Beta2-Glycoprotein I: Implications for a Regulatory Role in Thrombotic Thrombocytopenic Purpura.
Authors: de Laat Bas , Kremer Hovinga Johanna A. , Pos Wouter , Groot Evelyn , Dong Jing-Fei , Lammle Bernard , Voorberg Jan , de Groot Philip G. 
Published: October 2019

Introduction: Thrombotic thrombocytopenic purpura (TTP) is serologically characterized by a severe deficiency of ADAMTS-13 (von Willebrand factor (vWF)-cleaving protease) activity and a pentad of clinical symptoms; thrombocytopenia, microangiopathic hemolytic anemia, neurological symptoms, renal dysfunction and fever. Due to the absence of ADAMTS-13 activity, ultra-large vWF multimers can be detected in plasma of TTP patients. It is thought that these ultra-large vWF-multimers have a more active (=platelet-binding) conformation than normal vWF-multimers. One of the poorly understood observations in TTP is that some patients have no ADAMTS-13 activity for years without suffering from TTP-related clinical symptoms. Apparently, there are additional factors that regulate the activity of vWF and thereby possibly influences disease activity in TTP patients. It was recently published that plasma protein beta2-glycoprotein I (b2GPI) binds activated vWF and thereby impairs its platelet binding capacity. Given the presence of large, active vWF multimers in TTP, we speculated on a role for b2GPI in acute TTP.

Methods: In a population of 38 patients suffering from acute acquired TTP blood samples were drawn. ADAMTS-13 activity, ADAMTS-13 inhibitor levels, anti-ADAMTS-13 IgG titre (Technoclone, Vienna Austria) and b2GPI plasma concentration were measured. Furthermore, we tested the influence of b2GPI on platelet adhesion onto vWF under flow in a flow based-assay (as described by Dong et al., Blood. 2002;100:4033–9); Endothelial cells were stimulated with histamine and subsequently perfused with platelets in the presence/absence of b2GPI.

Results: The median b2GPI plasma concentration in all 38 patients was significantly lower than in controls (87.6ug/ml +/− 7.7 vs 169.0ug/ml +/− 16.7, p=0.001). Furthermore, we found a significant difference in b2GPI plasma levels between patients with a high ADAMTS-13 inhibitor levels (bethesda titre > 3, n=19) and patients having low inhibitor levels (bethesda titre < 3, n=19) of 81.3ug/ml +/− 8.4 compared to 107.5ug/ml +/− 16.1 (p=0.03). In addition, a negative correlation with a trend towards significance was observed between the anti ADAMTS-13-IgG titre and the b2GPI plasma concentration (R=−0.28, p=0.09). In the flow-based assay, we found a concentration dependent inhibition of b2GPI on platelet adhesion. B2GPI coated beads bound directly to the freshly released vWF in a regular (string-like) pattern indicating specific binding of b2GPI to vWF.

Conclusion: In this study, we show that b2GPI was able to inhibit platelet adhesion on freshly released VWF on endothelial cells. TTP patients seem to have lower b2GPI levels than healthy controls and the lowest levels were observed in the patient group with high functional ADAMTS-13 inhibitor titres. Together, our findings suggest that the inhibitory effect of b2GPI on vWF-platelet adhesion is attenuated in TTP possibly resulting in increased disease activity.