It seems that the virus does not infect the brain directly. The scientists found evidence that antibodies – proteins produced by the immune system in response to viruses and other invaders – are involved in an attack on the cells lining the brain’s blood vessels, leading to inflammation and damage. The study was published in the journal Brain.
Brain tissue autopsy
“Patients often develop neurological complications with COVID-19, but the underlying pathophysiological process is not well understood,” Avindra Nath, MD, stated in a National Institutes of Health news release. Dr. Nath, who specializes in neuroimmunology, is the clinical director at the National Institute of Neurological Disorders and Stroke (NINDS) and the senior author of the study. “We had previously shown blood vessel damage and inflammation in patients’ brains at autopsy, but we didn’t understand the cause of the damage. I think in this paper we’ve gained important insight into the cascade of events.”
In this study, Dr. Nath and his team examined brain tissue from a subset of patients from their previous study. The nine individuals, ages 24-73 years, died shortly after contracting COVID-19. They were chosen because structural brain scans showed signs of blood vessel damage in the brain. The samples were compared with those from 10 controls. The team looked at neuroinflammation and immune responses using immunohistochemistry.
As in their earlier study, researchers found signs of leaky blood vessels based on the presence of blood proteins that normally do not cross the blood-brain barrier. This suggests that the tight junctions between the endothelial cells in the blood-brain barrier have been damaged.
Neurologic symptoms’ molecular basis
Dr. Nath and his colleagues discovered deposits of immune complexes on the surface of the cells. This finding is evidence that damage to endothelial cells was likely due to an immune response.
These observations suggest an antibody-mediated attack that activates endothelial cells. When endothelial cells are activated, they express proteins called adhesion molecules that cause platelets to stick together.
“Activation of the endothelial cells brings platelets that stick to the blood vessel walls, causing clots to form and leakage to occur. At the same time, the tight junctions between the endothelial cells get disrupted, causing them to leak,” Dr. Nath explained. “Once leakage occurs, immune cells such as macrophages may come to repair the damage, setting up inflammation. This, in turn, causes damage to neurons.”
Researchers found that in areas with damage to the endothelial cells, more than 300 genes showed decreased expression, whereas six genes were increased. These genes were associated with oxidative stress, DNA damage, and metabolic dysregulation. As the NIH news release notes, this may provide clues to the molecular basis of neurologic symptoms related to COVID-19 and offer potential therapeutic targets.
Together, these findings give insight into the immune response damaging the brain after COVID-19 infection. But it remains unclear what antigen the immune response is targeting, because the virus itself was not detected in the brain. It is possible that antibodies against the SARS-CoV-2 spike protein could bind to the angiotensin-converting enzyme 2 receptor used by the virus to enter cells. More research is needed to explore this hypothesis.
‘Brain fog’ explained?
The study may also have implications for understanding and treating long-term neurologic symptoms after COVID-19, which include headache, fatigue, loss of taste and smell, sleep problems, and “brain fog.” Had the patients in the study survived, the researchers believe they would likely have developed long COVID.
“It is quite possible that this same immune response persists in long COVID patients, resulting in neuronal injury,” said Dr. Nath. “There could be a small, indolent immune response that is continuing, which means that immune-modulating therapies might help these patients. So, these findings have very important therapeutic implications.”
The results suggest that treatments designed to prevent the development of the immune complexes observed in the study could be potential therapies for post-COVID neurologic symptoms.
This study was supported by the NINDS Division of Intramural Research (NS003130) and K23NS109284, the Roy J. Carver Foundation, and the Iowa Neuroscience Institute.
A version of this article first appeared on Medscape.com. This article was translated from Medscape French edition.