Updated: Jun 5, 2020
Science magazine discusses a paper just published in Cell describes a study that shows CD4+ and CD8+ T-cells in the immune system of 100% and 70% of recovered COVID-19 patients, respectively, many of which target the spike protein characteristic of coronaviruses as well as other peptides made by the virus.
Interesting, these T-cells were also seen in 40-60% of unexposed individuals from samples before COVID-19, perhaps from exposure to other coronaviruses, including some that comprise the common cold.
It is not yet known what this means yet -- it could mean that this subset of individuals are the ones who are able to handle SARS-CoV-2 exposure without ever showing symptoms of COVID-19, or only mild symptoms, but it could also be true that these individuals if infected might experience a more severe form of COVID-19 because an overactive immune system.
This is one of the tricky problems with this disease -- having a strong immune response to the virus is good early to fight it off before it spreads and moves down to the lungs and elsewhere, but if it does spread many of the worst symptoms and medical problems in the severe cases involve inflammatory responses that suggest an overactive and out of control immune system.
Further studies will help to clarify these issues, and figure out whether the presence of these T-cells in an individual can be used to determine if they are at less risk of infection, or possibly at risk for more severe disease should they get infection. If that question is answered, these could be potentially useful biomarkers for prevention and management and use alongside antibody blood tests.