T cells help where antibodies don't: Immunity to COVID-19 may be higher than previously thought

In the six months since it was first reported to the World Health Organization (WHO) in China, COVID-19 has made over 10 million people sick and claimed more than half a million lives all over the world.

Dig a little deeper, though, and you'll find that about five and a half million patients have also recovered from this viral infection over the last six months.

Epidemiologists and medical researchers have been proposing theories on immunity to COVID-19: how it works, who is immune and for how long, what does it mean for our health in the long term, and its impact on public policy and health measures.

The latest among these theories is based on the findings of a research team from Sweden who think that the number of people immune to the disease may be higher than previously thought.

Their reasoning: you don't need to have antibodies in your system to prove that you had this coronavirus infection, and that you were successful in beating it.

In other words, it is possible that the T cells of the immune system (in some people) can conjure enough power to identify and fight SARS-CoV-2 (the coronavirus that causes COVID-19) and give future immunity against it too.

So far, a majority of the studies on matters related to immunity, recovery and treatment of COVID-19 patients have depended on antibody tests. Antibody tests are done to identify who among the people recovering from the disease have developed antibodies to fight the disease.

Read more: ELISA antibody test

Antibody testing has been critical in identifying not only patients who have recovered from the disease but also those who may have been asymptomatic and would not have found out they had the disease if they weren't tested.

By using this method, doctors and healthcare workers across the world have been able to identify who can donate blood plasma for COVID-19—plasma from recovered patients is rich in antibodies. The Indian Council of Medical Research is running a multi-centre trial to see if transfusing this blood plasma in moderately sick COVID-19 patients can help them recover. This is known as convalescent plasma therapy.

However, the new research from the scientists in Sweden shifts the focus from antibodies on to how T cells in the body fight off infections such as COVID-19—and how T cells could prove to be critical towards long-term immunity from the respiratory infection.

Function of T cells in the immune system
New research focuses on COVID-19 immunity from T cells

Function of T cells in the immune system

To begin with, it is important to understand the role T cells play in our immune system to launch the right immune response against different kinds of pathogens, like bacteria, viruses or parasites.

Also known as T lymphocytes, T cells are a type of white blood cells. They play a key role in the body's immune system, which adapts to different kinds of invaders entering the body. T cells help identify and then kill off pathogens at different parts of the body.

T cells are also of different types and have different roles to play throughout the body. These include identifying pathogen and killing infected or faulty cells (killer T cells). T cells also alert the body in case there is a need to produce antibodies (helper T cells). That is why they are also known as the body's immune warriors.

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New research focuses on COVID-19 immunity from T cells

The Swedish study, published earlier this week in the preprint server bioRxiv, suggests that the immunity of the general population may be higher than what has been ascertained from antibody tests. This is because antibody tests only look for specific antibodies that the body may or may not have produced in response to an antigen such as the SARS-CoV-2 virus.

T cells are a critical part of the body's immune system—even if a person doesn't have the requisite antibodies to prevent harm from the coronavirus, T cells can still come forward and fight off the disease in the absence of antibodies.

The study included immunological analyses of samples from over 200 people, most of whom had shown mild to no signs of infection. The control group also included people who had donated blood to patients in the duration of the study.

One of the main findings of the study was that both patients and their close contacts (like family members living in the same household but who were asymptomatic) showed T cell immunity against COVID-19. Another 30% of the blood donors from the test period had T cells that could target the novel coronavirus.

This suggests that previous studies projecting numbers of people immune to COVID-19 may have been flawed, as they did not take T cell immune response into account. However, it was unclear as to how much immunity these T cells are able to provide against this infection.

Another positive outcome of the study was that where antibodies are known to fight off infection and last for only three to six months, the T cell response to SARS-CoV-2 bodes well for long-term immunity in people. Two previous studies have also corroborated this with their findings.

An earlier study, published as “Presence of SARS-CoV-2 reactive T cells in COVID-19 patients and healthy donors” in the preprint server medRxiv in April 2020, found that helper T cells identified and targeted the spike protein in SARS-CoV-2 in 15 out of the 18 patients in the study—all participants in the study had been hospitalised with COVID-19.

Another study published in the journal Cell in May 2020 went in-depth, to find out about adaptive immunity to the coronavirus, particularly in the bid for vaccine development. It identified COVID-19 specific T cells CD8+ and CD4+ in 70% and 100% of patients, respectively. The two different types of T cells can be identified as helper T cells that react to different proteins in the virus, and killer T cells that target and eliminate the virus from the system.

Read more: CD4 test

The findings provide an encouraging outlook in the battle against COVID-19, as they suggest that immunity to COVID-19 isn't limited to the presence of resistant antibodies. Even those who have not developed antibodies in response to the infection may be able to fight it off. Additionally, a strong T cell-driven immune response could help provide immunity to a population for a longer period of time.