Pathogenic IgG from long COVID patients with neurological sequelae triggers sensitive but not cognitive impairments upon transfer into mice

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Approximately 30% of long COVID patients still experience neurological symptoms (brain fog, pain, chronic fatigue) more than 4 months after the onset of COVID-19. This condition, known as 'neurological long COVID', remains poorly understood and might be explained by a persisting autoimmune response against nervous-derived self-antigens. The aim of this study is to determine whether IgG autoantibodies from long COVID patients with neurological sequelae can bind to central or peripheral nervous system epitopes and trigger neuropsychiatric symptoms upon passive transfer into mice, thereby mirroring patient-reported manifestations. Long COVID patients meeting the 2021 consensus WHO definition were included following a standardized neuropsychological assessment, while excluding patients with a medical history of autoimmune and neurological disorders. Age- and sex-matched asymptomatic individuals were used as healthy controls. Total IgGs were isolated using protein G purification and injected intraperitoneally into C57Bl6/J mice for four consecutive days. During the two weeks post-injections, behavioral tests assessed mechanical allodynia, thermal hyperalgesia, spatial working memory, depression, and anxiety. Mice injected with IgG from long COVID patients showed no difference with the control group in terms of anxiety or depression behaviors, short- or long-term spatial memories. However, they displayed a transient decrease of paw withdrawal threshold and thermal hypersensitivity during the first week. This effect was abolished when IgG-depleted serum or papain-digested IgGs were transferred. IgG from long COVID patients accumulated in the lumbar dorsal root ganglia of injected mice and colocalized with proprioceptive and nociceptive sensory neurons, without inducing local neuroinflammation or astrogliosis. When applied onto human post-mortem DRG tissue, patient-derived IgG also exhibited immunoaffinity for sensory neuron somata. These data demonstrate that IgGs from long COVID patients bind to peripheral sensory neurons and induce pain-related symptoms in mice. Our findings also support the hypothesis that autoantibodies mediate pain-related pathophysiology in the spectrum of long COVID symptoms.

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