Even as the COVID-19 pandemic continues to spread across the world, researchers are still trying to understand the many ways in which it can manifest and its pathogenesis. A new report by researchers at the University of Michigan and published on the preprint server medRxiv* in June 2020 shows that one possible mechanism of disease in COVID-19 is the production of antiphospholipid antibodies.
Antibody and virus - visual concept of the immune system. Illustration Credit: Peter Schreiber / Shutterstock
COVID-19 and Coagulopathy
Severe COVID-19 is correlated with the appearance of abnormalities in the coagulation parameters like D-dimer and fibrinogen degradation products (FDP). The D-dimer level is described in some studies as an independent predictor of COVID-19 mortality. Arterial clots and, more commonly, venous thromboembolism (VTE), have also been described in this condition, almost undoubtedly underdiagnosed due to the difficulty of imaging blood vessels when patients are under isolation nursing and the fact that the D-dimer level is already elevated.
The strange thing about COVID-19 coagulation defects is the normal levels of most coagulation factors, as well as the platelet counts and fibrinogen concentration. The mechanisms of coagulation could include the hyperactive cytokine response by which white blood cells, vascular endothelium, and platelets are activated, leading to arterial blockage, hypoxia of the area supplied, and direct viral infection of cells. High concentrations of neutrophil extracellular traps (NETs) in the blood of several hospitalized COVID-19 patients also contribute to increased coagulation.
The Antiphospholipid Syndrome (APS)
The antiphospholipid syndrome (APS) is an acquired disorder of coagulation that affects millions of people worldwide. The affected individuals have autoantibodies to phospholipids and phospholipid-binding proteins, including prothrombin and beta-2-glycoprotein I (β2GPI). These bind to cell surfaces and activate platelets, neutrophils, and endothelium, triggering the release of molecules that cause thrombosis in all types and sizes of blood vessels. Its diagnosis involves testing for anticardiolipin and anti- β2GPI antibodies, and lupus anticoagulant (LAC) testing.
Catastrophic APS (CAPS) is the most severe form of APS and the rarest, causing both inflammation and thrombosis in multiple organs at the same time. The most commonly affected organs were the kidneys, in over 70%, the lungs and brain in about 60%, and the heart and skin in about 50%.
Various other human viruses are known to produce antiphospholipid (aPL) antibodies, including HIV and HCV. About a third of these patients will have persistent aPL for months, at least. Recent research shows that 116/163 published cases of aPL occurring after viral infection were associated with thrombosis. While this may be unduly high due to publication bias, the possibility of clotting events obviously exists.
Whether severe and even less symptomatic cases of COVID-19 are associated with clotting is the question. The current study focused on testing for aPL in a large group of COVID-19 patients and for procoagulation properties of IgG from these patients.
Testing for aPL in COVID-19
The researchers tested eight types of aPL in 172 hospitalized COVID-19 patients, of which 89 (52%) were positive for one or more aPL, with two-thirds of these having moderate to high levels.
LAC testing was not done because fresh plasma was not available. The most common aPL was anti-phosphatidylserine/prothrombin (anti-PS/PT) IgG and anticardiolipin IgM, at almost a quarter of patients each, and anti-PS/PT IgM in a fifth. Almost 25% of patients had more than one aPL.
They also found that aPL associated with COVID-19 correlated closely with clinical parameters of neutrophil activation such as calprotectin, and with kidney function as assessed by the estimated glomerular filtration rate (eGFR).
The lungs are almost always affected in COVID-19, and the researchers suggest that this could be due to the induction of local immunity from virally infected cells, including possibly the endothelium. This could act to enhance the action of circulating aPL, resulting in very severe lung injury by both thrombotic and inflammatory mechanisms.
NETs have been found to be involved in the disease process in APS, and the researchers in the current study have previously reported the release of NETs from neutrophils on exposure to aPL or serum from patients with APS. These neutrophils are more prone to adhesion to the endothelium because of the higher expression of integrin Mac-1. This leads to a higher NET formation as a result of neutrophil-endothelium interaction and a higher risk of thrombosis with earlier disease.
The researchers found that IgG from aPL-positive COVID-19 patients led to the release of NET just like that from patients known to have active APS, and in contrast to IgG from COVID-19 patients without aPL antibodies. These antibodies, when taken from patients with a high aPL titer, also led to increased clot formation in mouse models, along with an increase in circulating NET remnants in the bloodstream.
Applications of the Study
The study, therefore, indicates the potential of therapies that reduce the formation of NET, such as selective adenosine A2A receptor agonists that prevent the release of NETs in response to aPL, and reduces the chances of thrombosis in the large veins. Another such drug is dipyridamole.
The study also adds to the early literature demonstrating that IgGs with a high level of anti-PS/PT are NET triggers in vitro and enhance thrombosis in vivo. Moreover, it adds a level of explanation to the use of anticoagulants like heparin, corticosteroids, and plasmapheresis, and these do improve the outcome, especially in the critically ill COVID-19 patients.
However, their use could be refined by restricting their use to those patients with high aPL titers. The screening of convalescent serum for aPL or other autoantibodies could also help define the mechanisms by which these are useful in COVID-19 patients.
The study has limitations, including the fact that fresh plasma was unavailable, and therefore, LAC testing was not possible. Also, aPL measurements were scattered over the course of the hospital stay, which limits the availability of the timeline of aPL evolution in the context of the disease. The study concludes, “Testing aPL, including anti-PS/PT, may lead to improved risk stratification and personalization of treatment.”
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.