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 Recent studies show that some recovered patients who tested negative
for coronavirus antibodies did develop T cells in response to their
COVID-19 infection. While the studies are small and have yet to be
reviewed by outside experts, some scientists now say that people who
experience a mild illness, or no symptoms at all, from the new
coronavirus, may be eliminating the infection through this T cell
response.
The findings add to the evidence that an effective COVID-19 vaccine
will need to prompt T cells to work in addition to producing
antibodies, and may have implications for several treatments in
development. They may also shed light on how immunity to new
exposure to infection could work.
"There is mounting evidence that people exposed to the virus have a
transient (short-lived) antibody response, or have a T cell response
in spite of a minor or absent antibody response," Dr Alessandro
Sette, professor and member of the La Jolla Institute’s Infectious
Disease and Vaccine Center in California, told Reuters.
When a virus gets past the body's initial defenses - which include
infection-fighting white blood cells - a more specific "adaptive"
response kicks in, triggering production of cells that target the
invader. These include antibodies that can recognize a virus and
lock onto it, preventing its entry into a person's cells, as well as
T cells that can kill both invaders and the cells they have
infected.
Six months into a global COVID-19 pandemic that has infected more
than 12 million people, questions remain about whether the antibody
response to this virus is robust and lasts over time. That could
mean T cells have a more important role in offering protection
against the illness.
"T cells are often important in controlling viral infections. We are
seeing evidence of that," John Wherry, director of the University of
Pennsylvania's Institute for Immunology, told Reuters.
A recent small French study
https://www.medrxiv.org/
content/10.1101/2020.06.21.20132449v1, not yet reviewed by experts,
found that six out of eight family members in close contact with
relatives who had COVID-19 developed a T cell response, but did not
test positive for antibodies.
A Swedish study
https://www.biorxiv.org/content/
10.1101/2020.06.29.174888v1.full.pdf of 200 people found a strong T
cell response in most individuals who had mild illness or no
symptoms following coronavirus infection, regardless of whether they
showed an antibody response. The finding suggests that coronavirus
infection rates may be higher than what has been studied using
antibody tests alone, the researchers said.
MEMORIES OF CORONAVIRUS
The focus on T cell responses could also shed light on the prospect
for longer-term immunity.
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There is some evidence that T cells developed after exposure to other
coronaviruses that cause the common cold could help fight off the new virus,
known as SARS-CoV-2.
A study led by the La Jolla Institute detected T cells that reacted to
SARS-CoV-2 in about half of stored blood samples collected between 2015 and
2018, suggesting that the immune system cells developed after previous infection
with circulating common cold coronaviruses, and that they might help protect
against the new virus.
"It is a potentially encouraging piece of evidence," Wherry said.
Vaccine candidates against COVID-19 currently in the works aim to generate
antibody and T cell responses, and the recent findings highlight the importance
of gauging the T cell response seen in human clinical trials.
"We believe that the optimal vaccine design would be one that induces both an
antibody and T cell response," Sette said.
After an infection or vaccination, the immune system retains a number of
"memory" cells that are already primed to quickly attack the same virus in case
of a future infection.
Many countries are using blood tests that look for antibodies to estimate how
many people have been infected with the new virus even if they never showed
symptoms. But scientists still do not know how antibody levels correlate to
exposure to the virus or how long they may last. There are also questions about
exactly which combination of immune-system cells will result in significant
protection.
Measuring the level of memory T cells is much more complicated, especially if
the cells are in lymph nodes or other hard-to-access areas of the body. In
addition, T-cell responses are highly variable.
"It is a lot easier to collect antibody data," said Dr Daniela Weiskopf, an
assistant professor at the La Jolla Institute.
(Reporting by Deena Beasley in Los Angeles; Editing by Michele Gershberg and
Matthew Lewis)
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