A high percentage of 85 to 90 percent of those infected with SARS-Cov-2 remain without or with mild symptoms. Most of them don’t even notice the infection. It has long been suspected that there is immunity here. Researchers from the United States and Australia have now presented the results of their studies on the mechanisms of immunity from previous infections with common cold coronaviruses.
There are many unknowns about human immune responses to the SARS-CoV-2 virus. SARS-CoV-2-reactive T cells have already been found in several studies in non-exposed persons, which indicates an already existing cross-reactive T cell memory in humans. Further results have now been found in Sciencemag published as a preprint.
The nature and functioning of the T cells that provide immunity has been speculative. Human blood samples taken prior to the discovery of the SARS-CoV-2 virus in 2019 showed cross-reactivity to SARS-CoV-2 and the cold coronaviruses HCoV-OC43, HCoV in a number of existing CD4 + T memory cells -229E, HCoV-NL63 or HCoV-HKU1. Therefore, a T cell memory for coronaviruses that cause colds can account for at least part of the extensive immunity observed in SARS-Cov-2 infections.
Previous study results
Studies examining the human immune response against SARS-CoV-2 have started to characterize SARS-CoV-2-specific T cell responses, and several studies have shown significant activation of T cell subsets in acute COVID-19 -Patients described.
For example, T-cell studies conducted in five different countries reported that 20-50% of those who were not exposed to SARS-CoV-2 showed significant T-cell reactivity to SARS viruses. The studies came from the USA, the Netherlands, Germany, Singapore and Great Britain. The generally observed pattern was that the T cell reactivity found in non-exposed persons was mediated predominantly by CD4 + T cells (T helper cells). It was assumed that this phenomenon was due to the presence of human cold coronaviruses such as HCoV-OC43, HCoV-HKU1, HCoV-NL63 or HCoV-229E. These HCoVs have a similar structure and components to SARS-CoV-2, are widespread in the general population and are usually responsible for mild symptoms.
This cross-reactive T cell immunity to SARS-CoV-2 has far-reaching implications in that it explains aspects of the different clinical outcomes of COVID-19, affects epidemiological models of herd immunity, and could affect the performance of COVID-19 vaccine candidates.
The study results
Samples from unexposed subjects, which were collected between March 2015 and March 2018, were used for the study, long before SARS-CoV-2 was distributed worldwide. The unexposed subjects were confirmed to be negative for SARS-CoV-2.
SARS-CoV-2 reactive T cells were tested for their reactions against the various “spikes” and their components. In 82 of 88 cases (93.2%), the cells that responded to the SARS-CoV-2 were clearly CD4 + T cells (T helper cells). In four cases (4.5%) the responding cells were CD8 + T cells (the suppressor or killer cells) and in two cases (2.3%) the responses from both CD4 + and CD8 + T cells were detected .
The investigation was also able to clarify which parts (epitopes) of the SARS-Cov-2 were recognized by the existing T cells. These virus parts are the same for the common cold virus and SARS and can therefore be recognized by T cells that were created by your two virus types and thus also fought.