Anti-beta2-glycoprotein I antibodies in complex with beta2-glycoprotein I can activate platelets in a dysregulated manner via glycoprotein Ib-IX-V.
OBJECTIVE
Results of previous studies suggest that anti-beta2-glycoprotein I (anti-beta2GPI) antibodies in complex with beta2GPI activate platelets in a dysregulated manner, potentially contributing to the prothrombotic tendency associated with the antiphospholipid syndrome (APS). We undertook this study to investigate the possible contribution of the GPIb-IX-V receptor to platelet activation mediated by the anti-beta2GPI antibody-beta2GPI complex.


METHODS
In vitro methods were used in the present study. The interaction between beta2GPI and the GPIbalpha subunit of the GPIb-IX-V receptor was delineated using direct binding and competitive inhibition assays. The interaction between the anti-beta2GPI antibody-beta2GPI complex and platelets was studied using a novel method in which the Fc portion of the antibody was immobilized using protein A coated onto a microtiter plate. Platelet activation was assessed by two methods; one involved measuring thromboxane B2 production and the other involved assessment of the activation of the phosphatidylinositol 3-kinase/Akt/glycogen synthase kinase 3beta intracellular signaling pathway. The contribution of the GPIbalpha receptor to platelet activation induced by the anti-beta2GPI antibody-beta2GPI complex was assessed by observing the influence of 2 anti-GPIbalpha antibodies (AK2 and SZ2) directed against distinct epitopes.


RESULTS
This study showed that beta(2)GPI could bind to the GPIbalpha receptor. The anti-beta2GPI antibody-beta2GPI complex was able to activate platelets, and this effect was inhibited by anti-GPIbalpha antibody directed against epitope Leu-36-Gln-59, but not by anti-GPIbalpha antibody directed against residues Tyr-276-Glu-282.


CONCLUSIONS
Our findings show that inappropriate platelet activation by the anti-beta2GPI antibody-beta2GPI complex via the GPIbalpha receptor may contribute to the prothrombotic tendency associated with APS.
Shi
Tong
T
St. George Hospital, University of New South Wales, Sydney, New South Wales, Australia.
Giannakopoulos
Bill
B
Yan
Xiaokai
X
Yu
Pei
P
Berndt
Michael C
MC
Andrews
Robert K
RK
Rivera
Juan
J
Iverson
G Michael
GM
Cockerill
Keith A
KA
Linnik
Matthew D
MD
Krilis
Steven A
SA
D019138:Antibodies, Blocking / Q000494:pharmacology; D001323:Autoantibodies / Q000276:immunology; D001666:Binding Sites, Antibody / Q000276:immunology; D001792:Blood Platelets / Q000187:drug effects / Q000276:immunology* / Q000378:metabolism; D006023:Glycoproteins / Q000276:immunology* / Q000378:metabolism; D006801:Humans; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D015539:Platelet Activation / Q000187:drug effects / Q000276:immunology*; D019038:Platelet Glycoprotein GPIb-IX Complex / Q000276:immunology* / Q000378:metabolism; D013929:Thromboxane B2 / Q000378:metabolism; D053482:beta 2-Glycoprotein I
D019138:Antibodies, Blocking; D001323:Autoantibodies; D006023:Glycoproteins; D019038:Platelet Glycoprotein GPIb-IX Complex; D053482:beta 2-Glycoprotein I; C023592:glycocalicin; D013929:Thromboxane B2; D019869:Phosphatidylinositol 3-Kinases