Much remains unknown about SARS-CoV-2, the new coronavirus responsible for the on-going COVID-19 pandemic. Two important questions revolve around why some people become severely ill while others experience no symptoms, and the associated implications for long-term immunity. Findings from a recent study suggest that prior exposure to related coronaviruses may affect the production of T cells that affect disease severity and immunity.
SARS-CoV-2 belongs to the Coronaviridae family of RNA viruses. There are six coronaviruses known to infect humans, having effects that range from severe pneumonia (SARS-CoV-1 and MERS-CoV) to the “common cold”.
A healthy person infected with a virus mounts a coordinated immune response involving distinct cell types that work together to neutralize the invading pathogen. The adaptive immune system includes T cells that kill virus-infected cells and display antigens (viral fragments) to B cells, which in turn produce antibodies against the virus. Research demonstrates that T cells persist in the body longer than antibodies. For example, 11 years after recovering from SARS-CoV-1 infection, memory T cells were still present, while antibodies were undetectable two to three years after the disease resolved.
Proteins are highly conserved among related coronaviruses. The current study investigated whether the blood of people who had recovered from SARS-CoV-1 or SARS-CoV-2 contained immune factors that would react to isolated SARS-CoV-2 viral proteins.
The authors of the study identified SARS-CoV-2 virus-specific memory T cells in the blood of people who had recovered from COVID-19. The authors then analyzed the blood of 15 SARS-CoV-1 resolvers, 17 years after infection. They observed that all individuals possessed viral-specific T cells that responded not only to SARS-CoV-1 viral proteins but also to SARS-CoV-2 proteins. These results suggest that T cells provide long-lasting protection and offer some cross-protection between SARS-CoV-1 and SARS-CoV-2. The researchers even detected SARS-CoV-2 specific T cells in 19 out of 37 people unexposed to either SARS-CoV-1 or SARS-CoV-2, suggesting they had encountered a related coronavirus, possibly one responsible for the “common cold”.
These findings suggest that T cells produced against SARS-CoV-2 will provide long-term immunity. This research also supports the idea that prior infection with a related coronavirus affords some immunity against SARS-CoV-2 infection, and might explain the variations observed in infection rates and disease severity.