Researchers detect robust T cell response in patients with mild COVID-19 infection

According to scientists, mild cases of COVID-19 (coronavirus disease 2019) pandemic can activate strong responses from memory T cells, even when detectable virus-specific antibody responses are absent.

Image Credit: Andrii Vodolazhskyi/Shutterstock.com

The study was published in the Cell journal on August 14^th, 2020.

According to the study’s authors, memory T cell responses produced through natural exposure or an infection caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2 for short) could be a major immune component to inhibit the recurrent episodes of this severe infection. SARS-CoV-2 is the virus responsible for causing the COVID-19 pandemic.

We are currently facing the biggest global health emergency in decades. In the absence of a protective vaccine, it is critical to determine if exposed or infected people, especially those with asymptomatic or very mild forms of the disease who likely act inadvertently as the major transmitters, develop robust adaptive immune responses against SARS-CoV-2.”

Marcus Buggert, Study Senior Author, Karolinska Institute

So far, only limited proof of reinfection cases in humans is available with the earlier documented COVID-19 cases. A majority of the studies conducted on immune protection against the SARS-CoV-2 in human beings have targeted the induction of neutralizing antibodies.

However, the reactions of antibodies tend to diminish and could not be detected in all patients, specifically those who have mild forms of the COVID-19 infection.

Mice study has demonstrated that memory T cell responses induced by vaccines, and which can last for several years, protect against the corresponding SARS-CoV-1 virus, even when detectable antibodies are absent.

To date, it was vague how the responses from SARS-CoV-2-specific T cells are associated with the clinical course of COVID-19 infection in human beings or with the response of antibodies.

To bridge this knowledge gap, Buggert and his colleagues evaluated SARS-CoV-2-specific T cells as well as antibody responses in over 200 Swedish people over the full spectrum of exposure, disease, and infection.

In the acute phase of infection, the responses of T cells were linked to numerous clinical markers of the severity of the disease. Once the patients recovered from the COVID-19 infection, SARS-CoV-2-specific memory T cell responses were successfully detected.

People who recovered from the severe forms of COVID-19 were found to have the most robust T cell responses. In the meantime, progressively lower responses of T cells were noted in those who recovered from very mild forms of COVID-19, and in members of the family exposed to the virus.

As expected, all 23 patients who recovered from the severe forms of COVID-19 developed both T cell and SARS-CoV-2-specific antibody responses.

But to the researchers’ amazement, SARS-CoV-2-specific memory T cell responses were identified months after the COVID-19 infection in family members exposed to the virus and in a majority of the individuals who have a history of very mild forms of COVID-19, at times in the absence of SARS-CoV-2-specific antibodies.

Among the 28 family members exposed to the virus, only 17 (slightly more than half) were found to have detectable antibody responses, while almost all (26/28) family members displayed T cell responses.

Among the 31 people who recovered from the mild forms of COVID-19, nearly all were found to have detectable antibody responses (27/31) and subsequently developed T cell responses (30/31).

Our findings suggest that the reliance on antibody responses may underestimate the extent of population-level immunity against SARS-CoV-2. The obvious next step is to determine whether robust memory T cell responses in the absence of detectable antibodies can protect against COVID-19 in the long-term.”

Marcus Buggert, Study Senior Author, Karolinska Institute