'Killer T cells' dodge variants to help prevent severe COVID-19

A scanning electron micrograph of a human T lymphocyte (also called a T cell) from the immune system of a healthy donor, in this National Institute of Allergy and Infectious Disease (NIAID) handout photo obtained March 30, 2021. (Photo by Handout / National Institute of Allergy and Infectious Diseases / AFP)

by French Press Agency - AFP

Mar 30, 2021 11:46 pm

In good news about vaccines that came out Tuesday, "killer T cells" remain mostly unaffected by coronavirus variants that may dodge antibodies or reinfect those who have recovered, a new study shows.

The finding is encouraging because although these white blood cells are not a first-line defense against infection, they can help prevent severe disease.

Scientists at the National Institutes of Health and Johns Hopkins University analyzed blood samples from 30 people who had contracted and recovered from COVID-19 prior to the emergence of variants.

They published their findings in Open Forum Infectious Diseases, an Oxford University Press journal.

The team wanted to know whether these cells, known by their technical name CD8+ T cells, could still recognize the three major variants of SARS-CoV-2 that are circling the globe: B.1.1.7, first found in Britain; B.1.351, identified in South Africa and B.1.1.248, first seen in Brazil.

What makes each of these variants unique is the mutations they carry, especially in a region of the virus' spike protein, structures that stud its surface and allow it to invade cells.

It has already been shown that mutations to this region of the spike protein make some variants less recognizable to neutralizing antibodies – infection-fighting proteins produced by the immune system's B cells.

This seems to be particularly true, for instance, of B.1.351, according to research on the impact of current generation COVID-19 vaccines that may need to be adapted as more variants surface.

Sandile Cele – a doctoral candidate at the University of Kwazulu-Natal who figured out how to grow the South African variant of the coronavirus in the laboratory, which enabled them to test it and discover that people previously infected with COVID-19 don't produce antibodies against the mutant version – poses in a lab in Durban, South Africa, March 4, 2021. (AP Photo)

Neutralizing antibodies are custom-made to fit an antigen or a specific structure of a pathogen.

In the case of the coronavirus, this is the spike protein, which the antibodies bind to, preventing the virus from infecting cells.

Killer T cells, on the other hand, look for telltale signs of cells that have already been infected with pathogens they have previously encountered and then kill those cells.

In the new study, the researchers found that the killer T cell responses remained largely intact and could recognize virtually all mutations in the variants studied.

The researchers noted that larger studies are needed to confirm the results, but said that it nevertheless demonstrated that killer T cells are less susceptible to the more than 4,000 variants in the coronavirus than neutralizing antibodies are.

Antibodies are still important to prevent infection in the first place – and the reduced efficacy of vaccines to the variants seems to be evidence of this.

But a killer T cell response that kicks in later and aids in clearing off the disease helps explain why the vaccines seem to be able to prevent severe disease and hospitalization, even though their efficacy at stopping infection by variants is reduced.