- Some people are barely affected by the new coronavirus, while others become gravely ill.
- Scientists have investigated why this happens.
- Their results indicate the answer may lie in previous exposure to common cold viruses.
While some patients with Covid-19 require hospitalisation, the majority of people who do become infected experience mild symptoms and can recover at home.
Researchers in a new study, published in the journal Science, suspect this may have to do with previous infections with common cold viruses that may be able to train our immune system to keep track of viruses they've previously come across, and give the cells a head start in recognising and targeting these repeat invaders.
For the study, the team from the La Jolla Institute for Immunology (LJI) in La Jolla, California, analysed blood samples collected from people in Wuhan, China, long before the Covid-19 outbreak (between 2015 and 2018).
T-cells and SARS-CoV-2
T-cells play a key role in the body's immune defence.
Professor Thomas Scriba, deputy director of immunology and laboratory director at the University of Cape Town, previously explained to Health24 that T-cells are "natural killers". Their job is to find infected cells in the human body and destroy them.
In the study, while analysing the blood samples that contained T-cells, the researchers found that they reacted to more than 100 specific sites on SARS-CoV-2, including parts of the "spike" protein (one of the major structural proteins of the virus that it uses to bind to and invade human cells).
More than this, they explained that these T-cells also reacted to similar sites on four different coronaviruses that cause common cold infections. Based on their findings, they hypothesise that this existing immune system "memory" may explain why some people end up having milder Covid-19 infections than others.
Building on previous findings
According to a news release by LJI, the new study built on a recent paper published in the journal Cell, from the Sette Lab and the lab of LJI Professor Shane Crotty. This study showed that the immune systems of 40% to 60% of people who were never exposed to SARS-CoV-2 ended up having T-cells that recognised the new coronavirus.
Researchers of this finding suspect that in these individuals past exposure to other coronaviruses, such as those that cause the common cold, had somehow primed their T-cells in a way that they could recognise and act against SARS-CoV-2, and this new research appear to confirm their discovery.
"We knew there was pre-existing reactivity, and 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," said study co-lead author Alessandro Sette, a professor at LJI in the statement.
Study co-lead author Daniela Weiskopf, assistant professor at LJI, also commented: "We have now proven that, in some people, pre-existing T-cell memory against common cold coronaviruses can cross-recognise SARS-CoV-2, down to the exact molecular structures," with Sette explaining that immune reactivity may translate to different degrees of protection against Covid-19.
Sette said: "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."
However, the team also wrote that their hypothesis was "highly speculative" and required additional research, as the role of T-cells in fighting SARS-CoV-2, specifically, were not yet fully understood. In fact, these cells were just one part of the very complex immune system.