Millions of people who have lost their sense of smell after contracting Covid may have an abnormal immune response that destroys nasal cells, researchers said.
Doctors analyzed nasal tissue of Covid patients and found that those with long-term problems with their sense of smell had inflammatory immune cells within the delicate lining of the nose, which likely wiped out vital sensory neurons.
Tissues from the lining of the nose “contain unique immune cells that produce inflammatory signals, along with fewer olfactory neurons,” said Dr. Bradley Goldstein, an associate professor of neuroscience at Duke University in North Carolina. The unusual immune response was seen only in patients whose sense of smell persisted for several months.
He added, “There appears to be an unresolved local immune response, which is seen by the subtle olfactory cells.”
Since doctors noted that many Covid patients lost their sense of smell, it was unclear whether the virus was harming sensory cells in the nose, areas of the brain that process olfactory information, or both.
Researchers studied biopsy tissue from the lining of the noses of 24 Covid patients, nine of whom had lost their sense of smell for at least four months. Histology from the latter group revealed that T cells involved in inflammation penetrated the lining of the nose where odorant neurons are located. The unusual immune response was seen even though the patients did not have detectable COVID, suggesting it persisted after the infection cleared.
When the researchers looked at the number of sensory neurons involved in the sense of smell, they found that those who had experienced prolonged loss of smell had significantly fewer numbers, possibly due to damage to the thin tissue of the lining of the nose by T-cell-driven T-cells. ignition. Similar errant immune responses may explain other symptoms of long Covid, Goldstein said.
Researchers reported in the British Medical Journal this year that at least 5% of people who lost their sense of smell during infection with Covid do not quickly or fully regain their sense of smell, which works out to about 15 million people globally. “Right now, we don’t have specific, effective treatments,” Goldstein said. “To develop treatments, we need to understand the pathobiology of the problem: what is damaged and where.”
Writing in the journal Science Translational Medicine, the researchers explain how the findings may pave the way for new treatments for post-Covid anosmia. One option is to locally block inflammation-causing immune cells in the lining of the nose, a part of the body that is most easily accessible with creams and sprays. “We are encouraged by these results and hope that new treatments will emerge,” Goldstein said.
Danny Altmann, professor of immunology at Imperial College London, said the work was an “important addition to deciphering the many disease tricks of SARS-CoV-2”.
“As we’ve seen before, profound changes in symptoms can occur in the absence of detectable live virus at the scene,” he said. “Loss of smell has been one of the major mysteries and these findings provide an answer, along with previous findings of changes in the olfactory bulb in the nervous system.”
“In patients with Covid-19, it has been shown that persistent problems with the sense of smell are associated with a decrease in the areas of the brain associated with our sense of smell,” said Dr. Gwenell Daoud, a neuroscientist who has studied the effects of Covid on the brain at the university. Oxford. “It is known that inflammatory processes persist in the brain after infection with Sars-Cov-2, regardless of whether the virus itself is present, and this biopsy study now provides further evidence that such specific brain loss could be related to continued inflammation and loss of Olfactory neurons in the nasal cavity itself.
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