Background and aims In antiphospholipid syndrome (APS), antibodies reactive to CL-beta2-GPI are known to be the important pathogenic factor, but the mechanism of the interaction between the antibodies and cells, and the reason why APS is highly associated with SLE are not fully elucidated.

Methods Since we obtained a monoclonal antibody WB-6 which shows reactivity to CL-beta2-GPI and induces a prothrombotic state in normal mice by tissue factor expression upon circulating monocytes, we tried to clarify how this antibody interacts with live cells.

Results In the current study, we found unexpectedly that WB-6 reacted with DNA by direct-binding ELISA which was confirmed by inhibition ELISA. The result of epitope mapping on the domain 1 of beta2-GPI suggested that WB-6 binds to the arginine- and lysine-rich peptides close to the N-terminal of beta2-GPI, not directly but indirectly via DNA. Incubation of endothelial cell lines or monocytic THP-1 cells with WB-6 revealed that WB-6 enter into the live cells. Because pre-treatment of the cells with DNase 1 significantly reduced the internalisation, and addition of extracellular DNA into the culture significantly increased the internalisation, this phenomenon is likely to be resulted from interaction of WB-6 and cell surface DNA.

Conclusions These results suggest that some anti-DNA antibodies show dual reactivity with CL-beta2-GPI via DNA, and this may contribute to the high percentage of association with SLE in APS. Such an antibody can enter live cells with DNA, and activate intracellular DNA sensors to induce tissue factor expression, without participation of the cell surface bata2-GPI and its still controversial receptors.