Explanation Proposed for Long-COVID Symptoms in the CNS

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BOSTON — The neurologic symptoms of long COVID appear to be explained by a phenomenon known as antigenic imprinting, which involves a misdirected immune response to the SARS-CoV2 virus, according to a collaborative study presented at the 2023 annual meeting of the American Academy of Neurology.

Already documented in several other viral infections, such as influenza and human immunodeficiency virus, antigenic imprinting results in production of antibodies to previously encountered viral infections rather than to the immediate threat, according to Marianna Spatola, MD, PhD, a research fellow at the Ragon Institute, Harvard University, Cambridge, Mass.

Original antigenic sin

In the case of persistent neurologic symptoms after COVID, a condition known as neuroPASC (neurological postacute sequelae of SARS-CoV2 infection), antibodies produced for previously encountered coronaviruses rather than for SARS-CoV2 might explain most or all cases, according to the data Dr. Spatola presented.

The evidence for this explanation was drawn from a study of 112 patients evaluated months after an acute episode of COVID-19. Of these, 18 patients had persistent neurologic dysfunction. When compared with the 94 whose infection resolved without sequelae, the patients with prolonged neurologic impairments had relatively low systemic antibody response to SARS-CoV2. However, they showed relatively high antibody responses against other coronaviruses.

This is a pattern consistent with antigenic imprinting, a concept first described more than 60 years ago as original antigenic sin. When the immune system becomes imprinted with an antigen from the first encountered virus from a family of pathogens, it governs all subsequent antibody responses, according to several published studies that have described and evaluated this concept.

Additional evidence

In Dr. Spatola's study, other differences, particularly in regard to the cerebrospinal fluid (CSF), further supported the role of antigenic imprinting as a cause of neuroPASC. For one, those with elevated immune responses to other common coronaviruses rather than SARS-CoV2 in the CSF relative to the periphery were more likely to have a bad outcome in regard to neurologic symptoms.

Moreover, the CSF in neuroPASC patients "was characterized by increased IgG1 and absence of IgM, suggesting compartmentalized humoral responses within the CSF through selective transfer of antibodies from the serum to the CSF across the blood-brain barrier rather than through intrathecal synthesis," Dr. Spatola reported.

In the case of COVID-19, the propensity for antigenic imprinting is not difficult to understand.

"The common cold coronaviruses are pretty similar to SARS-CoV2, but they are not exactly the same," Dr. Spatola said. Her work and studies by others suggest that when antigenic imprinting occurs, "it prevents full maturation of the antibody response."

NeuroPASC is one of many manifestations of long COVID, but Dr. Spatola pointed out that the immune response in the CSF is unique and the causes of prolonged neurologic impairment after COVID-19 are likely to involve different mechanisms than other long-COVID symptoms.

"Antibodies in the brain are functionally different," said Dr. Spatola, noting for example that antibody-directed defenses against viral threats show a greater relative reliance on phagocytosis. This might become important in the development of therapeutics for neurologic symptoms of long COVID.

A different phenomenon

The manifestations of neuroPASC are heterogeneous and can include confusion, cognitive dysfunction, headache, encephalitis, and other impairments. Neurologic symptoms occur during acute SARS-CoV2 infections, but neuroPASC appears to be a different phenomenon. These symptoms, which develop after the initial respiratory disease has resolved, were attributed by Dr. Spatola to persistent inflammation that is not necessarily directly related to ongoing infection.

"The reason why some patients develop neuroPASC is unknown, but I think the evidence has pointed to a role for the immune system rather than the virus itself," Dr. Spatola said.

Currently, neuroPASC is a clinical diagnosis but Dr. Spatola and her coinvestigators are conducting research to identify biomarkers. A viable diagnostic test is not expected imminently. They have identified 150 different features with potential relevance to neuroPASC.

In their comparison of those who did relative to those who did not develop neuroPASC, the initial studies were undertaken 2-4 months after the acute COVID-19 symptoms had resolved. The patients with neuroPASC and those without neurologic sequelae have now been followed for 6-8 months, which Dr. Spatola said was too short to draw firm conclusions about outcomes.

An evolving concept

Despite the small sample size of this study, these are "very interesting data" for considering the pathogenesis of neuroPASC, which is "a concept that is still evolving," according to Natalia S. Rost, MD, chief of the stroke division, department of neurology, Massachusetts General Hospital, Boston.

Applied to SARS-CoV2, the concept of original antigenic sin "is new" but Dr. Rost said that it might help differentiate neuroPASC from acute neurologic symptoms of COVID-19, which include stroke. She indicated that the work performed by Dr. Spatola and others might eventually explain the pathology while leading to treatment strategies. She cautioned that the concepts explored in this study "need to be further developed" through larger sample sizes and the exploration of other variables that support the hypothesis.

Dr. Spatola and Dr. Rost report no potential conflicts of interest.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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