Scientists who have spent months focused on the role of antibodies in fighting Covid-19 are beginning to suspect that a lesser-known part of the immune system is just as crucial: T cells.
Evidence is emerging that T cells, which can “remember” past infections and kill pathogens if they reappear, have a major influence on how long patients remain resistant to Covid-19 reinfection.
The cells, whose size and complexity are small dwarf antibodies, also appear to affect vaccine performance and even the level of community immunity required to suppress new waves of disease.
“The antibodies look a little precarious and transient in the blood, while there is a lot of evidence that T cells are durable,” said Mala Maini, professor of viral immunology at University College London.
People who recovered from Sars, the disease most closely related to Covid-19, in 2003 still show cellular immunity to that coronavirus 17 years later.
T cells, which circulate in the blood, can protect people who have been infected and recovered from the new coronavirus but who do not have detectable antibodies soon after.
Immunity to any infection arises from a complicated interaction of different cells and proteins, such as antibodies, that are produced in various human tissues. Some are designed to recognize invading germs. Others have the job of destroying them.
T cells come in several different types, including killer T cells, helper T cells, and memory T cells. Then there are B cells, another essential category of white blood cells. Among other functions, B cells are the antibody factories of the immune system.
Al Edwards, an associate professor at the University of Reading School of Pharmacy, offers an analogy. “T cells are testing the virus while antibodies are feeling the virus,” he said. “T cells can promote antibody responses, and antibody responses can promote a T cell response. These two systems work together.”
“Even if you run out of detectable circulating antibodies, that doesn’t necessarily mean you don’t have protective immunity, because you likely have immune memory cells (B and T cells) that can quickly kick in to start a new immune response if you re-encounter yourself. with the virus, “added Professor Maini at UCL.” So you could get a milder infection. “
Tom Evans, chief scientist at Vaccitech, the Oxford University vaccine launch company, said: “You can think of the human immune system as an orchestra that plays together and needs coordinated performance from all musicians and their instruments. . It makes no scientific sense to talk about antibodies or T cells on their own. “
As data from clinical trials of potential Covid-19 vaccines emerge, the extent to which they evoke T-cell immunity will be a focus of attention.
Advocates of viral vaccines, which use a genetically harmless virus to transport coronavirus antigens to human cells, are already suggesting that their method is more effective in increasing the T-cell response than an alternative approach, which injects coronavirus genes. in the form of RNA or DNA in human cells.
More evidence is expected Monday when Oxford scientists present the first clinical trial results of their ChAdOx1 vaccine, which is based on a chimpanzee adenovirus. But it remains to be seen whether the combination of neutralizing antibodies and T cells generated by the vaccine will provide strong and lasting immune protection.
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One reason that antibodies have been the focus of attention is that they are much easier to measure on diagnostic tests than T cells, which are almost 10,000 times larger. The problem is that the antibodies sometimes fade quickly, particularly in people who have had mild or no symptoms of Covid-19, making them an unreliable indicator of a past infection.
“Antibodies are protein molecules that circulate in the blood, which can be measured in direct analysis [blood test]Said Herb Sewell, professor of immunology at the University of Nottingham. “For T cells, they have to be removed from the blood, kept alive and exposed to analysis.”
Technology for mass testing of T-cell immunity is unlikely to be available in the near future. However, the first laboratory studies of the general immune response to Sars-Cov-2, the virus that causes Covid-19, are beginning to report results. One, led by Jennifer Juno at the University of Melbourne and published in Nature Medicine, studied 41 Australians with mild to moderate symptoms.
“Among the cohort, we found. . . a wide range of antibody responses, “he said.” Some high and low, but strong antibody responses were associated. . . a subset of a subset of the T cells that were most effective in helping to drive a better antibody response. “
Several studies suggest that T cells produced by other coronaviruses, which cause only mild cold-like illness, may also recognize Sars-Cov-2 and provide some protection against Covid-19.
This phenomenon may contribute to what some scientists have called “immune dark matter,” which could make Sars-Cov-2 collective immunity achievable with an infection rate as low as 20 percent, rather than the level of 60 percent often mentioned. But the idea is controversial, and much more evidence will be required before gaining wide scientific acceptance.
