How common cold virus boosts immune system to fight Covid

Scientists have found direct evidence that exposure to common cold coronaviruses can train T cells to fight SARS-CoV-2, the virus causing Covid-19.

In fact, prior exposure to a common cold coronavirus could also partially protect mice from lung damage during a subsequent SARS-CoV-2 infection, revealed the team from La Jolla Institute for Immunology (LJI), US.

The new research, published in the journal Nature Communications, provides an important first look at how "cross-reactive" T cells -- which can fight multiple viruses from the same family -- develop in an animal model.

"We are learning how these immune cells develop and function," said study co-leader LJI Research Instructor Annie Elong Ngono.

The team is now working to develop novel vaccines purposefully designed to harness these powerful T cells. Those vaccines would protect against SARS-CoV-2 and provide immunity against several other coronaviruses with pandemic potential.

"Our research will help scientists design and improve 'pan-coronavirus' vaccines that elicit broad, cross-protective responses," said LJI Professor Sujan Shresta.

T cells tend to be specialists. They learn to hunt down specific molecular targets, called epitopes, that belong to specific pathogens.

"Cross-reactive" T cells are important for human health because they recognize epitope targets on different—but closely related—pathogens, such as different members of the coronavirus family. This viral family includes common cold coronaviruses and serious pathogens such as SARS-CoV-2.

For the study, the researchers used mouse strains that can produce the exact same variety of T cells as the ones found in humans.

The researchers infected these mice with one of the most widespread common cold coronaviruses, called OC43. SARS-CoV-2 and OC43 are both betacoronaviruses.

The scientists found that mice infected with OC43 produced CD4+ "helper" T cells and CD8+ "killer" T cells that cross-reacted with SARS-CoV-2. Those cells targeted the same epitopes as T cells collected from humans with SARS-CoV-2 exposure.

Next, the researchers developed a model of sequential infection -- with OC43 infection followed by SARS-CoV-2 in these humanised mice.

They examined whether the cross-reactive T cells actually helped protect the mice from severe Covid.

Cross-reactive CD4+ "helper" T cells did indeed help counteract the virus's assault on the respiratory system.

Mice with previous OC43 exposure showed lower levels of SARS-CoV-2 infection in their airways and were less likely to develop pneumonia and lung damage. Cross-reactive T cells really did help prevent severe disease.