Scientists, including one of Indian-origin, have found that people with severe COVID-19 are left with more of the immune system's protective "memory" T cells needed for fighting reinfection with the novel coronavirus.
The study, published in the journal Science Immunology, assessed the expression of individual genes of a subset of T cells isolated from both COVID-19 patients and non-exposed donors.
Analysing more than 80,000 CD8 T cells, the scientists, including those from La Jolla Institute of Immunology (LJI) in the US, believe the study is the first to describe these cells that fight the SARS-CoV-2 virus in "high resolution" detail.
While antibodies offer one mode of protection against pathogens, the cells of the immune system, including the CD8 T cells are responsible for destroying virus-infected host cells, the researchers explained.
In particular, they said "memory" CD8 T cells are important for protecting the body from reinfection against many viruses.
In the research, the scientists assessed CD8 T cells from 39 COVID-19 patients and 10 subjects who had never been exposed to the virus whose blood samples were given before the pandemic.
Of the 39 patients, they found that 17 had a milder case that did not require hospitalisation, while 13 had been admitted for treatment and nine had needed additional ICU support.
{{/usCountry}}Of the 39 patients, they found that 17 had a milder case that did not require hospitalisation, while 13 had been admitted for treatment and nine had needed additional ICU support.
{{/usCountry}}"The data from this study suggest people with severe COVID-19 cases may have stronger long-term immunity," said study co-leader LJI Professor Pandurangan Vijayanand.
While the researchers expected to see a stronger CD8 T cell response in the mildly infected patients -- since these are cases where the immune system was equipped to fight off a severe infection -- the study showed the opposite.
The scientists observed weaker CD8 T cell responses in patients with milder COVID-19 cases, and the strongest responses in the severely ill patients who required hospitalisation or ICU support.
"There is an inverse link between how poorly T cells work and how bad the infection is. I think that was quite unexpected," explained Christian H Ottensmeier, another leader of the study from LJI.
In the milder cases, the scientists said there were molecular signs of T cell "exhaustion" -- a phenomenon in which the cells receive so much stimulation during a viral attack that they are less effective in doing their jobs.
Citing one of the limitations of the study, the scientists said it relied only on the CD8 T cells found in blood samples to make the inference.
In future studies, they hope to uncover how T cells in tissues hit hardest by SARS-CoV-2, such as the lungs, react to the virus.
The researchers believe this would be an important step since memory T cells which provide long-term immunity need to live in the tissues.
"This study is very much a first step in understanding the spectrum of immune responses against infectious agents," Ottensmeier added.