The roles of von Willebrand factor discussed

Evidence suggests von Willebrand factor (VWF) has important biological roles beyond haemostasis and inflammation, the Haematology Association of Ireland Annual Meeting heard.

At the meeting, Prof James O’Donnell, Consultant Haematologist, St James’s Hospital, Dublin, delivered a talk entitled ‘Roles for VWF beyond blood clotting’.

“I am going to talk about roles for VWF in sepsis and inflammation,” Prof O’Donnell said. “But they are not limited to that, VWF has also been implicated in wound healing, cellular perforation, blood brain barrier permeability, and tumour cell biology.

“What’s important is that those novel functions of VWF have also been shown to impact on disease pathology,” he said.

Cerebral malaria

The first example was the potential role for VWF in cerebral malaria. An estimated 300 to 500 million cases and 1.5 to 2.7 million deaths occur each year due to malaria.

“Unfortunately, most of them happen in children under the age of five living in endemic areas, notably in Sub-Saharan Africa,” Prof O’Donnell said.

Plasmodium (P) falciparum is responsible for most of this mortality, he stated, adding that its most feared complication was cerebral malaria.

“The children develop diffuse encephalopathy, severe headache, followed by drowsiness, confusion, and coma. It still has a case fatality rate of approximately 30 per cent,” Prof O’Donnell said.

“In spite of all that mortality globally, we don’t really understand the pathophysiology underlying cerebral malaria very well at all.

“What we do know is that malaria-infected red cells adhere to endothelial cells. At the site of adhesion of the infected red cells, that leads to the sequestration of more red cells, and then that leads to the collusion of microvasculature, particularly in the brain.”

Prof O’Donnell said he became interested in the idea that perhaps VWF could be involved in the pathogenesis of P falciparum malaria, particularly in the ‘site of adhesion’ step.

He cited a 2007 Dutch study in The Journal of Infectious Diseases (De Mast et al) that showed VWF was the first protein to be elevated in the blood.

“So, by four days after inoculation, the volunteers had significantly increased plasma VWF levels. That was well before any of the malaria parasites were visible in the peripheral blood,” he said. Another element that occurred was a pathological accumulation of ultra-large VWF multimers, the meeting heard.

“Those multimers accumulate because the ADAMTS13 is significantly reduced in P falciparum malaria,” he said. “This got us thinking, what is the significance of the early marked elevation of VWF? Is it simply a biomarker for endothelial cell activation? Or could the VWF actually be involved in the pathogenesis?

“That latter hypothesis is entirely plausible,” he said.

ADAMTS13 (a distintegrin and metalloproteinase with a Thrombospondin Type 1 motif, member 13) is an enzyme that cleaves the large, multimeric forms of VWF.

The meeting heard that platelets play a critical role in modulating cytoadherence and sequestration.

Citing research by Bridges et al, Prof O’Donnell said under shear test conditions in the lab, activated endothelial cells released elongated VWF strings from Weibel-Palade bodies.

“If we then flow platelets over the top, you get these long platelet-decorated VWF strings,” Prof O’Donnell said.

The researchers were able to show that if they then flowed P falciparum infected red cells over the activated endothelial cells, the platelet-decorated VWF strings were able to catch circulating P falciparum infected red cells.

“That’s important because this is a new mechanism through which VWF decorated strings could lead to sequestration of malaria-infected red cells, and could do so early in the course of malaria,” he said.

Covid-19

Prof O’Donnell stated the second example where VWF and ADAMTS13 might be involved in pathogenesis was Covid-19.

“As you all know the early reports that came out of China right back in February 2020 suggested there was blood clotting activation in patients presenting with Covid-19,” he said.

He highlighted that an Irish Covid vasculopathy study was set up, and samples were collected at the beginning of the pandemic. In keeping with reports from China, researchers noticed people presenting with severe Covid-19 at St James’s Hospital also had elevated D-dimers on admission.

Over time there was a progressive increase in D-dimers in Covid-19 patients with poor outcomes.

“What was different from what was being reported from Wuhan was we were not seeing DIC (disseminated intravascular coagulation),” Prof O’Donnell said. The meeting heard that DIC was rare in Irish patients with severe Covid-19, unlike Wuhan.

“We saw it in less than 5 per cent of the patients that presented with acute Covid. This is right back at the beginning,” he said. “Subsequently, other groups have followed this up, including in Boston, and they had the exact same conclusion.

“This led to the hypothesis that maybe what was happening in Covid-19 coagulopathy was different to regular sepsis. In that the hypercoagulation in Covid-19… instead of having a DIC, maybe the coagulopathy was pulmonary intravascular coagulopathy,” he said.

The meeting heard that coagulation activation in Covid-19 was multifactorial.

“One of the things that the data is pointing to conclusively, at this stage, is that there is a critical role for endothelial cells in Covid-19 coagulopathy,” he said. “In the postmortem studies, all of the sites have the same finding, that in the lungs, there’s severe endothelial cell damage, endothelial cell swelling, apoptosis, and death, and there’s many reasons why the endothelial cells could be damaged in Covid lungs,” he said.

He said that the VWF levels were “just astonishing”.

“It was even higher than we’re seeing in the children with cerebral malaria, and again the levels correlated with the severity of the illness in Covid-19. And just like malaria, the ADAMTS13 levels were significantly reduced,” he said. “So the Covid-19 blood findings are like cerebral malaria, except more so. So is VWF playing a role in Covid-19 pathogenesis as well.

“Those postmortem studies have clearly shown that not only has there been endothelial cell damage in the lungs, but there’s also dissemination of microthrombi, through the small blood vessels of the lungs.”

He added that Covid-19-induced thrombi were associated with megakaryocytes and rich in VWF.

In long Covid, Prof O’Donnell said persistent coagulation activation was being observed in about 25 per cent to 30 per cent of patients.

“In particular we are seeing that VWF levels are increased in 25 per cent to 30 per cent of these people with long Covid and correlating with some of the clinical outcomes.”

Prof O’Donnell said he was intrigued with how VWF could be influencing sepsis and inflammation, and “this is what we have been working on intensely for about three to four years”.

“So as well as cerebral malaria and Covid-19, there is evidence of VWF in other types of sepsis and inflammation,” he said, highlighting sickle cell disease. He said that the most important question was how does VWF influence inflammatory responses?

He said that the researchers hypothesised that the binding of multimeric VWF, which potentially interacts with multiple macrophage receptors, might directly influence macrophage biology.

“The key finding that we had was VWF doesn’t just bind to macrophages, when it binds to macrophages it triggers pro-inflammatory intracellular signalling pathways,” he said.

He said that VWF had numerous roles within the body, including in the regulation of complement activation “so I suspect this story has a long way to go”.

“VWF binds to a lot of things, and so consequently VWF has the potential to function as a biological shuttle. So I suspect we are going to be hearing more and more about its non-haemostatic functions.”

Prof O’Donnell hoped that in the future the HAI would be hearing about VWF’s other non-vascular effects, notably in relation to cancer. He added that anti-VWF targeted therapies might be useful well beyond the scope of haemostasis. Furthermore, he said, clinicians were working to try and determine whether women with heavy menstrual bleeding might have more than just VWF-related bleeding phenotype.

He concluded his presentation by saying that there was “overwhelming evidence” that VWF has roles beyond haemostasis, notably in linking primary haemostasis and innate immunity. The data suggested that through multiple different mechanisms VWF can impact inflammatory responses.

Accumulating data suggested that VWF has important biological roles beyond haemostasis and inflammation, the meeting was told.

“That accumulating data is of clinical importance and also likely impacts VWD [von Willebrand disease] pathology, but also contributes to the pathogenesis of other important diseases,” Prof O’Donnell said.