Acute and chronic manifestations due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection have continued to stimulate an ever-growing body of research. Much has been already published about the acute phase symptoms (i.e., anorexia, myalgia, productive sputum, fever, exhaustion, dyspnoea) of Coronavirus Disease 2019 (COVID-19), as initial non-specific upper airway symptoms due to viral infections before developing into pneumonia (1–3). From the neurological point of view, manifestations of the central and peripheral nervous systems, such as meningitis, encephalitis, myelitis, Guillain-Barré syndrome, Miller Fisher syndrome and cerebrovascular diseases, have been reported (4–10). In particular, recent scientific literature has revealed anosmia and dysgeusia as typical symptoms of the acute phase, which persist for weeks or months after infection and can be included among the so-called “long COVID symptoms” (11, 12). Prominent among the long-term consequences of COVID-19 are neuropsychiatric sequelae, also referred to as “brain fog” (13). The brain fog summarizes a range of variegate neurological complications, including inattention, short-term memory loss and reduced mental acuity (14). Impaired cognition, attention, concentration, and sleep are the commonly reported neuropsychiatric manifestations associated with the post-recovery phase of COVID-19 (14). This cognitive impairment may last for weeks or even months after the acute phase. To understand the pathophysiology of the brain fog, clinicians are currently trying to identify structural, metabolic and perfusion alterations in the brain using advanced neuroimaging techniques (15–19). Many ideas have been proposed to explain the aetiology of brain fog-associated symptoms, such as SARS-CoV-2 neuro-invasion, abnormal systemic and neuroimmunological response, cytokine overactivation, neuroglial dysfunction, virus-induced coagulopathy and endotheliopathy; however, the exact pathogenic mechanisms remain largely unknown (13, 20–23).

The important role of the complement system (C) in the pathogenesis of COVID-19 has emerged since the beginning of the COVID-19 (24–26). In general, C1q, a major player in the complement-mediated neuroinflammation (27, 28), can be produced by microglia cells; in concert with TNF-α and IL-1α, it can induce the polarization through a A1 phenotype, which is the neurotoxic and pro-inflammatory phenotype of astrocytes (29, 30). Reactive astrocytes increase the expression level of many genes of the classic pathway, such as C1r, C1s, C3, and C4, which are harmful for the neurovascular unit (28). However, little is known about the of role of the C system in neuroinflammation due to long COVID.

A recent study demonstrated that the subcomponent of the C lectin pathway, mannan-binding lectin (MBL), can bind to glycosylated SARS-CoV-2 spike protein, activating the C, and inhibiting SARS-CoV-2 cell entry in vitro (31). Single-nucleotide polymorphisms (SNPs) of MBL2 impact serum levels and functional activity of MBL (32). Interestingly, Stravalaci et al. demonstrated that genetic polymorphisms at the MBL2 locus are associated with COVID-19 severity (31). Therefore, MBL may act as a double-edged sword in the resistance to infection as well as in the pathogenesis of COVID-19. MBL deficiency is the most frequent C immunodeficiency, occurring in more than 10% of general population (33, 34). Interestingly, MBL polymorphisms have been reported as predisposing factors for susceptibly to infectious as well as systemic diseases such as systemic lupus erythematosus, rheumatoid arthritis and sepsis (33, 35–37).

Since the pathogenesis of the neurological “long COVID syndrome” remains largely unclear (20) and the C cascade is known to be involved in brain development, homeostasis, injury and regeneration (38), the aim of the study was to evaluate a correlation between the neurological “long COVID syndrome” and the C pathway. In particular, we determined MBL levels and activity in the sera of a cohort of COVID-19 patients affected by the “long COVID syndrome”. Thus, we divided the cohort into patients who developed brain fog and those who complained of hyposmia/hypogeusia as persistent symptoms, comparing them with healthy volunteers.

The term long COVID refers to symptomatic manifestations persisting at least 12 weeks after SARS-CoV-2 infection (40, 41). It can reveal in both sexes and at all ages, despite being more frequently present in women and in older people (42). Many individuals infected with SARS-CoV-2 developed one or more long-lasting symptoms which strongly impair daily function and quality of life. Among the range of clinical manifestations associated with long COVID, neurological issues are the most frequent (7–9, 41), occurring approximatively in one third of long COVID patients (43–47). The most common neurological symptoms comprise fatigue, headache, attention disorder, cognitive impairment, ageusia, anosmia, memory loss, dizziness (41). Therefore, not only acute phase symptoms of COVID-19, but also post-COVID sequelae, is a matter of concern for the healthcare system as well as clinicians.

One of the most common concerns about long COVID is the frequent onset of the so called “brain fog”. Brain fog has been previously associated with viral aftermath (48), but also with chronic fatigue syndrome (49), chemotherapy treatments (50), fibromyalgia and other chronic conditions (51). Since brain fog is considered as one of the most debilitating post-COVID symptoms, understanding its pathophysiological mechanisms is an urgent clinical need.

Multiple explanations have been offered towards the aetiology of symptoms associated with brain fog, such as viral neuro-invasion, virus-induced coagulopathy and endotheliopathy, and abnormal immunological response (20). A recent study has implicated long-term tissue damage and unresolved inflammation due to viral persistence and lymphopenia as the main cause of long COVID (43). Interestingly, chronic inflammation, especially neuroinflammation, is frequently the driving force behind COVID-associated cognitive impairment (52). In order to assess the contribution of inflammation as well as endothelial dysfunction, we first measured the serum levels of C-reactive protein (CRP) and Vascular Cell Adhesion Molecule-1 (sVCAM-1), respectively. sVCAM-1 is considered a marker of chronic cerebral blood flow dysregulation due to cerebral endothelial damage (53), whereas CRP is an acute phase reactant, a well-known systemic marker for inflammation (54). Our data showed a significantly higher level of CRP in BF +ve sera compared to both BF -ve and CTRL groups. These results partially confirmed the observations of Mandal et al. who reported elevated levels of CRP in long COVID-19 patients (55). Thus, we specifically noted a chronic condition of inflammation with higer levels of inflammatory biomarkers in BF +ve patients. On the contrary, sVCAM-1 levels were higher in the patients who had recovered from COVID-19 (BF +ve and BF -ve) compared to the CTRL group, confirming that SARS-CoV-2 infection can be considered an endothelial disease.

The C system is a protective factor in the early stages of SARS-CoV-2 infection enhancing virus elimination; however, an excessive or aberrant activation of the C, leading to hyperinflammation and endothelial injury, has extensively been attributed to severe COVID-19 pathogenesis (56, 57). Interestingly, MBL may also function as a double-edged sword in COVID-19 (31). MBL is a key player in the innate immune defense system, which allows us to fight various pathogens by direct recognition and neutralization, acts both as an opsonin for pathogens as well as a recognition molecule in C activation via the lectin pathway. MBL is capable of binding human immunodeficiency virus-1 (HIV-1) and hepatitis virus B (HBV), contributing to host susceptibility to infection and disease progression (58, 59). Reduced levels of circulating MBL are associated with an increased risk of invasive pneumococcal disease, other bacterial infections (e.g., Staphylococcus aureus, Pseudomonas aeruginosa, Clostridium difficile), sepsis, and death from pneumonia (60–62).

Around 10-20% of humans harbour point mutations in the MBL gene (MBL2), which are associated with low MBL activity and consequent higher incidence of infections. In fact, polymorphisms in MBL2 promoter and coding sequences adversely affect plasma levels, oligomeric state and ligand binding ability of MBL (63). Interestingly, genetic polymorphisms at the MBL2 locus have previously been associated with susceptibility to SARS-CoV-2 infection (64), and more recently, with COVID-19 severity (31, 65). Due to its preponderant and ambivalent role in infection and pathogenesis, we aimed at investigating a potential contribution of MBL to long COVID, especially brain fog onset. Our study found significantly lower levels of MBL and lower lectin pathway activation in the sera of BF +ve patients compared to BF –ve group, whereas the activation of classical and alternative pathways was not compromised. The frequency of patients with low or no MBL levels in the BF +ve group (31.3%) seems higher compared to those registered in the general population (>10%), revealing a possible connection between low MBL levels and brain fog onset. Thus, low levels of MBL not only reflect a reduced capacity to inhibit SARS-CoV-2 infection (31), but it can also predispose individuals to more severe symptoms during acute phase (65) as well as long COVID brain fog onset.

MBL level and lectin pathway activity are significantly reduced in subjects experiencing brain fog as a neuropsychiatric sequela in the post-acute phase of COVID-19. Thus, long COVID-associated brain fog can be listed among the variegate manifestations of increased susceptibility to infections and diseases induced by MBL deficiency.

The original contributions presented in the study are included in the article/supplementary material. Further inquiries can be directed to the corresponding authors.

The studies involving human participants were reviewed and approved by the local ethics committee (CEUR-2021-OS-48). The patients/participants provided their written informed consent to participate in this study.

Conceptualization, GF, RB, CA and PM. Methodology MT, GF, AB and GM. Resources, GF, LR, ML, and PM. Data curation, CA, MT, AM, GM, and LR. Writing—original draft preparation, GF, CA, LR, AB, AM, M.L and PM. Writing—review and editing, RB, AB, UK, GF, LR and PM. supervision, RB and PM.