Previous infections with common cold viruses can train the immune system to recognize SARS-CoV-2, the virus that causes COVID-19, according to a new study.
The study, published Aug. 4 in the journal Science, found that immune cells known as T cells that recognize common cold coronaviruses also recognize specific sites on SARS-CoV-2 — including parts of the infamous "spike" protein it uses to bind to and invade human cells.
This existing immune system "memory" may explain why some people have milder COVID-19 infections compared with others; however, the authors stress that this hypothesis is "highly speculative" and requires more research to confirm. That's because it's unknown exactly how big a role T cells play in fighting COVID-19 — T cells are just one part of a complex menagerie of molecules and cells that makes up our immune system.
"We have now proven that, in some people, preexisting T-cell memory against common cold coronaviruses can cross-recognize SARS-CoV-2, down to the exact molecular structures," study co-lead author Daniela Weiskopf, assistant professor at La Jolla Institute for Immunology in La Jolla, California, said in a statement.
It's possible that this "immune reactivity may translate to different degrees of protection" against COVID-19, study co-lead author Alessandro Sette, a professor at La Jolla Institute for Immunology, said in the statement. "Having a strong T-cell response, or a better T-cell response may give you the opportunity to mount a much quicker and stronger response."
Previous studies have shown that upwards of 50% of people never exposed to COVID-19 have T cells that recognize SARS-CoV-2. This ability has been seen in people around the world, in the Netherlands, Germany, the United Kingdom and Singapore. Scientists hypothesized that this existing immunity could be due to previous infections with other coronaviruses, specifically those that cause common cold infections.
In the new study, the researchers analyzed blood samples collected from people between 2015 and 2018, well before COVID-19 first emerged in Wuhan, China.
These blood samples contained T cells that reacted to more than 100 specific sites on SARS-CoV-2. The researchers showed that these T cells also reacted to similar sites on four different coronaviruses that cause common cold infections.
"This study provides very strong direct molecular evidence that memory T cells can 'see' sequences that are very similar between common cold coronaviruses and SARS-CoV-2," Sette said.
In addition to binding to the spike protein, the T cells also recognized other viral proteins beyond the spike.
Currently, most COVID-19 vaccine candidates target the spike protein, but the new findings suggest that including other proteins in a vaccine, besides the spike, might harness this T cell cross reactivity and potentially enhance the vaccine's potency, the researchers said, although much more research would be needed to show this.
The authors note that their findings of cross-reactivity with T cells are different from what has been seen with neutralizing antibodies — another weapon of the immune system that blocks a pathogen from infecting cells. Neutralizing antibodies against common cold viruses are specific to those viruses and don't show cross-reactivity with SARS-CoV-2, according to previous studies, the authors said.
Originally published on Live Science.