**Abstract**

We have found that a novel autoantibody against β2-glycoprotein I (β2GPI)/human leukocyte antigen (HLA) class II complexes (anti-β2GPI/HLA-DR) is involved in the pathogenesis of antiphospholipid syndrome (APS). It was also found that many APS patients who were negative for conventional antiphospholipid antibodies (aPLs) possessed anti-β2GPI/HLA-DR. These results suggested that anti-β2GPI/HLA-DR measurements may be more sensitive for diagnosing APS than conventional aPLs tests. Recurrent pregnancy loss (RPL) is one of the clinical manifestations of APS. Therefore, a prospective, multicenter, cross-sectional study were conducted to assess whether anti-β2GPI/HLA-DR is also associated with RPL. This study of 227 couples with RPL revealed that 22.9% (52/227) of RPL women tested positive for anti-β2GPI/HLA-DR, and 24 (19.8%) of the 121 couples with unexplained RPL tested positive for anti-β2GPI/HLA-DR. Interestingly, thirty-five of the 52 (67.3%) RPL patients who were positive for anti-β2GPI/HLA-DR possessed no conventional aPLs of criteria. This novel autoantibody against β2GPI/HLA class II complexes may be a major risk factor for RPL, and it may be a promising biomarker for diagnosing APS.

**Keywords:** Autoantibody, β2-glycoprotein I, HLA class II, recurrent pregnancy loss
