NMDAR [
78] antibodies. Interestingly, in our study, both MOG and NMDAR are associated with Spike protein, the most common antigen presented in SARS-CoV-2 vaccination, which may indicate the relevance of MM in post-COVID ADEM manifestations.
In this cohort, 52.45% of the COVID-19 ICU patients presented cognitive impairment during the post-acute phase of COVID-19. Notably, NMDAR, GAD65 and MPO may be involved not only in acute encephalitis or demyelinating events but also in neurocognitive and psychiatric manifestations, frequently seen in long COVID patients [
69], [
79]–[
81]. Pathological results in cognitive screening were associated with the presence of antibodies against NMDAR and GAD65 in CSF of long COVID patients [
82]. The MM between these autoantibodies and SARS-CoV-2 antigens may be a prominent asset to understanding the pathogenesis of long COVID cognitive and psychiatric symptoms.
Genetic susceptibility seems to explain the heterogeneity of response to immune tolerance breakdown and molecular mimicry between autoantigens and viral proteins [
49]. Due to the limited knowledge about genetic susceptibility to explain mechanisms involved in the pathophysiology of AE, we choose
HLA variants with known associations with CNS autoimmune diseases [
58], [
83]–[
87]. Interestingly enough, all eleven combinations with significant linear and three-dimensional identity presented at least one epitope with strong or intermediate binding capacity to the chosen HLA subtypes. In this manner, the investigation of the connection between
HLA alleles related to CNS autoimmune diseases and the MM found in this paper can strengthen the results and possibly help to elucidate the pathophysiology of these manifestations.
It is worth to highlight the results regarding the S protein. Spike or its fragments, has the ability to cross the blood-brain barrier (BBB), irrespective of the presence of the viral RNA [
88]. Furthermore, some cases have reported an association of CNS demyelination events with the use of vaccines with S protein as the main antigen for the generation of immunological memory, which has become a major concern for health authorities worldwide [
89]. Thus, the MM regarding S may be more common than the others described in this article. Indeed, both MOG and NMDAR1, which presented significant linear and three-dimensional overlap with Spike, have been associated with COVID-19 in a more expressive way than the other autoimmune affection [
15], [
68], [
69], and have been related to COVID-19 vaccination [
70], [
90]–[
92]. One of our patients triggered IDD following COVID-19 vaccination. Although is not possible to affirm causality, MM must be considered as a possible mechanism for this phenomenon.
As limitations of this study, is important to mention that is a theoretical work, however it is based on our cohort findings and recent literature studies regarding SARS-CoV-2. Besides, it uses validated software’s to give results as close as possible to reality. Additionally, the study used a limited number of HLA alleles in the prediction of T-cell binding capacity, only the most common HLA alleles on the literature associated with CNS autoimmune diseases, in order to increase the specificity of the results. Thereby, is possible that some epitopes of rarer HLA weren’t included in this study. Further studies are needed to validate the in silico work described here, as well as to understand probable genetic susceptibility some individuals must develop such manifestations.
Author Contributions
Conceptualization, EGG. SVAL, MMS and FLFD.; methodology, EGG, FLFD, JVRV, RAF; software, EGG and FLFD; validation, ALS, RAF, JVRV, CFT, VGCC, VCSRP, MMS and FLFD.; formal analysis, EGG.; investigation, EGG, ALS, LAD, SVAL, CFT, VGCC, VCSRP, MMS and JVRV; resources, SVAL; data curation, ALS, EGG, RAF, LAD, FLFD, JVRV, CFT, VGCC, VCSRP, MMS and SVAL; writing—original draft preparation, EGG.; writing—review and editing, ASL, RAF, SVAL, FLFD and JVRV.; visualization, EGG, LAD, ASL, RAF, FLFD, JVRV, CFT, VGCC, VCSRP, MMS and SVAL; supervision, FLFD, MMS and SVAL; project administration, SVAL; funding acquisition, SVAL. All authors have read and agreed to the published version of the manuscript.
Data Availability Statement
The results presented in this article are supported by data in other articles published in MDPI journals. The clinical profile and risk factors for severe COVID-19 in our cohort of hospitalized patients, comparing the First and Second Pandemic Waves was published on the Journal of Clinical Medicine in 2023, under the doi: 10.3390/jcm12072568. Disease severity was associated with older age, pre-existing neurological comorbidities, higher viral load, and dyspnea. Laboratory biomarkers related to white blood cells, coagulation, cellular injury, inflammation, renal, and liver injuries were significantly associated with severe COVID-19. During the second wave of the pandemic, the necessity of invasive respiratory support was higher, and more individuals with COVID-19 developed acute hepatitis, suggesting that the progression of the second wave resulted in an increase in severe cases.
We used transcriptome analysis of these patients to understand key genes and cellular mechanisms that are most affected by the severe outcome of COVID-19. Transcriptomic analysis revealed 1009 up-regulated and 501 down-regulated genes in the SARS group, with 10% of both being composed of long non-coding RNA. Ribosome and cell cycle pathways were enriched among down-regulated genes. The most connected proteins among the differentially expressed genes involved transport dysregulation, proteasome degradation, interferon response, cytokinesis failure, and host translation inhibition. Furthermore, interactome analysis showed Fibrillarin to be one of the key genes affected by SARS-CoV-2. This protein interacts directly with the N protein and long non-coding RNAs affecting transcription, translation, and ribosomal processes. This work was published on the International Journal of Molecular Sciences in 2022, under the doi: 10.3390/ijms232113588.
We also published a study that aimed to establish a relationship between miRNA and neurological manifestations in our cohort of COVID-19 patients co-infected with HHV-6 and evaluate miRNAs as potential biomarkers. miRNA analysis by real-time polymerase chain reaction (qPCR) revealed miRNAs associated with neuroinflammation were highly expressed in patients with neurological disorders and HHV-6 detection. When compared with the group of patients without detection of HHVs DNA and without neurological alterations, the group with detection of HHV-6 DNA and neurological alteration, displayed significant differences in the expression of mir-21, mir-146a, miR-155 and miR-let-7b (p < 0.01). This work was published on the International Journal of Molecular Sciences in 2023, under the doi: 10.3390/ijms241311201.
One of our authors (Salvio, AL) also analyzed the Effectiveness of Household Disinfection Techniques to Remove SARS-CoV-2 from Cloth Masks. The study showed that all biocidal treatments successfully disinfected the tissue tested. This work was published on Pathogens in 2022, under the doi: 10.3390/pathogens11080916.