SARS-CoV-2 spreads through aerial and, to a lesser extent, fecal-oral route[23-25]. It infects target cells[26] by the interaction of the S1 subunit of the S protein with the angiotensin-converting enzyme 2 (ACE2) expressed on plasma membrane[27]. The transmembrane serine protease 2 (TMPRSS2) is also of crucial importance in cleaving the viral S protein and facilitating the envelope fusion with the plasma membrane[28]. Animal models showed that ACE2 is broadly expressed in cells of the respiratory apparatus[29,30], including goblet and Clara cells, type I and type II pneumocytes, ciliated and non-ciliated cells, endothelial and smooth muscle cells. In humans, the expression of ACE2 follows a similar pattern, being also detectable in many other organs (gallbladder, heart muscle, kidney, epididymis, breast, ovary, prostate, pancreas)[31]. The expression of TMPRSS2 is even broader than that of ACE2 across different human tissues[32,33].

In nasal cavities, the mucosal secretory and ciliated cells are more susceptible to SARS-CoV-2 compared to other nasal cells, due to a major expression of ACE2[33]. SARS-CoV-2 may also attack olfactory cells placed at the top of the nasal cavities and reach, through this way, the central nervous system[34]. Notably, in a study conducted on 417 European patients suffering from COVID-19, anosmia and dysgeusia have been reported in more than 80% of individuals[35].

By infecting type II pneumocytes, SARS-CoV-2 may deplete the alveoli of surfactant leading to cavity collapse[36]. Furthermore, SARS-CoV-2 may invade lung stem cells, which reside at the bronco-alveolar junction and express ACE2[37]. Viral or immune-mediated cytolysis may deprive lungs of progenitors of Clara cells and pneumocytes and impair the lung regenerative properties.

In the gastrointestinal tract, ACE2 and TMPRSS2[25] are particularly abundant in the epithelial cells of colon and ileum[32]. According to one study, goblet, Paneth and enterochromaffin cells have undetectable expression of ACE2 in human gut[38]. ACE2 has instead been found in salivary glands and tongue[31,39], and SARS-CoV-2 infection of these cells may be responsible for gustatory dysfunction[35].

Both lung and intestine harbor a population of immune cells at the localized sites that represent the first-line defense against pathogens. Immune cells may aggregate to form proper lymphoid organs (Waldeyer's tonsil ring in the upper airways[40] and Peyer's patches in small bowel[41]) or be scattered in sub-epithelial connective tissue, interweaving an intricate network with epithelial cells, vessels and fibroblasts.

Mucosal immunity is of fundamental importance in counteracting the entry of pathogens into the body and acts, therefore, during the very first phase of infections. In addition, mucosal immunity may also contribute to controlling the Phyla composition of resident microbiota, which, in turn, may be influenced by external agents, including SARS-CoV-2[42], or also influence the interaction with these pathogenic agents.

The innate immune system is highly represented in both respiratory and intestinal mucosa and continuously scavenges and counteracts potentially dangerous stimuli through a complex interplay of cells and soluble mediators[43]. These include resident macrophages and monocytes, natural killer (NK) cells, innate lymphoid cells, polymorphonuclear and dendritic cells, cytokines, chemokines, and the complement system. Intracellular agents, like viruses, are usually cleared through direct phagocytosis and cytolysis of infected cells. In cases where pathogen clearance is impaired, this can lead to persistence of antigenic stimulation, and to a shift in the immune response from innate to adaptive immunity. The latter is characterized by higher rapidity and robustness, due to antigen selectivity and memory[44].

The interferon pathway: Cells belonging to the monocyte-macrophage system are able to directly recognize viral motifs through pattern recognition receptors (PRR), including the endosomal toll-like receptors (TLR), the cytosolic platforms retinoic-acid inducible gene I (RIG-I), the nucleotide oligomerization domain-like receptors (NLR) and melanoma differentiation-associated protein (MDA)5, and to set an antiviral response, mainly explicated through the production of IFN[45]. Three different types of IFN have been characterized so far: type I IFN comprises IFNα and IFNβ, type II IFNγ and type III IFNλ, all of which act via a Janus kinase-signal transducer and activator of transcription proteins (JAK-STAT) signaling pathway[46]. Plasmacytoid dendritic cells (pDC) are the main source of type I and II IFN, while type III IFN are mostly released by epithelial cells at barrier interfaces of the respiratory and gastrointestinal tracts[45]. The antiviral property of IFN is explicated through transcription of IFN-stimulated genes (ISG), able, in turn, to potentially prevent each step of viral lifecycle[47]. Viruses have evolutionary developed many escape mechanisms against the IFN antiviral pathway. In this regard, it has been shown that SARS-CoV-2 open reading frame (ORF)6 can impede the transcription of some ISG[27]. Notably, the hyper-production of IFN (mainly type I) dominates the pathogenesis of some autoimmune diseases, like systemic lupus erythematosus (SLE) and primary Sjögren's syndrome (SS)[48], and may link the etiology of these diseases to a primitive viral infection[49,50].

The inflammasome pathway: In addition to IFN response, viral proteins and nucleic acids can activate the inflammasome platforms in cells belonging to the monocyte-macrophage system with the following production of IL-1 and IL-18[51,52], having a systemic pro-inflammatory effect. Specifically, viral pathogen-associated molecular patterns (PAMPs) of RNA viruses, including SARS-CoV-2, can activate the NOD-like receptor family pyrin domain (PYD)-containing 3 (NLRP3), while their RNA fragments can sensitize RIG-I and MDA5 and activate mitochondrial antiviral signaling (MAV) platforms in the cytosol. These events culminate in the intra-nuclear translocation of nuclear factor kB (NF-kB), the transcription of genes coding for pro-IL-1β and pro-IL-18 and the final conversion of these precursors into active cytokines by means of inflammasome-activated caspases[52]. Inflammasome platforms dominate the pathogenesis of auto-inflammatory syndromes, a group of genetically induced rheumatic diseases characterized by recurrent episodes of fever and other systemic manifestations, often triggered by external infections[53]. Recent evidence links autoinflammation to other multifactorial rheumatic diseases, including Behçet's syndrome, seronegative arthritis and Still's disease[54-56].

HMGB-1 and viral clearance: Viral nucleic acids can further complex with high mobility group box 1 protein (HMGB-1) released by necrotic cells and activated immune cells, and bind the receptor for advanced glycation end-products (RAGE), TLR2, TLR4 and TLR9 in either monocyte-macrophage cells or lymphocytes. This cascade of events culminates in lysosomal membrane disruption, pyroptosis, cytokine release and activation of autoreactive T and B cells[10,57,58]. Furthermore, the release of HMGB-1 in lungs can trigger acute inflammation through the secretion of pro-inflammatory cytokines and the recruitment of neutrophils in the interstitial and alveolar space[59]. The increase in HMGB-1 inversely correlates with ACE2, and drugs targeting HMGB-1 may be promising candidates in treating pulmonary manifestations of COVID-19[58].

Neutrophils and viral clearance: A recent study enrolling 8 COVID-19 patients, 146 community-acquired pneumonia patients and 20 controls evidenced a distinct metatranscriptomic profile in the bronchoalveolar lavage fluid (BALF) of COVID-19 patients[60]. Specifically, neutrophils were the most abundant cells and the chemokines CXCL17, CXCL2, CXCL8, CCL2 and CCL7, attracting neutrophils and monocytes, and systemic cytokines (IL-1β and type I IFN) were also hyper-expressed. Remarkably, the authors described a positive correlation between the inflammatory burden and the viral load, both of which tended to attenuate in the late stages of the disease and to persist in fatal cases. Neutrophils are a source of transforming growth factor (TGF)β, whose secretion in lung has been associated with interstitial fibrosis[61]. Moreover, neutrophils are typically increased in peripheral blood samples of COVID-19 patients and may contribute to systemic inflammation and thrombosis through the process of neutrophil extracellular trap (NET)osis[62]. The process of NETosis also plays a crucial role in the pathogenesis of some autoimmune diseases, like SLE and anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis[63,64]. Noteworthy, both NETosis and inflammasome activation are related to an impaired removal of reactive oxygen species (ROS), which may arise in lung during COVID-19, following the unbalanced ACE1/ACE2 ratio[65]. It was shown that type II pneumocytes of the elderly have a noted down-regulation in expression of superoxide dismutase (SOD)3[66], and the inadequate clearance of ROS may be at the basis of the severe forms of COVID-19 pneumonia observed in older when compared to younger individuals. Recent studies showed that sera of COVID-19 individuals may trigger NETosis in vitro, and that NETs may be considered to be a marker of a more severe course of disease[67,68].

The complement system and viral clearance: The complement system takes part in the antiviral response through inactivation of virions, the recruitment of immune cells, viral opsonization, cytolysis, and the induction of B and T cell responses[69]. Its activation may follow classical, alternative and mannose-binding protein pathways[70]. The glycoproteins S of SARS-CoV can interact with mannose binding lectin and trigger the complement cascade[69], inducing a direct (cytolysis of infected cells) or indirect (immune complex-mediated) viral clearance. However, the persistent activation of the complement system may lead to detrimental consequences, like organ fibrosis and musculoskeletal inflammation[69]. In SLE, the complement system cascade, triggered by autoantibodies and immune complexes via the classical pathway, is typically hyper-activated and consumption of its components represents a hallmark of the disease[71]. Hypo-complementemia has been associated with SLE disease activity and specific organ involvements, like glomerulonephritis[72]. Some viruses, like Parvovirus B19, notoriously associated to SLE[73], may affect the function of the complement system and indirectly influence the clinical course of autoimmune diseases[73,74]. Data concerning the interplay between SARS-CoV-2 and the complement system are controversial. According to one study, SARS-CoV-2 seems not to affect the serum levels of C3 and C4[75]. However, it was recently reported that the intravenous (i.v.) administration of the C3-inhibitor AMY-101 for 14 consecutive days ameliorated the laboratory and clinical picture of a 71-year-old patient affected by COVID-19 ARDS[76].

An efficient adaptive immune response is a basic requirement for a definitive viral clearance, as well as for the prevention of re-infection. According to the antigenic nature (intracellular vs extracellular), the adaptive immune response can follow a cellular or humoral pathway, with T and B lymphocytes being protagonists of the two scenarios, respectively.

T cell immunity: Dendritic cells act as connectors between the innate and adaptive immune system, since they may directly set an unselective pro-inflammatory response after interaction with pathogens or behave as antigen presenting cells (APC) and, thus, clonally activating lymphocytes. Proteins of SARS-CoV-2 envelope, like S, E and M, contain several epitopes which, once recognized by dendritic cells, can trigger an adaptive immune response[27]. Epitopes derived from the digestion of intracellular antigens are usually presented by the major histocompatibility complex (MHC) class I to CD8+ T lymphocytes, which ultimately act by means of a direct cytotoxic mechanism. Additionally, CD4+ T helper (h) cells may also be activated following the stimulation of their T cell receptors (TCR) by epitopes presented by MHC class II. According to the cytokine milieu, Th cells further differentiate in Th1, Th2, Th17, Th22, Th9, Th follicular (f) or T regulatory (reg) lymphocytes, giving rise to different immunologic cascades[77].

Peripheral lymphopenia is typically associated with COVID-19 and represents a predictive biomarker of morbidity and mortality[4]. SARS-CoV-2 may spread into lymphocytes through the interaction with CD147 molecule[78]. In T lymphocytes, CD147 seems to preside over the reorganization of lipid rafts and inhibits TCR-mediated proliferation[79]. The engagement of the SARS-CoV-2 S protein to CD147 expressed on T lymphocytes may result either in lymphopenia or in an impairment of T cell physiologic functions. Noteworthy, CD147 seems to be hyper-expressed in CD3+ T lymphocytes of SLE individuals compared to healthy controls, predisposing these patients to SARS-CoV-2 cytopathic effect on white blood cells[80]. Although the role of SARS-CoV-2 in human secondary and tertiary lymphoid organs is unclear, it has been shown that a virulent strain of bovine CoV may induce depletion of T cells in calf Peyer's patches and mesenteric lymph nodes[81]. In SARS-CoV-2 infected people, lymphopenia especially affects T and NK lymphocyte populations and memory cells[27]. In addition, CD8+ T and NK lymphocytes display an exhausted phenotype[82]. Among CD4+ T lymphocyte subsets, an unbalance in the Th17/Treg ratio has also been described in sick patients[83]. The increase in serum IL-17 and IFNγ, as well as the detection of multinucleated giant cells at the histological examination of lung specimens of COVID-19 patients, suggest the activation of Th1 and Th17 responses[27]. The expansion of autoreactive Th1 and Th17 lymphocytes at the detriment of Treg cells also lies at the basis of many autoimmune diseases[84,85].

B cell immunity: Th2 lymphocytes cooperate with B cells in supporting the final maturation in plasma cells and the secretion of specific antiviral antibodies, which may belong to different isotypic classes (IgM, IgG, IgA). Additionally, follicular dendritic cells can also stimulate B cells in a MHC-independent manner, by inducing the cross-linking of their B cell receptors (BCR) in response to multimerized antigens[86]. Although a depletion in CD19+ cells has been reported[27,87], COVID-19 patients are usually able to mount a humoral response and synthesize serum antibodies against SARS-CoV-2 antigens[88,89].

According to a study on 173 COVID-19 patients, more than 90% of them developed anti-SARS-CoV-2 IgM and approximately 79% patients had detectable IgG 15 days from the onset of the disease[89]. Notably, humoral immunity was not always paralleled by viral clearance, suggesting that other immunologic mechanisms may be additionally needed to effectively counteract the infection. Moreover, the authors found a significant positive correlation between higher antibody titer measured after 2 weeks of disease and clinical severity. This unusual association may rely on the activation of the complement system by immune complexes, the stimulation of immune cells (especially monocytes and macrophages) through the interaction of the fragment crystallizable (Fc) of antibodies with Fc receptors (FcR)[90,91] or antibody-dependent enhancement of viral entry[92]. FcR may be blocked by treating patients with i.v. human polyclonal Ig (IVIG)[93]: Of interest, this therapeutic approach proved to be effective in several reported cases of SARS-CoV-2 pneumonia, especially when started early[94,95]. IVIG play an immunomodulatory role, being therefore indicated for the treatment of autoimmune diseases[96]. IVIG, in fact, counteract the expansion and the activation of T, B and dendritic cells, interrupt the TLR signaling cascade and rebalance cytokine milieu[97], restoring immune tolerance. Many of these effects are mediated by the interaction of polyclonal IgG with FcγRIIb expressed on the plasma membrane of cells belonging to the monocyte-macrophage line[98], which is known for imparting an inhibitory effect on either the innate or adaptive immune response[99]. A polyclonal hyper-gammaglobulinemia is often found in patients affected by autoimmune diseases sustained by an aberrant humoral response, including rheumatoid arthritis (RA) and SLE[100]. Similarly to IVIG, polyclonal antibodies may exert an immunomodulatory role in infected rheumatic patients[101]. Natural antibodies with different specificities for pathogen conserved patterns are able to rapidly neutralize microorganisms and maintain B cell memory reservoir[102]. It is however unclear whether up-regulation of the humoral response may protect rheumatic patients from complications due to SARS-CoV-2 infection.

Despite the controversial findings regarding humoral immunity, the transfusion of sick patients with anti-SARS-CoV-2 neutralizing antibodies of already recovered individuals was shown to be an effective and safe therapeutic option[103,104]. Doubts still remain concerning optimal dosages and timing, donor recruitment and Ig screening assays[105].

Both the innate and acquired immune system can contribute in generating a cytokine storm syndrome or hemophagocytic lymphohistiocytosis, a potentially fatal condition triggered by the uncontrolled release of many pro-inflammatory mediators. This clinical condition can have a genetic etiology or follow infectious, autoimmune or cancer disorders. In rheumatic patients, a cytokine storm syndrome, also known as secondary macrophage activation syndrome (MAS), is more common during the pediatric age. MAS sub-clinically occurs in 30%-40% of patients suffering from juvenile idiopathic arthritis (JIA), and may have a fatal course in 10% of cases[106]. Minor prevalence and incidence rates have been reported in SLE (0.9%-9% of cases), Kawasaki syndrome (1%-2% of cases)[107,108] and adult Still's disease (10% of cases)[106]. In rheumatic patients, MAS can be triggered in 1/3 of cases by external infections exacerbating a pre-existent inflammatory background. The subsequent generation of PAMPs or damage-associated molecular patterns (DAMPs) may stimulate macrophages, T lymphocytes and inflammasome platforms, with the final transition of macrophages into histiocytes responsible for hemophagocytosis, tissue repair and fibrosis.

MAS symptoms include high fever, lymphadenopathy, hepato-splenomegaly, hyper-coagulation, visceral hemorrhages and multiorgan impairment (especially of liver, central nervous system and kidney). This medical condition also occurs in approximately 3%-4% of septicemic individuals[109]. Polymorphic variants of the PRR, cytokine gene expression kinetics, virus characteristics and an unbalance between pro-inflammatory and anti-inflammatory mediators have been all considered risk factors[110].

In COVID-19, the development of a cytokine storm syndrome has been reported to occur after an interval of 2-3 weeks since the onset of the disease and to be associated with multiorgan failure, ARDS, disseminated intravascular coagulation (DIC)[111] and death[27]. Released cytokines include IL-1β, IL-6, IL-18, IL-33, IFNγ and tumor necrosis factor–α (TNF-α)[112], which have both a local and systemic effect and are secreted by a panel of activated immune and non-immune cells (B and T lymphocytes, NK cells, macrophages, dendritic cells, neutrophils, monocytes, epithelial and endothelial cells). COVID-19 patients with augmented level of serum ferritin, TNF-α and IL-6 and lymphopenia have been considered to be at higher risk of developing high-grade systemic inflammation and MAS[87].

In lung parenchyma, cytokine storm syndrome eventually results in interstitial fibrosis, promoted by the hyper-activation of macrophages and lymphocytes. This event occurs in the most severe cases of COVID-19 and is associated with high morbidity and mortality[113]. Of note, lung fibrosis is characterized by distinctive histological patterns found in several rheumatic diseases, including systemic sclerosis (SSc) and RA[114]. Similarly to what happens in rheumatic lung fibrotic disease[115], the aberrant secretion of TGFβ by macrophages and lymphocytes plays a central role in extracellular matrix remodeling during COVID-19[61]. Finally, hyper-activation of cells belonging to the monocyte-macrophage system is also at the basis of the aberrant release of tissue factor (TF)[116], which activates the coagulation cascade through the extrinsic pathway and may trigger DIC.

APL antibodies, including anti-cardiolipin and anti-β2-glycoprotein Ig, have been detected in the sera of COVID-19 patients, but their contribution to thrombosis is uncertain[117,118]. Although benign aPL may appear during infections as a transient epiphenomenon[119], Zhang et al[19] reported 3 cases of ischemia of upper and lower limbs and cerebral infarcts in COVID-19 patients associated with positivity of anti-cardiolipin and anti-β2-glycoprotein IgA and IgG, that may orient towards an anti-phospholipid syndrome (APS). Contrary to anti-cardiolipin antibodies, anti-β2-glycoprotein antibodies are classically regarded as thrombogenic and represent a distinctive tract of APS. These antibodies may also appear during bacterial infections[120], but in these cases they display different biomolecular properties, being not able to elicit the coagulation cascade. However, since infections may trigger APS de novo[121] and even complicate a pre-existing APS with a catastrophic syndrome[120], follow-up is important to ensure whether such immunologic response might spontaneously recover or rather result into full-blown autoimmunity. Other authors have reported the presence of anti-52 kDa and anti-60 kDa Ro-SSA antibodies, antinuclear antibodies (ANA) and anti-citrullinated protein antibodies (ACPA) in the serum of 20%, 25%, 50% and 20% of COVID-19 patients, respectively[122,123]. Nevertheless, their pathogenicity is unknown, as well as the likelihood to induce autoimmune disorders in the long term. It is well-known that autoantibodies may be detectable in the serum of patients many years before the onset of autoimmune diseases[124,125], but it should not be ignored that autoantibody seropositivity may also be found in healthy individuals[126,127].

Neurological manifestations, involving central and peripheral nervous system have been reported in up to 36% of COVID-19 ARDS cases[128]. Brain neurons express ACE2[129] and can be infected by SARS-CoV-2 spreading via the olfactory cells and bulbs[35] and crossing the cribriform plate of the ethmoid bone[34]. Accordingly, neuronal damage may derive from direct viral cytotoxicity or from an immune-mediated attack[130]. Immune cells may recognize viral antigens expressed in infected cells, as well as cross-reactive self-antigens, like gangliosides. Neuronal inflammation may further generate DAMPs and autoantigens, thus amplifying the inflammatory cascade and predisposing to autoimmunity[131]. This pathway is likely exploited by several viral agents that contribute to the onset of autoimmune diseases, like multiple sclerosis or autoimmune peripheral neuropathy[132-135].

Pilotto et al[136] reported the case of a 60-year-old man presenting with COVID-19 neurologic symptoms. Nasopharyngeal swab was positive for SARS-CoV-2 and an increase in serum D-dimer was found along with a bilateral interstitial pneumonia. Cerebrospinal fluid was negative for SARS-CoV-2 and showed lymphocytosis and an increase in IL-6, IL-8, TNF-α and β2-microglobulin. Despite a treatment with antiviral agents, antibiotics and anti-malarials, the patient recovered only after the administration of i.v. pulses of steroids. Altogether, these findings suggest that neuronal damage might have been secondary to the immune response rather than to a viral cytopathic effect.

Guillain-Barrè[135] syndrome consists of an acute flaccid paralysis, symmetrically affecting upper and lower limbs. This medical condition develops after a respiratory or gastrointestinal infection in 2/3 of the cases and pathogenesis is immune-mediated. Specifically, the most accredited mechanism is the formation of cross-reacting anti-ganglioside antibodies that can induce a demyelinating polyneuropathy or a motor axonal neuropathy[135]. Cases of Guillain-Barrè syndrome with an unpredictable course have been reported prior or following the classical symptoms of SARS-CoV-2 infection[12,137]. Lymphopenia, thrombocytopenia and anamnesis may orient towards COVID-19 in those cases in which Guillain-Barrè syndrome is the primitive manifestation.

A recent meta-analysis, focusing on the neuropsychiatric manifestations occurring in patients undergoing severe coronavirus infections, showed that post-traumatic stress disorder, depression or anxiety, memory impairment, fatigue, insomnia and sleep disturbances were frequent in the post-illness stage (follow-up 60 days to 12 years) of SARS and Middle East respiratory syndrome (MERS)[138]. These symptoms also characterize two rheumatic dysfunctional diseases, namely fibromyalgia (FM) and myalgic encephalomyelitis[139]. These two clinical conditions share many similarities and may coexist; however, contrary to FM, myalgic encephalomyelitis has often been associated with previous infections and may be the result of an autoimmune process[140]. Although the development of an algodysfunctional syndrome may be plausible in COVID-19 recovered patients, epidemiologic data are still lacking.

A number of papers described the occurrence of immune-mediated blood cytopenia in COVID-19 individuals. Lazarian et al[13] reported 7 cases of autoimmune hemolytic anemia sustained by both warm and cold IgG. This condition occurred after a mean of 9 days in middle-aged patients affected by COVID-19 pneumonia. Of note, the majority of patients had coexistent lympho-proliferative disorders. Another two cases of hemolytic anemia sustained by cold agglutinins during COVID-19 have been described in the literature. The first patient was a 46-year-old woman, with a past history of immune thrombocytopenic purpura and splenectomy and testing negative for autoantibodies, who developed a rapidly fatal course of the disease[141]. The second patient was a 62-year-old oncologic man, who was hospitalized due to SARS-CoV-2-related fever, asthenia, respiratory symptoms and a mild hemolytic anemia precipitating in the following two weeks of follow-up[142].

Other authors reported a case of thrombocytopenic purpura occurring in a 65-year-old woman eight days after the onset of COVID-19 symptoms[14]. Anti-platelet antibodies were not detected and the bone marrow aspiration showed a normal cellularity.

Evans syndrome, consisting of the combination of autoimmune hemolytic anemia and thrombocytopenia and often following viral infections, has also been reported in a patient affected by COVID-19[143]. In this case, thrombocytopenia preceded hemolytic anemia and was already present at admission along with fever and respiratory symptoms. Given the current recommendations discouraging the use of steroids in COVID-19 ill patients[144,145], these hematologic complications were successfully managed with IVIG.

The contribution of viral agents to the pathogenesis of autoimmune myositis has been so far postulated and supported by the hyper-expression of MHC class I and the presence of CD8+ T lymphocyte foci at the histological examination of muscle biopsies of patients affected by polymyositis and necrotizing myopathy[146]. An interesting study identified three SARS-CoV-2 highly specific immunogenic epitopes in the antibody epitope repertoire of 20 adult individuals affected by dermatomyositis[147]. Epitopes mapped to SARS-CoV-2 2'-O-ribose methyltransferase, 3'-to-5' exonuclease proteins and RNA-dependent RNA polymerase, the latter being associated with HLA-A*01:01-restricted CD8+ T cell expansion.

To date, only one case of COVID-19-related myositis has been reported[15]. Myositis appeared as the first clinical manifestation and was followed after 4 days by fever and respiratory distress. The patient complained of hyposthenia of the muscles of the pelvic girdle, with clinical, laboratory and imaging findings being undistinguishable from those commonly found in autoimmune myositis. SARS-CoV-2 was detected in BAL but not in nasopharyngeal swab. Additionally, the patient was lymphopenic while autoantibodies were absent. Notably, respiratory symptoms and bilateral lower lobe ground-glass opacities at lung computer tomography (CT) scan are present in up to 40% of autoimmune myositis cases[148], making differential diagnosis with COVID-19 challenging in case of negative nasopharyngeal swab.

A few cases of COVID-19-associated myocarditis have also been reported. Myocarditis, consisting of the inflammation of myocardium, is a severe and potentially fatal complication of many autoimmune diseases, including acute rheumatic fever, RA, sarcoidosis, vasculitis and connective tissue diseases[149-151]. Additionally, myocarditis may be the result of viral infections, mostly sustained by Coxsackievirus B and adenovirus[150]. Myocarditis may manifest as acute coronary syndrome, arrhythmia, sudden death or heart failure. Viruses proliferating inside myocardiocytes can induce a direct tissue injury or trigger an immune response and inflammation, followed by myocardium remodeling and fibrosis. These events subsequently lead to systolic and diastolic dysfunction[149]. As endothelial and myocardial cells express ACE2, SARS-CoV-2 may directly invade these cells and hamper the beneficial effect played by ACE2 in counteracting heart failure[152]. In addition, the balance between IL-17 and IFNγ is crucial in dictating the final course (acute vs chronic damage) of myocarditis[153].

An acute COVID-19-associated eosinophilic myocarditis was described in a 17-year-old male patient without any prior risk factors[16]. Myocarditis appeared 2 days after the onset of a clinical picture characterized by gastrointestinal symptoms, headache and dizziness. SARS-CoV-2 was isolated in the nasopharyngeal swab. Eosinophilic myocarditis has been associated with drug and allergen hypersensitivity, ANCA vasculitis[149,151], and less frequently with viral infections[153].

Another case of COVID-19 myocarditis was described in a 57-year-old male subject, affected by arterial hypertension[154]. The patient developed ARDS and acute myocardial failure, characterized by an increase in serum troponin I and N-terminal pro-B-type natriuretic peptide (NT-proBNP), tachycardia at electrocardiogram, diffuse hypo-kinesis without pericardial effusion at transthoracic echocardiogram, and myocardial edema at cardiac magnetic resonance imaging (MRI). A combinatory therapeutic strategy based on the use of corticosteroids, tocilizumab and the experimental compound aldose reductase inhibitor was effective in inducing myocardial recovery, underlining that heart damage was mostly immune-mediated.

Patients with suspected or confirmed COVID-19 have been reported to suffer from polyhedral cutaneous manifestations[155]. Among them, chilblains have been described in several case series[17,156]. Chilblains are localized red or purple cutaneous lesions due to an inflammatory reaction triggered by the exposure to cold temperatures[157]. These manifestations can be found in SLE and other connective tissue diseases or vasculitis[158,159], and be related to an excessive production of type I IFN[160]. Autoimmune chilblains mostly occur in fertile or middle-aged woman, are associated with typical autoantibody patterns and tend to chronically persist, being less influenced by variations in external temperature[161]. On the contrary, COVID-19 chilblains were mostly reported in pediatric male patients approximately 2 weeks from disease onset. The lesions were mainly localized in the lower extremities of the toes or feet and rapidly disappeared following the recovery of the underlining infectious disease. Tests for SARS-CoV-2 at the oropharyngeal swab were positive in a few cases[17,156]. Given the chronologic interval, it may be argued that skin lesions following COVID-19 might be the result of the antiviral response characterized by the hyper-production of IFN.

Kawasaki syndrome is an autoimmune necrotizing vasculitis of medium- and small-sized vessels, affecting coronary arteries in more than 20% of cases[162]. The disease occurs in pediatric patients and classically starts with fever, mucocutaneous manifestations and cervical lymphadenopathy. Thus, external infections of the upper and lower respiratory tract, especially sustained by RNA viruses[163], have been suspected as triggering factors. This hypothesis is supported by the detection of CD8+ T aggregates and oligoclonal IgA in coronaries, along with the hyper-expression of ISG, and the presence of cytosolic inclusion bodies in ciliated bronchial epithelium[163].

An increased incidence of Kawasaki vasculitis-like syndrome has been reported in Bergamo province, which has been one of the most affected territories by SARS-CoV-2 infection in Italy. Here, Verdoni et al[164] described 10 cases of Kawasaki-like disease developing in children (mean age 7.5 years) between February 2020 and April 2020. Five of them had a classical syndrome with mucocutaneous manifestations and lymphadenopathy, while the others had incomplete forms. Compared to Kawasaki cases attending their medical Institute prior to the COVID-19 pandemic, the authors found a higher incidence of cardiac involvement, MAS and shock syndrome requiring adjunctive steroidal treatment. Additional cases of Kawasaki vasculitis, mainly occurring as incomplete forms, have been reported worldwide in young COVID-19 patients[18,165-167]. It is still debated whether this clinical condition represents a true Kawasaki vasculitis or rather reflects nonspecific post-infectious immune-mediated manifestations. Notably, in these patients, classical symptoms of COVID-19, including cough and dyspnea, appeared less frequent, while other manifestations like gastrointestinal symptoms[18,168] or cardiac hypotension were prevalent[165,166]. Heart involvement mostly consisted of pericardial effusion and myocarditis rather than coronary vasculitis[168]. Besides the detection of SARS-CoV-2 in nasopharyngeal swab, some children were found positive for group A Streptococcus[165,166] at the nasopha-ryngeal rapid test. Group A Streptococcus is responsible for rheumatic fever and pancarditis through a molecular mimicry mechanism[169] due to structural homologies between epitopes of the streptococcal N-acetyl-beta-D-glucosamine and M protein and heart valve endothelium, basement membrane and cardiac myosin, respectively[170]. Thus, a synergistic effect played by streptococci and SARS-CoV-2 in setting an immune response eventually responsible for this incomplete Kawasaki-like syndrome may be hypothesized.

So far, in United Kingdom, the term "pediatric inflammatory multisystem syndrome temporally associated with SARS-CoV-2" (PIMS-TS) was coined in order to distinguish this clinical condition from other apparently overlapping diseases[171]. PIMS, also known as multisystem inflammatory syndrome in children (MIS-C), is characterized by a variegate spectrum of clinical manifestations that may include incomplete forms of Kawasaki vasculitis, MAS, myocarditis and toxic shock. Epidemiological data showed that PIMS-TS usually follows COVID-19 symptoms after a delay of at least two weeks; is more common among Afro-Caribbean descendants; manifests with a more frequent cardiac, gastrointestinal and hematological involvement compared to Kawasaki syndrome; and, finally, appears more refractory to IVIG treatment than classical vasculitis, often requiring the addition of glucocorticoids or IL-6 inhibitors[172]. Belhadjer et al[173] recently reported a case series of 35 pediatric patients admitted to French and Swiss hospitals with fever, cardiogenic shock or signs of left ventricular dysfunction. SARS-CoV-2 infection was confirmed in 88.5% of them, prevalently by anti-SARS-CoV-2 IgG serodiagnosis. Interestingly, COVID-19 manifested with atypical symptoms, like gastrointestinal disorders and meningeal irritation, and, despite the clinical severity at admission, most of patients rapidly recovered with i.v. inotropic agents, steroids, anticoagulants and IVIG, whilst 3 patients needed a treatment with anakinra. From March 1 to May 17 2020, a total of 156 PIMS diagnoses were notified to the French National surveillance[174]. Among them, 79 were clearly associated with SARS-CoV-2 infection and were characterized by a high prevalence of incomplete forms of Kawasaki disease, myocarditis and MAS, requiring critical care intervention in 67% of cases. Similarly, a retrospective study in United Kingdom collected clinical data of 58 cases of PIMS-TS, following an atypical presentation of COVID-19[175]. Noteworthy, only 13 cases fulfilled the classification criteria of the American Heart Association for Kawasaki disease[176], whilst a hyper-inflammatory status leading to cardiogenic shock in 29 patients was reported. These results are in line with those reported by larger retrospective United States cohort analyses including 186 and 191 juvenile/pediatric cases of MIS-C, respectively[177,178]. Gastrointestinal symptoms were the most commonly reported followed by cardiovascular, hematological, mucocutaneous, ocular and pulmonary manifestations.