Scientists
studying survivors of recent outbreak in west Africa say some women never
contracted the virus despite having Ebola antibodies
A study of Ebola survivors in west Africa has found a group of
women who appear to be immune to the deadly virus.
The discovery was made by a team of British and European
scientists who are studying Ebola survivors in Guinea.
“These are phenomenal women who have had a horrendous story
to tell. They have had lots of contact with the virus, clearing up vomit,
diarrhoea, sleeping with children with Ebola overnight, and they never
presented with Ebola symptoms and somehow
they have an immune response to the virus,” said Prof Miles Carroll, a
virologist at the UK government’s chemical and biological weapons research
facility at Porton Down, Wiltshire.
Early data from the research also offers a theory as to why
survivors have not contracted Ebola for a second time even when the virus
lingers in places such as the testes, spinal cord and eye chamber.
The research bodes well for the 16,000 survivors in Sierra
Leone, Liberia and Guinea, although British nurse Pauline
Cafferkey has shocked experts by suffering complications from an earlier Ebola
infection.
She is critically ill eight months after being discharged
from hospital having apparently recovered from the infection.
The most eye-catching finding in the research came from the
parallel study of women who failed to show symptoms of Ebola even though they
had been in direct contact with people who had died of the illness.
Ebola is highly contagious and is transmitted through bodily
fluids including saliva, blood, urine, vomit and faeces, once a patient becomes
ill.
The team studied 60 women, 25 of whom were in Guéckédou, the
town where the most recent Ebola epidemic in west Africa is believed to have
started.
Blood tests showed one woman in this subset had Ebola
antibodies even though she had never contracted the virus.
“We tested her blood eight months later in May 2015 and she
had a phenomenally strong neutralising antibody response to Ebola,” said
Carroll, who is also head of research at Public Health England’s national
infection service.
Not enough research has been done to determine why these
women are immune, but Carroll said the ongoing study will try to establish
which part of the immune system kicks in to fight off the virus.
“It may be that these people are genetically unique and have
an innate response strong enough to fight Ebola before it can get a foothold,”
said Carroll. The findings suggest there are people who are exposed to the
virus but never become infected.
Before the epidemic, it was widely assumed that Ebola
survivors would have lifetime immunity to the virus but there was limited scientific
evidence to prove this.
Carroll’s project, which is ongoing and not yet published in
a scientific journal, will test the theory that survivors develop long-term
immunity because, for a limited time, the virus lingers in parts of the body
the immune system cannot reach. He believes that when it leaches back into the
blood it restimulates the immune response.
His team found that the immune response in survivors is
approximately 10 times stronger in survivors than in those who received a
vaccine being tested in Guinea.
The team also discovered that the strength of the immune
response did not appear to diminish over the year they studied the individuals.
With EU funding, Carroll and his collaborator, German
scientist Prof Stephan Günther, started looking at blood and semen samples from
Ebola survivors in Guinea from 2014. They discovered, as others have, that the
virus continued to be present in the semen, but not in the blood of
approximately one third of their sample.
With a further £2m funding from the US Food and Drugs
Administration and the assistance of German virologist Dr Stephan Becker, the
researchers have now selected 130 other survivors for a three-year programme
researching the infectivity in blood as well as the state of their health.
Blood samples were taken in May and August and immune
analysis suggests the virus continues to re-enter the blood of survivors but is
instantly killed off.
“What is likely to be happening is the virus is lingering
around these immune-privileged sites like the gonads and the spinal cord but
continually leaching into the body, but the survivor never experiences symptoms
because it is instantly soaked up by antibodies and T cells [part of the body’s
immune response],” said Carroll.
He said the team found “really strong
neutralising antibody response” that was 10 times higher than the response in
those who had taken part in a vaccination programme.
“Quite likely what is happening is the reason
they were so strong was the virus was leaching again and again into the
bloodstream and this was boosting the immune system,” said Carroll.
And the immune response
did not appear to get weaker with time. “The naturally acquired immunity is
exceptionally strong,” he said.
Prof Ian Goodfellow,
head of virology at the University of Cambridge, said nothing about Ebola
surprises him.
“All our knowledge of
Ebola up to this epidemic has been based on very small cohorts. We have
absolutely no idea how a host’s genetic make-up affects the immune response.
He said the women may
have been genetically predisposed to being Ebola immune or exposure to a
similar virus in the past may have conferred a degree of immunity.
He said previous
research has shown the Ebola virus in Sierra Leone’s population before this
outbreak and this could also be the case in Guinea.
“We all vary in our
ability to be affected by a virus that can be genetic or adaptive. It is one of
the reasons why we survive as a human race,” said Good fellow.
