TLR2 Is One of the Endothelial Receptors for β2-Glycoprotein I

During the antiphospholipid syndrome, β2-gpI interacts with phospholipids on endothelial cell (EC) surface to allow the binding of autoantibodies. However, induced-pathogenic intracellular signals suggest that β2-gpI associates also with a receptor that is still not clearly identified. TLR2 and TLR4 have long been suspected, yet interactions between TLRs and β2-gpI have never been unequivocally proven. The aim of the study was to identify the TLR directly involved in the binding of β2-gpI on EC surface. β2-gpI was not synthesized and secreted by ECs in vitro, but rather taken up from FCS. This uptake occurred through association with TLR2 and TLR4 which partitioned together in the lipid rafts of ECs. After coimmunoprecipitation, mass-spectrometry identification of peptides demonstrated that TLR2, but not TLR4, was implicated in the β2-gpI retention. These results were further confirmed by plasmon resonance-based studies. Finally, siRNA were used to obtain TLR2-deficient ECs that lost their ability to bind biotinylated β2-gpI and to trigger downstream phosphorylation of kinases and activation of NFκB. TLR4 may upregulate TLR2 expression, thereby contributing to β2-gpI uptake. However, our data demonstrate that direct binding of β2-gpI on EC surface occurs through direct interaction with TLR2. Furthermore, signaling for anti–β2-gpI may be envisioned as a multiprotein complex concentrated in lipid rafts on the EC membrane.