**4. The discovery of a novel autoantibody against β2GPI/HLA-DR complex in APS**

We found that 293 T cells co-transfected with β2GPI and HLA-DR expressed both β2GPI and HLA-DR on the cell surface by flow cytometry analysis

*A Novel Autoantibody against β2-Glycoprotein I/HLA Class II Complexes in Antiphospholipid… DOI: http://dx.doi.org/10.5772/intechopen.97511*

**Figure 1.**

*Monoclonal anti-phospholipid antibody binds to* β*2GPI/HLA-DR complex on the cell surface. 293 T cells transfected with only* β*2GPI did not express* β*2GPI on the cell surface, and human monoclonal antiphospholipid antibody (EY2C9) did not bind to these cells (the upper 3 histograms and 1 scheme). When* β*2GPI was co-transfected with HLA-DR into 293 T cells,* β*2GPI was expressed on the cell surface and was recognized by EY2C9 monoclonal antibody (the lower 3 histograms and 1 scheme). Abbreviations: HLA, human leukocyte antigen;* β*2GPI,* β*2-glycoprotein I; aPL mAb, anti-phospholipid monoclonal antibody.*

(**Figure 1**) [4]. Conversely, 293 T cells transfected with only β2GPI did not express β2GPI on the cell surface, because β2GPI is a secreted protein (**Figure 1**) [4]. Immunoprecipitation and immunoblotting experiments revealed that full-length β2GPI proteins, but not peptide fragments of β2GPI, formed a complex with HLA-DR, and that these full-length β2GPI/HLA-DR complexes were present on the cell surface [4].

Furthermore, flow cytometry analysis revealed that not only the monoclonal antiphospholipid antibody derived from an APS patient (EY2C9), but also antibodies in the sera of APS patients can bind to the β2GPI/HLA-DR complexes, even in the absence of phospholipids [4].
