Infection with SARS-CoV-2 can cause severe symptoms in some patients, while other are entirely asymptomatic. The reason why this is the case is one of the many mysteries of the virus.
There are likely to be many factors that contribute to the development of a severe reaction to the virus and why others only experience mild symptoms or none at all. One theory proposed is that prior exposure to other coronaviruses may convey a level of protection. One study, recently published in the journal Nature, suggests previous exposure to coronaviruses with structural similarities to SARS-CoV-2 may provide protection. The T-helper cells of the immune system in certain individuals may have a memory of other coronaviruses and recognize spike proteins of SARS-CoV-2 and go on the attack.
A team of researchers isolated T-helper cells from blood samples taken from 18 patients diagnosed with COVID-19 and 68 individuals who had not been exposed to SARS-CoV-2. The immune cells were then stimulated by exposing them to synthetic fragments of the spike proteins present on the surface of the SARS-CoV-2 virus, which are used by the virus to gain entry into cells.
The researchers exposed the t-helper cells in both groups to the synthetic virus fragments to see if they activated the cells. The T-helper cells were activated in 85% of the samples from the COVID-19 group, which was as expected, as the immune system in those patients was currently fighting the virus. What was not expected was 35% of samples taken from individuals who had not been exposed to SARS-CoV-2 also saw T-helper cells activated by the spike proteins.
Further analysis revealed the T-helper cells in COVID-19 patients and healthy individuals reacted to different parts of the spike protein. The T-helper cells of COVID-19 patients reacted to the full length of the spike protein whereas the T-helper cells of healthy individuals were only activated by certain sections of the spike protein. Those sections of the spike protein were found in other coronaviruses that cause the common cold.
This finding strongly suggests that individuals who have contracted a cold caused by certain strains of coronaviruses similar to SARS-CoV-2 may have a degree of immunity. The researchers also demonstrated that the T-helper cells of the healthy patients reacted to several other coronaviruses that cause the common cold.
“One of the characteristics of T-helper cells is that they are not only activated by a pathogen with an ‘exact fit,’ but also by pathogens with ‘sufficient similarity’,” explained Claudia Giesecke-Thiel, PhD, one of the lead authors of the study and head of the Flow Cytometry Joint Facilities Scientific Service at the Max Planck Institute for Molecular Genetics.
While it is possible that individuals who have recently contracted a cold may have T-helper cells with memory of the virus which can provide a protective effect, further studies are required to determine if that is the case. However, exposure to other coronaviruses may not necessarily result in mild symptoms. “A recent bout of the common cold would probably result in less severe COVID-19 symptoms. However, it is also possible that cross-reactive immunity could lead to a misdirected immune response and potentially negative effects on the clinical course of COVID-19. We know this can occur with dengue fever, for instance,” said Co-author Leif Erik Sander, MD of the Dept. of Medicine, Div. of Infectious Diseases at Charité University Hospital Berlin
You can read more about the research in the paper – SARS-CoV-2-reactive T cells in healthy donors and patients with COVID-19- which was published in Nature on July 29, 2020. DOI: 10.1038/s41586-020-2598-9