Can the microthrombosis hypothesis explain the symptoms of long new crown? Long article "Nature"

Originally written by Cassandra Willyard

Some patients choose to try risky treatments because of a microthrombus hypothesis, and scientists have mixed opinions on the evidence behind it
.

Lara Hawthorne, an illustrator in the UK, has developed some strange symptoms after COVID-19 that she hopes weren't caused
by the coronavirus.
She was not seriously ill at first, she said: "I got three shots of the vaccine and should be well protected
.
But months later, a series of symptoms that debilitated her body still lingered: earache, tinnitus, congestion, headache, dizziness, palpitations, muscle pain, etc
.
Some days, Hawthorne's body was so weak that he couldn't even get out of bed
.
When she finally had the strength to go to the doctor, the doctor's diagnosis was the outcome she feared most: long COVID
.

With no antidote, Hawthorne felt increasingly desperate
.
She read an article in The Guardian about how blood clots can cause coronavirus symptoms, and she immediately contacted a physician in Germany who specializes in treating patients
with blood thinners and a blood filtration technique.
She hasn't heard back yet, and rumors have it that the doctor's appointment will take months, but if she had the opportunity to go to the local area to receive these unproven treatments, she might actually go
.
"I don't want to keep delaying my health when my health
is already so bad," she said.
”

Illustration by David Parkins

The long coronavirus has puzzled researchers: hundreds of studies have tried to reveal its mechanism, but little progress has been made (Related Reading:‍New crown sequelae: can the long new crown be cured?| the long article "Nature"‍）
。 Now, some scientists and a growing number of long-term coronavirus patients are beginning to support this hypothesis, which is yet to be proven: small, persistent blood clots may block blood flow to vital organs, causing all kinds of strange symptoms
in the human body.

Proponents of this theory, who call themselves #teamclots on Twitter, include Etheresia Pretorius, a physiologist at the University of Stellenbosch in South Africa, and Douglas Kell
, a systems biologist at the University of Liverpool in the United Kingdom.
The two of them led the first team
to visualize microthrombi in the blood of patients with long COVID.
They say the evidence shows that microthrombi is irrefutable, and they hope that the anticoagulant drug Hawthorne wants to try will be clinically tested
.
Pretorius is the author
of the article Hawthorne read in The Guardian.

However, many hematologists and COVID-19 researchers are concerned that enthusiasm for the thrombosis hypothesis has outgrown available data
.
They want to see larger studies and stronger causal evidence; They also expressed concern
about some people rushing to try unproven, potentially risky therapies.

On the coronavirus issue, "we now have some sporadic evidence," says Danny Altmann, an immunologist at Imperial, "and everyone wants to try to piece the pieces together and come up with some consensus
.
" But we're still far from there, and progress is too slow
.
”

Blood clots grow

Pretorius and Kell met each other
about 10 years ago.
Pretorius has previously studied the role of iron in blood clotting, but has not been able to cite some of Kell's research
.
After Kell's active contact, the two began to communicate
.
Pretorius said: "We talked on Skype and then decided to work together
.
"They found abnormal, dense blood clots in the bodies of patients with various diseases that could not be dissolved
for years.
They built on this research to theorize that some molecules — including iron, proteins, or bacterial cell wall fragments — may induce the formation
of these abnormal blood clots.

Thrombosis is a complex process, but fibrinogen, a cigar-shaped soluble protein, plays an important role
in it.
Fibrinogen moves freely through the blood
.
If trauma occurs, the cells release thrombin, which cleaves fibrinogen into an insoluble protein
called fibrin.
Fibrin fragments become entangled into a network that promotes the formation of blood clots and helps stop bleeding
.

Under a microscope, the net normally looks like "a plate of delicious pasta," Kell said
.
But the blood clots the team found in many inflammatory diseases don't look the same
.
They're "ugly, sticky and dark," Kell says, "like you glue all the half-cooked pasta together
.
" Kell, Pretorius, and colleagues have shown that these fibrin proteins are misfolded [1] into a sticky version
of "amyloid.
" Jell says just a small number of misfolds can trigger disaster
.
"If the first one changes its configuration, the rest will also be drawn according to the gourd", much like prions, which are also contagious misfolded proteins that cause Creutzfeldt–Jakob disease
.

Pretorius first found these bizarre, densely packed blood clots in the blood of patients with a certain clotting disorder [2], but she and Kell later observed this phenomenon in a variety of diseases [1], including diabetes, Alzheimer's disease, Parkinson's disease, and so on
.
However, their hypothesis has not received much attention
until now.

When the pandemic hit in 2020, Klell and Pretorius almost immediately applied their approach to people infected with the virus
.
"We wanted to look at the clotting problem with COVID because that's what we've
been working on," Pretorius said.
They experimented with a special dye that fluoresces when bound to amyloid, including misfolded fibrin, and visualized
the luminescence under a microscope.
The team compared plasma samples from 13 healthy volunteers, 15 COVID-19 patients, 10 diabetics, and 11 patients with COVID-19 [3].

Pretorius said that for COVID and acute COVID-19, thrombosis "is much more than we have previously found in diabetes or other inflammatory diseases.
"
In another study [4], they analyzed the blood of 80 patients with long COVID and found microthrombi
in all samples.

So far, Pretorius, Kell and their colleagues are the only team
to have published the results of microthrombus studies in patients with long COVID.

But in other as-yet-unpublished studies, Caroline Dalton, a neuroscientist at Sheffield Hallam University's Centre for Biomolecular Sciences, has repeated the results
.
She and her colleagues used a slightly different method—using an automated microscope imaging scanner—to count the number of
blood clots in the blood.
The team compared three groups of people, each with about 25 people: one group known to have never had COVID-19, one group who had COVID-19 and recovered, and one group who had COVID-19 and had a long COVID patient
.
All 3 groups had microclots, but people who had never had COVID-19 had fewer and smaller blood clots; Patients with long new crowns have more blood clots and are also larger; People with a history of infection fall somewhere in between
.
The team's hypothesis is that a new coronavirus infection suddenly forms a large number of microthrombi in the body, but these microthrombs disappear
over time.
But for patients with long new crowns, these blood clots seem to be present
forever.

Dalton also found that fatigue scores also seemed to correlate with the number of microthrombi — at least in some people
.
This boosted our confidence that our subjects were mechanically related to the disease,"
Dalton said.

In many ways, the new crown resembles another unexplained disease: chronic fatigue syndrome, also known as myalgic encephalomyelitis (ME/CFS).

Maureen Hanson, director of the ME/CFS Collaborative Research Center at Cornell University's National Institutes of Health (NIH), believes that Pretorius and Kell's study has refocused attention on a hypothesis developed in the 1980s that abnormal blood clots cause symptoms
.
Pretorius, Kell, and colleagues found amyloid blood clots in the blood of ME/CFS patients, but in much smaller numbers than they found in patients with long COVID [5].

Pretorius says thrombosis may only be part of the cause
of ME/CFS.

The mystery of microclots

The question of where exactly these microthrombs come from is not fully clear
.
But Pretorius and Kell believe that the spike protein that the new coronavirus uses to enter cells may be an inducing factor
in patients with long new crowns.
After adding the spike protein to the plasma of healthy volunteers in the laboratory, they found that this step alone was enough to induce the formation of these abnormal blood clots [6].

There is evidence that the spike protein may indeed be involved
.
In a preprint paper published in June [7], researchers at Harvard University reported the discovery of the spike protein
in the blood of patients with long COVID.
Another paper published by a Swedish team [8] suggests that certain peptides in the spike protein can form amyloid fragments on their own—at least in test
tubes.
The misfolded fragments may provide a template, says author Sofie Nyström, a protein chemist at Linköping University
.

A study subject's microthrombus (green) before infection with the novel coronavirus (left four panels)
and after the onset of long coronavirus symptoms (right four panels).
Source: E.
Pretorius et al.
/Cardiovasc.
Diabetol.
(CC BY 4.
0)

A team in California found that fibrin does bind
to the spike protein.
A 2021 preprint paper [9] states that when these two proteins are combined, fibrin can exacerbate inflammation and form thrombes
that are more difficult to dissolve.
But how these puzzle pieces should be pieced together remains unclear
.

If the spike protein is the cause of abnormal blood clots, can a new crown vaccine containing the spike protein or containing instructions to form the spike protein also induce the formation of these blood clots? There is no direct evidence that the spike protein from the vaccine helps form blood clots, but Pretorius and Kell have received funding
from the Medical Research Council of South Africa to study the problem.
(Rare thrombotic events associated with the Oxford-AstraZeneca vaccine are thought to be caused by another mechanism; Nature 596, 479–481; 2021）

Concerns about the safety of coronavirus vaccines can cause panic, says
Per Hammarström, a protein chemist at Linköping University and co-author of Nyström.
"We don't want to sensationalize, but if it's really a medical problem, even if it's just about some people, we're going to point it
out.
" Gregory Poland, head of the vaccine research team at Mayo Clinic in Minnesota, agrees that the issue is important
.
"My guess is that we'll end up finding that the spike protein and the new coronavirus have all kinds of surprising pathophysiological features," he said, "and I don't know
how many of these claims about the new crown vaccine are true or wrong.
" ”

Data famine

Many researchers believe that the theory that microthrombus may cause a new crown is quite credible and worth studying
.
This theory also seems to coincide with other data on thrombosis
.
Researchers know that people with COVID-19 are more likely to develop blood clots, especially those
with severe disease.
The new coronavirus can infect the endothelial cells of blood vessels 100,000 kilometers long in the human body, causing inflammation and damage
that induce thrombosis.

These blood clots may have an impact
on physiological function.
Danny Jonigk, a pathologist at the Hannover Medical School in Germany, and colleagues found microthrombi in tissue samples from COVID-19 deceased people and observed branches of capillaries that maintain the flow of oxygen-rich blood by forming new branches [10].

The downside here is that these branches disturb blood flow, promoting the formation of new blood clots
.

Many laboratories have also found that for some people, the tendency to form blood clots continues months after
the initial infection.
James O'Donnell, a blood and thrombosis specialist at Trinity University Dublin, and colleagues found [11] that about 25% of people who recovered from COVID-19 showed a "very pronounced and unusual" increase in blood clots
.

It's still unclear whether these abnormal thrombotic reactions are causing any of the symptoms of COVID, "or is it just another anomaly associated with COVID?" O'Donnell said
.

Alex Spyropoulos, a hematologist at the Feinstein Institute for Medical Research in New York, said the microthrombus hypothesis suggests "a very succinct mechanism.
"
But he noted that more research is needed to link laboratory markers to clinical symptoms
.
Spyropoulos said: "It's disturbing that these authors and others are
a little too confident.
”

Jeffrey Weitz, a blood and thrombosis expert at McMaster University in Canada, noted that the method used by the Pretorius team to find microthrombi is "not a standard method at all.
"
He explains, "I'd like to see confirmation
from other researchers.
"Microthrombi are difficult to detect
.
Pathologists can find them in tissue samples, but hematologists generally look for markers of abnormal thrombosis, not the thrombus itself
.

Other larger studies of the long coronavirus have not been able to find signs of
thrombosis.
Michael Sneller, an infectious disease expert at the National Institutes of Health, and colleagues conducted a comprehensive examination of 189 people infected with the new coronavirus, some with persistent symptoms, others without, and 120 controls [12].

They didn't specifically look for microclots, but if they blocked capillaries, Sneller should be able to find some evidence — such as tissue damage
in organs where capillaries are concentrated, such as the lungs and kidneys.
Microthrombi can also damage red blood cells, leading to anemia
.
But Sneller and his colleagues didn't find these signs
in any of their lab tests.

Kerll and Pretorius say that just because the study didn't find any evidence of microthrombi doesn't mean microthrombi doesn't exist
.
A key issue surrounding the coronavirus is that "the results of all the tests end up within the normal range," Pretorius said, "but these are patients who are in a hurry, and they don't have a
diagnosis.
" She hopes other researchers will read their paper and try to replicate their results
.
"After that, we'll be able to talk about it
," she said.
She also said that the final decisive causal evidence will be that patients with long new crowns will gradually improve after receiving anticoagulant therapy
.

There is some limited evidence
in this regard.
Kell, Pretorius, and other researchers, including physician Gert Jacobus Laubscher at Stellenbosch University, noted in an earlier version of a preprint paper published in December 2021 that 24 patients with long COVID who were treated with a combination of two antiplatelet drugs and one anticoagulant experienced some remission [13].

Subjects reported that their main symptoms disappeared and fatigue decreased
.
Their microthrombus was also reduced
.
Pretorius and Kell are collecting more data in order to officially publish the results
.
But other doctors are already using these drugs to treat patients with new coronavirus
.
Some even offer a dialysis-like technique to filter fibrinogen and other inflammatory molecules
in the blood.
For O'Donnell, this type of therapy is not yet mature
.
He agrees that some patients with long COVID are indeed more likely to develop blood clots, but he said that scaling from a small study to treating a large number of people will not appear in
my 2022 plans.
Sneller
agrees.
"Anticoagulation therapy is not completely harmless
.
You're essentially interfering with your blood's ability to clot," he said, potentially putting even small traumas at risk
.

Kell said he was tired of waiting for consensus on a long-term coronavirus treatment
.
"These patients are deeply afflicted
," he said.
They are in bad health and anxious
inside.
Altmann understands the annoyance
.
Almost every day, he gets emails asking, "Have clinical trials of these drugs been done?" Why wait so long? But he also said that even in the days when the pandemic is hot, researchers must follow
the process.
I'm not belittling anyone's data
.
I just want to say we're not there yet," he said, "let's
connect the dots first.
" ”

References:

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B.
, Laubscher, G.
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& Pretorius, E.
 Biochem.
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 479, 537–559 (2022).

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, Briedenhann, S.
, Marx, J.
& Franz, R.
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 et al.
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 et al.
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, Kruger, A.
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 J.
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The original article was published in the News Features section of Nature on August 24, 2022, as Could tiny blood clots cause long COVID's puzzling symptoms

© nature

doi: 10.
1038/d41586-022-02286-7