**8. Autoantibody against β2GPI/HLA-DR complex is a promising novel biomarker for APS**

In our previous study, we measured serum levels of anti-β2GPI/HLA-DR in stored sera from 120 patients with APS, most of whom had a history of vascular thrombosis, and found that 83% of the 120 patients had autoantibodies directed against β2GPI/HLA-DR complexes. Furthermore, about 50% of the APS patients who tested positive for anti-β2GPI/HLA-DR (< 99th percentile values measured in sera of 100 healthy subjects) were negative for both IgG aCLs and IgG aβ2GPI antibodies [4]. Another recent study also showed that 27% of 111 patients with idiopathic chronic limb ulcers who were negative for aPLs possessed anti-β2GPI/ HLA-DR [20]. These results suggest that anti-β2GPI/HLA-DR are associated with APS manifestations, even in patients who do not meet the diagnostic criteria for APS because they are negative for conventional aPLs.

The latest prospective, multicenter, cross-sectional study, of 227 couples with recurrent pregnancy loss (RPL), which is one of the clinical manifestations of APS,

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

revealed that 22.9% (52/227) of women with RPL tested positive for anti-β2GPI/ HLA-DR (< 99th percentile values measured in sera of 208 healthy, fertile control women) [19]. In this study, anti-β2GPI/HLA-DR were detected most frequently in women with RPL among other commonly recognized risk factors for RPL, i.e., uterine malformation, thyroid dysfunction, chromosomal abnormality, aPLs positive, low factor XII activity, low protein S activity, and low protein C activity (**Figure 2**). Importantly, 53.3% (121/227) of women with RPL had no commonly accepted risk factors for RPL, and 24 of these 121 (19.8%) women with unexplained RPL were positive for anti-β2GPI/HLA-DR (**Figure 2**). In addition, 45 of the 227 women with RPL (19.8%) were positive for at least one of the 5 conventional aPLs meeting the diagnostic criteria for APS in this study, i.e., IgG aCL (8.8%), IgM aCL (6.2%), IgG aβ2GPI (3.1%), IgM aβ2GPI (1.3%), and LAC (2.6%). The rate of positivity for anti-β2GPI/HLA-DR was the highest (22.9%) of the 5 aPLs that met the diagnostic criteria for APS. Notably, 35 (67.3%) of the 52 women with RPL who were positive for anti-β2GPI/HLA-DR, were negative for APS laboratory criteria (**Figure 3**).

On the other hand, the presence of multiple aPLs and LAC positivity has been reported to be strongly associated with the severity of clinical manifestations of APS [21–26]. In our study, all 3 women with RPL who had double or triple aPLs positivity were also positive for anti-β2GPI/HLA-DR, and the 2 with triple positivity had very high anti-β2GPI/HLA-DR levels (927.5 units and 330.7 units). First of both women experienced early-onset HELLP syndrome (hemolysis, elevated liver enzymes, and low platelets) at 14 weeks of gestation, and the second experienced a thromboembolism with cerebral infarction [19]. Multiple positivity for aPLs may be associated with higher levels of anti-β2GPI/HLA-DR, and these conditions may be closely associated with the severity of the clinical manifestations of APS.

#### **Figure 2.**

*Risk factors for recurrent pregnancy loss (RPL) among 227 women with RPL. All women with RPL enrolled in this study attended evaluations to identify commonly accepted risk factors for RPL. Black pie slices indicate the frequencies of women with RPL who were also positive for anti-β2GPI/HLA-DR (n = 52). Abbreviations: aPLs, antiphospholipid antibodies.*

#### **Figure 3.**

*Positivity for anti-*β*2-glycoprotein I /HLA-DR antibodies (anti-*β*2GPI/HLA-DR) and antiphospholipid antibodies (aPLs) in 227 women with recurrent pregnancy loss (RPL). Numbers in the Venn diagram represent the number of women who had unique or nonunique results in tests for aPLs and anti-*β*2GPI/HLA-DR. abbreviations: Ig, immunoglobulin; HLA, human leukocyte antigen; β2GPI, β2-glycoprotein I; aβ2GPI, anti-β2-glycoprotein I antibody; aCL, anti-cardiolipin antibody; LAC, lupus anticoagulant.*
