Platelets vs. blood vessels: what causes bleeding in fetuses and newborns with FNAIT?
In brief...
This study shows that impairment of blood vessel formation plays a major role in a devastating bleeding disorder of fetuses and newborns.
Blood platelets have long been the focus of FNAIT research. Platelets are small fragments that clot blood to prevent bleeding. Newborns with FNAIT have fewer platelets, which leaves them at risk of bleeding. The low platelet counts in newborns with FNAIT occurs because the mother forms antibodies that recognize the baby’s platelets. Antibodies are proteins that usually help protect the body against foreign matter such as bacteria and viruses. In FNAIT, the mother’s antibodies cross the placenta and bind to the baby’s platelets, identifying them as “foreign”. The baby’s platelets are then targeted by the immune system and cleared from the baby’s circulation, leading to a low platelet count
How low platelet counts lead to intracranial bleeding is not clear, and there is evidence suggesting that more is at play. In many cases of FNAIT, the mother’s antibodies bind to a protein called beta 3 integrin on the baby’s platelets, causing the immune system to attack the platelets. However, beta 3 integrin is also found in blood vessels when they are being formed. The process of forming new blood vessels (called angiogenesis) takes place during the development of the fetus, at a time when FNAIT is observed. What role beta 3 integrin plays in blood vessel formation is controversial. This study investigated the role of beta 3 integrin and other platelet proteins in FNAIT to try to understand the cause of intracranial bleeding.
What did the researchers do?
This research cannot be carried out in humans, so the symptoms of FNAIT were mimicked in laboratory mice. The study used genetically-modified female mice lacking either beta 3 integrin or GPIb alpha, another common platelet protein that can be recognized by maternal antibodies in FNAIT. These mice were immunized against platelets, causing them to produce antibodies against either beta 3 integrin or GPIb alpha. The mice were bred, and their offspring developed FNAIT. The symptoms and progression of the disease were compared between FNAIT caused by maternal antibodies against beta 3 integrin and FNAIT caused by antibodies against GPIb alpha. Platelet counts were measured in all models of FNAIT. High resolution imaging was used to monitor bleeding. In vitro cellular immune-chemistry experiments in mouse and in human cells were also conducted.
What did the researchers find?
Low platelet counts occurred in all models of FNAIT. When FNAIT was caused by antibodies against beta 3 integrin, intracranial bleeding occurred. Surprisingly, when FNAIT was caused by antibodies against GPIb alpha, intracranial bleeding was never found, although low platelet counts still occurred. When FNAIT was caused by antibodies against beta 3 integrin, but not by antibodies against GPIb alpha, there were symptoms that suggest blood vessel formation was affected. For example, when FNAIT was caused by antibodies against beta 3 integrin:
- There was a reduction in the density of blood vessels in the brain and in the retina of the eye.
- There were less cells lining the brain blood vessels due to increased cell death.
These symptoms were not seen when FNAIT was caused by antibodies against GPIb alpha. In vitro experiments in human cells showed that the same processes are likely happening in human FNAIT caused by antibodies against beta 3 integrin.
How can you use this research?
In this study, intracranial bleeds occurred when FNAIT was caused by antibodies against beta 3 integrin and not when FNAIT was caused by antibodies against GPIb alpha. This shows for the first time that intracranial bleeds are caused by more than low platelet counts. Antibodies against beta 3 integrin prevented proper formation of blood vessels in the brain and in the eye, strongly suggesting that the major cause of intracranial bleeding is impairment of blood vessel formation. It is possible that both impaired blood vessel formation and low platelet counts act together to cause intracranial bleeds in patients with FNAIT. This research changes the previously-held view of what causes intracranial bleeding in FNAIT. It opens other research questions for investigation: how beta 3 integrin contributes to blood vessel formation; and how platelets contribute to blood clotting in developing fetuses.
Treatment of FNAIT, particularly in fetuses in the womb, remains a clinical challenge. Currently, newborns with FNAIT are given platelet transfusions to raise their platelet counts. Mothers of FNAIT babies can be treated with a therapy called intravenous immunoglobulin (IVIG) in subsequent pregnancies to prevent FNAIT. This study suggests that giving IVIG, which is prepared from donated plasma and provided by Canadian Blood Services, may prevent intracranial bleeds associated with FNAIT. Studies such as this one point the way in developing new therapies for this devastating disease. Based on their research to understand FNAIT, the researchers have explored several new therapy ideas. Many of these show promise, including an approach to halt production of the mother’s anti-platelet antibodies, and an approach to prevent these antibodies from crossing the placenta. Although more research is needed to take these from the laboratory bench to the clinic, these findings will ultimately help improve care for fetuses and newborns affected with FNAIT.
About the research team
The research was conducted in the laboratory of Dr. Heyu Ni a Canadian Blood Services scientist and a professor in the departments of laboratory medicine and pathobiology, physiology and medicine at the University of Toronto. Dr. Ni is also a member of the Toronto platelet immunology group at the Keenan Research Centre for Biomedical Science at St. Michael’s Hospital, Toronto. He is the platform director for Hematology, Cancer, and Immunologic Diseases at St. Michael’s Hospital. The research team included members of Dr. Ni’s laboratories, as well as collaborators from the division of cardiology and departments of ophthalmology and vision science, obstetrics and gynaecology, physiology and medicine, all at the University of Toronto, Toronto, ON, as well as the department of laboratory medicine, University Hospital of Northern Norway, Tromsø, Norway.
This research unit is derived from the following publication(s)
[1] Yougbaré I, Lang S, Yang H, Chen P, Zhao X, Tai W-S, Zdravic D, Vadasz B, Li C, Piran S, Marshall A, Zhu G, Tiller H, Kjaer Killie M, Boyd S, Leong-Poi H, Wen X-Y, Skogen B, Adamson SL, Freedman J, Ni H: Maternal anti-platelet β3 integrin impair angiogenesis and cause intracranial hemorrhage. J Clin Invest. 2015
Acknowledgements: This research was supported financially by the Canadian Institutes of Health Research, with equipment funds from the Canadian Foundation for Innovation, St. Michael’s Hospital, and Canadian Blood Services, which is funded by the federal (Health Canada), provincial and territorial Ministries of Health. I. Yougbaré and H. Yang are recipients of Canadian Blood Services postdoctoral fellowship awards and D. Zdravic is the recipient of a Canadian Blood Services graduate fellowship award.
Keywords: Fetal and Neonatal Alloimmune Thrombocytopenia, angiogenesis, bleeding, platelet, antibody, IVIG, beta 3 integrin
Want to know more? Contact Dr. Heyu Ni at nih@smh.ca.
Research unit is a knowledge mobilization tool developed by Canadian Blood Services’ Centre for Innovation
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