Gut Microbiome Implicated in Antiphospholipid Syndrome
Researchers have identified a gut bacterium that can trigger the antiphospholipid antibodies seen in patients with antiphospholipid syndrome (APS), an underrecognized autoimmune disease that can lead to abnormal blood clot formation in arteries and veins. Until now, exactly what triggers these antibodies or what triggers some people with these antibodies to have blood clots has been unknown. In the new study, published in Cell Host & Microbe (2019;26:1-14), researchers say that the human gut microbiome and specifically Roseburia intestinalis is a trigger for autoantibodies in patients who are susceptible to the disease.
“In APS, we haven’t known what triggers the antiphospholipid antibodies. This study sheds light on an important area to understand the mechanisms of APS,” said Doruk Erkan, MD, MPH, a rheumatologist at the Barbara Volcker Center for Women and Rheumatic Diseases, Hospital for Special Surgery, in New York City, an APS expert, and a coauthor of the research. “Millions of microorganisms exist in our gastrointestinal system, and usually these microorganisms are harmless. However if they elicit systemic immune responses, these microorganisms can be problematic by triggering autoimmunity outside the gut.”
APS can occur in patients with or without other systemic autoimmune diseases, such as lupus. In addition to blood clots, APS increases the risk for pregnancy complications, low platelet counts, anemia, kidney disease, and heart valve disease. Antiphospholipid antibodies mainly bind to the antigen β2-glycoprotein-I, a phospholipid binding protein in the blood, and this binding triggers blood clotting and inflammation.
In the new study, the researchers investigated whether APS can be triggered by gut microorganisms, because other research has implicated the role of certain gut bacteria in autoimmune diseases such as lupus. The investigators searched a bacterial protein database to identify bacteria that had a similar snippet in their molecules to the APS-associated self molecule β2-glycoprotein-I targeted in patients by their immune system. This snippet, called epitope, is the part of an antigen molecule to which an antibody attaches itself. They found that core epitopes of β2-glycoprotein-I are conserved in Roseburia intestinalis, paving the way for cross reactivity. In other words, antibodies produced against this bacteria can also dock and bind on β2-glycoprotein-I.
The researchers then recruited, from HSS and Yale, 20 healthy controls and 15 individuals who were persistently antiphospholipid antibody-positive. Cassyane Aguiar, MD a previous HSS fellow and currently a pediatric rheumatologist at Children’s Hospital for the King’s Daughters, Norfolk, VA, was also part of the research team helping with the HSS patient recruitment and follow-up. They collected blood and stool samples at baseline (month 1), and then at month 2 and month 3, to capture microbial and immune marker variability over time. They gathered information on health status, medications, 24-hour diet history, and APS-relevant genetic and immune markers such as other antibodies against self molecules.
Based on studies completed in the laboratory of Martin Kriegel, MD, PhD, at Yale School of Medicine, researchers found that Roseburia intestinalis was capable of eliciting an immune response in humans and that T cells from APS patients respond systemically to the bacteria in a manner that depended on the patient’s genetic predisposition. They found long-lasting T and B cell immune responses against this bug in APS patients. Studies in mice further supported this hypothesis and linked the cross-reactive immune responses with tissue damage. “Benign cross-reactive immune responses to gut microbes is probably very common given that we have so many of them living inside of us,” said Dr. Kriegel, who is the lead investigator on the project. “Some of them, however, can lead to damaging immune responses in predisposed individuals, which we have shown here.”
“This study is important because if we can eliminate Roseburia, then we may be able to decrease the levels of antiphospholipid antibodies,” said Dr. Erkan. “The similar epitopes between β2-glycoprotein-I and the gut microbiome can help us better understand the mechanisms of antiphospholipid antibody production and also open doors for future treatment options.” This includes the use of antibiotics or the development of vaccines for APS and APS-associated autoimmune diseases.
According to Michael Lockshin, MD, Director of Barbara Volcker Center for Women and Rheumatic Diseases, at Hospital for Special Surgery, moving the research forward will require verifying that Roseburia intestinalis is a major, if not the most important, trigger for antiphospholipid antibodies. Since most people with this bacterium do not develop antiphospholipid syndrome, the next step will be to determine why patients develop disease.
“The microbiome itself and the concept of cross-reactivity is hard to study in humans,” said Dr. Kriegel. “We have used molecular approaches and supported our culture dish findings with animal models to show that cross-reactivity induced by a common gut bacterium in APS patients can lead to tissue damage. This study thus went far beyond the typical microbiome surveying studies and made causal links to disease, something still lacking in many studies in the field.”
The current study was a collaborative effort with researchers from HSS; Yale School of Medicine, in New Haven, Connecticut; Benaroya Research Institute, in Seattle, Washington; the Frederick National Laboratory for Cancer Research, in Frederick, Maryland; and Sackler School of Medicine, in Tel Aviv, Israel.
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