THU0206 ANTI-D4GDI ANTIB ODIES ACTIVATE PLATELETS IN VITRO: A POSSIBLE LINK WITH THROMBOCYTOPENIA IN PRIMARY ANTIPHOSPHOLIPID SYNDROME

Background: Thrombocytopenia is a manifestation associated with Primary Antiphospholipid Syndrome (PAPS) and many studies have stressed the leading role played by platelets in the pathogenesis of Antiphospholipid Syndrome (APS) (1). Platelets are highly specialized cells and their activation involves a series of rapid rearrangements of the actin cytoskeleton (2). Recently, we described the presence of autoantibodies against D4GDI (Rho GDP Dissociation Inhibitor Beta, ARHGDIB) in the serum of a large subset of SLE patients and we observed that anti-D4GDI antibodies activated the cytoskeleton remodeling of lymphocytes by inhibiting D4GDI and allowing the upregulation of Rho GTPases, such as Rac1(3). Proteomic and transcriptomic studies indicate that D4GDI is very abundant in platelets and small GTPases of the RHO family are critical regulators of actin dynamics in platelets (4). Objectives: The aim of the present study was to evaluate the presence of anti-D4GDI antibodies in PAPS sera and whether they can affect platelet activation, contributing to the thrombotic events and the thrombocytopenia of PAPS patients. Methods: 38 PAPS patients diagnosed according to the 2006 Sydney classification criteria were enrolled from the Lupus Clinic of the Sapienza University of Rome. 20 normal healthy subjects (NHS) served as controls. Sera were stored at −20 °C to performed an ELISA test using commercial D4GDI protein. Anti-D4GDI antibodies were purified from PAPS sera and used for in vitro treatment of platelets purified from NHS. Flow cytometry analysis was performed to determination of integrin αIIbβ3 activation, a well-established marker of platelet activation and adhesion. Results: We identified anti-D4GDI antibodies in sera from 18/38 (47%) patients with PAPS, but in no sera from NHS. Dividing the patients with PAPS according to the presence or absence of thrombocytopenia, we found a significant association between this hematologic manifestation and a higher titer of anti-D4GDI antibodies. Our in vitro results show a significant 30% increase in the activation of integrin αIIbβ3 upon stimulation of platelets from healthy donors preincubated with the antibody anti-D4GDI purified from the serum of APS patients. Interestingly the antibody does not only increase the overall integrin activation but also the rate/speed of integrin activation. Conclusion: We demonstrated that antibodies anti-D4GDI are present in the sera of PAPS patients and can prime platelet activation. Thus, explaining, at least in part, the pro-thrombotic state and the thrombocytopenia of PAPS patients. These findings may lead to improved diagnosis and treatment of APS. References: [1] Shechter Y, Tal Y, Greenberg A, Brenner B. Platelet activation in patients with antiphospholipid syndrome. Blood Coagul Fibrinolysis. 1998 Oct;9(7):653-7. [2] Seong-Hoon Yun, Eun-Hye Sim, Ri-Young Goh, Joo-In Park, and Jin-Yeong Han. Platelet Activation: The Mechanisms and Potential Biomarkers. BioMed Research International,Volume 2016, Article ID 9060143. Review [3] Barbati C, Alessandri C, Vomero M, Vona R, Colasanti T, Vacirca D, Camerini S, Crescenzi M, Pendolino M, Truglia S, Conti F, Garofalo T, Sorice M, Pierdominici M, Valesini G, Malorni W, Ortona E.Autoantibodies specific to D4GDI modulate Rho GTPase mediated cytoskeleton remodeling and induce autophagy in T lymphocytes. J Autoimmun. 2015 Apr;58:78-89. [4] Rowley JW, et al. Genome-wide RNA-seq analysis of human and mouse platelet transcriptomes. Blood. 2011;118(14). Disclosure of Interests: None declared