In a recent study posted to the medRxiv* preprint server, researchers determined the immune mechanisms associated with severe neurological manifestations of coronavirus disease 2019 (COVID-19).

Mounting evidence from COVID-19-related studies indicates that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is linked to prolonged neurological sequelae, termed Neuro-COVID. Immune alterations in the cerebrospinal fluid (CSF) have been reported in Neuro-COVID patients.

Further, previous studies have shown that overactive microglia were associated with detrimental immune responses in SARS-CoV-2 infection. Nevertheless, the fundamental pathophysiologic processes that cause central nervous system (CNS) derogation in COVID-19 are unknown. 

About the study

In the current prospective cross-sectional cohort study, the scientists evaluated whether there was a pattern of immune mechanisms in the plasma and CSF of SARS-CoV-2 patients that was severity-dependent. They also determined if these immune mechanisms were connected with brain imaging and clinical alterations. The study was conducted from August 2020 through April 2021. Subjects were recruited at two clinical sites in Zurich and Basel, Switzerland, from intensive care units (ICU), hospital wards, and outpatient clinics.

The inclusion criteria were individuals above 18 years and a COVID-19-positive test result. A total of 310 people were identified as potentially matching. However, 269 refused to participate, and one did not meet the inclusion criteria. Paired plasma and CSF samples and brain images were collected from the subjects. 

The SARS-CoV-2 cohort comprising 40 participants with a mean age of 54 and 42% females were prospectively categorized according to the severity of neurological symptoms, i.e., classes I, II, and III. 25 gender and age-matched plasma and CSF samples were collected from healthy controls (HCs) and inflammatory controls (ICs). Volumetric brain analysis was conducted using a healthy gender and age-matched control cohort of 36 subjects.

Results and discussion

The results show that SARS-CoV-2 patients had a non-inflammatory CSF profile but a class-incremental plasma cytokine storm. Patients with class II and III neurological symptoms associated with COVID-19 experienced severe neurological problems than class I. CSF lactate and glucose levels were high in class III patients, hinting at cerebral hypoxia, and the incidences of potential COVID-19-induced intracerebral hemorrhage and stroke in these patients supported this inference. Further, SARS-CoV-2-related death was high in the class III patients.

Patients with class III Neuro-COVID symptoms demonstrated polyclonal B cell responses enriched with immunoglobulin A (IgA) and IgG towards foreign antigens such as bovine serum antigen (BSA) and self-antigens like double-stranded deoxyribonucleic acid (dsDNA). These patients also had signs of blood-brain barrier (BBB) impairment, such as high levels of EN-receptor for advanced glycation end products (EN-RAGE), vascular endothelial growth factor A (VEGFA), and interleukin-8 (IL-8). In addition, higher CSF albumin, protein, IgM, IgA, IgG, CSF/plasma ratio, and the identification of peripherally synthesized spike (S)-antibodies (Abs) also supported the BBB impairment in class III patients.

Increased levels of anti-dsDNA IgG in the CSF were found in two of the four class III patients, emphasizing the connection between cardiovascular risk and severe SARS-CoV-2. Anti-gut microbial IgA Abs were also increased in patients with class III Neuro-COVID symptoms, suggesting gut barrier disruption in severe SARS-CoV-2 infection.

The elements causing microbiota dysbioses, such as diabetes, increased age, and hypertension, were high in class III relative to class I and II patients. Class III patients had increased levels of a decoy receptor named TNFRSF11B for receptor activator of nuclear factor kappa-Β ligand (RANKL), compared to ICs, which resulted in microglia overstimulation, and served as a biomarker predicting severe COVID-19. They also had elevated macrophage scavenger receptor 1 (MSR1) and cell surface transmembrane glycoprotein CD200 receptor 1 (CD200R1) levels.

Class III patients had significant abnormalities on the brain imaging, but the class I/II patients had no evidence of active neuroinflammation. Hepatocyte growth factor (HGF) was associated with tissue-regenerative responses in SARS-CoV-2-induced lung damage. This inference suggests that HGF might promote neuroregeneration following neuronal damage in COVID-19. 

Further, the decreased hepatocyte growth factor (HGF) of olfactory pathway regions correlated with CSF protein, leukocytes, and CSF/blood albumin ratio in SARS-CoV-2 patients. Choroid plexus volumes (CPVs) were also reduced in COVID-19 patients. The reduction in gray matter volumes (GMVs) in class III and II patients was due to the overexpression of programmed death-ligand 1 (PD-L1), an immune checkpoint protein.

Thus, the blockade of PD-L1 might counteract the immune malfunction and associated structural changes in the brain during SARS-CoV-2. The loss of GMVs in class II/III patients was associated with lower plasma levels of neuroprotective bone morphogenetic protein 4 (BMP-4) and growth differentiation factor 8 (GDF-8). This inference indicates that these GMV-related CSF/plasma parameters potentially serve as targets to avert long-lasting Neuro-COVID. 

Conclusions

The study findings illustrated Neuro-COVID-specific plasma and CSF changes offering insights into the pathomechanisms causing SARS-CoV-2-related neurological sequelae. The principal factors underlying severe Neuro-COVID are:

1) peripherally generated cytokine dysregulations,

2) compromised BBB with ingressing polyreactive auto-Abs,

3) microglia reactivity and neuronal injury, and

4) GMV loss. 

Overall, the study uncovered various possible targets for preventing SARS-CoV-2-related neurological complications.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Shanet Susan Alex

Shanet Susan Alex, a medical writer, based in Kerala, India, is a Doctor of Pharmacy graduate from Kerala University of Health Sciences. Her academic background is in clinical pharmacy and research, and she is passionate about medical writing. Shanet has published papers in the International Journal of Medical Science and Current Research (IJMSCR), the International Journal of Pharmacy (IJP), and the International Journal of Medical Science and Applied Research (IJMSAR). Apart from work, she enjoys listening to music and watching movies.