Immune cells known as T cells are a significant part of the immune response to SARS-CoV-2, the virus that causes COVID-19, according to new research from the US. The study also revealed that a substantial proportion of people who have not been infected with SARS-CoV-2 nevertheless have T cells that can recognise the virus.

“Our study was the first to show that COVID-19 triggers a strong immune response in most people,” says Alessandro Sette of the La Jolla Institute of Immunology in California, who led the research team.

Much of the discussion of the immune response to COVID-19 has focused on antibodies, protein molecules that bind to specific parts of a virus and thus stimulate activities that neutralize the virus. However, another major aspect of immunity is mediated by circulating T cells. One category of T cells, known as killer T cells, can bind to and destroy virus-infected cells. Another category, helper T cells, can assist in generating the antibody-based response.

Sette and his colleagues examined blood from 20 patients who had recovered from COVID-19. They found helper T cells that could recognise and act against the SARS-CoV-2 virus in all of the samples. They also found killer T cells that could target SARS-CoV-2 in 14 of the samples.

“Importantly, we saw a robust T-cell response against the spike protein of the virus, the target of most vaccines,” Sette says. However, they also found T cells that recognised other parts of the virus, revealing some aspects of the complexity of the anti-viral T-cell response.

Perhaps equally significant is their discovery of T-cell activity against the SARS-CoV-2 virus in around 50 percent of blood samples taken from people who could not have been exposed to this virus, since the samples dated from 2015-2018, before SARS-CoV-2 emerged. The researchers believe that this recognition may be due to previous infection with less harmful coronaviruses that cause common colds.

They say it is possible that this ‘cross-reactivity’ of immune response against coronaviruses may help explain the relatively mild illness experienced by some people with COVID-19. At present, however, there is no firm evidence for or against this speculation.

By learning more about the immune response to SARS-CoV-2 the researchers hope to contribute to the development of effective vaccines and treatments.

 “Hopefully we can learn how to push the immune response in the right direction,” says Sette.

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