Study links milder Covid-19 to previous encounters with similar, less virulent coronaviruses

In Covid-19 patients with mild symptoms, the researchers found they were more likely than sicker patients to show signs of previous infection with similar and less virulent coronaviruses. The study, conducted by researchers at Stanford University School of Medicine, was published in the journal Science Immunology.

The results suggest that people with Covid-19 may have milder symptoms if certain cells in their immune system “remember” previous encounters with seasonal coronaviruses – the ones that cause about a quarter of common colds children get. These immune cells are better equipped to mobilize quickly against SARS-CoV-2 if they have already encountered its “milder cousins,” the scientists concluded.

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They suggested their findings might help explain why some people, especially children, appear to be much more resistant to SARS-CoV-2 infection than others. They could also help predict which people are likely to develop the most serious symptoms of Covid-19.

The immune cells in question are killer T cells. The study showed that killer T cells taken from the sickest Covid-19 patients show less evidence of having had previous encounters with coronaviruses causing the common cold.

Human cells have regularly seen samples of each protein that they have turned into tiny pieces and display those pieces (called peptides) on their surface, so that T cells can inspect them. When a killer T cell notices a peptide that does not belong on a cell’s surface, it multiplies furiously and its many descendants are set off to destroy any cell carrying those peptides.

The researchers put together a panel of 24 different peptide sequences that were either unique to proteins made by SARS-CoV-2 or also found on similar proteins made by one or more (or even all) of the coronavirus strains causing the common cold. .

They analyzed blood samples taken from healthy donors before the pandemic started, meaning they had never encountered SARS-CoV-2 – although many were likely exposed to strains of the coronavirus that cause the common cold. They found that killer T cells from unexposed individuals targeting SARS-CoV-2 peptides that were shared with other coronaviruses were more likely to have proliferated than killer T cells targeting peptides found only on SARS. -CoV-2. The T cells targeting these shared peptide sequences had likely already encountered one or the other milder coronavirus strain and proliferated in response, they concluded.

– Source: Stanford University School of Medicine