3:34 p.m., July 24, 2020
To better understand the reaction of the immune system to Covid-19, researchers have chosen to focus not on antibodies but on T and B lymphocytes. Their results, published in “Nature”, are encouraging.
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Recently, studies have had the effect of a cold shower on the pandemic front. On July 11, for example, a British study pre-published on the site medRxiv (not yet validated by peers) concluded that the majority of patients affected by the new coronavirus developed antibodies … but that these dropped around 20 to 30 days after the first symptoms. But for some scientists, this decline in the number of antibodies is not necessarily incompatible with the hope of long-term immunity.
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Long-lasting memory T lymphocytes
Because antibodies are not the only cogs in the immune system. Several researchers have thus focused on T and B lymphocytes. Faced with a virus, B lymphocytes produce antibodies which will fight against the pathogen while T lymphocytes target already infected cells. After the disease, B and T lymphocytes called “memories” persist in the body and are reactivated in the event of a new infection. The immune system is therefore faster and more efficient.
Several studies are trying to determine how this “immune memory” works in the case of Covid-19. This is the case of Inserm MEMO-CoV-2 project, for example. Already, the results are encouraging. In a study published July 15 in the journal Nature, researchers observed the presence of “long-lasting memory T cells” capable of recognizing Sars-CoV-2 in patients recovered from Covid-19. “Surprisingly, we have also frequently detected T cells specific for Sars-CoV-2 in people with no history of Sars or Covid-19,” the authors write.
“A level of pre-existing immunity”
These observations swell hope for cross-immunity. This is the idea that infections linked to other coronaviruses, such as a common cold, could confer protection against Sars-CoV-2. In France, researchers from the Institut Pasteur, Inserm, AP-HP and the University of Paris looked into the question by comparing the level of antibodies against four seasonal coronaviruses in 775 children. , affected by Covid-19 or not. In their preliminary results, published on June 30 on medRxiv, they did not observe any difference.
The study published in Nature suggests the existence of this cross-immunity, but rather on the side of lymphocytes. “A level of pre-existing immunity against Sars-CoV-2 appears to exist in the general population,” comments one of the authors, Antonio Bertoletti, a virologist at the Singapore School of Medicine. If so, it will remain to be seen how much “memory” lymphocytes, associated with the antibodies, are needed to prevent re-infection and for how long. They could help to counter the disease or, at least, reduce the symptoms.
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