Discussion
Increasing evidence suggests the pathogenic role of anti-β2GPI-DI IgG in APS. In this study, we performed an integrated analysis of its clinical associations in a large Chinese APS cohort. The major findings were its significant associations with extra-criteria manifestations, especially thrombocytopenia and APS nephropathy.
In our study, the prevalence and diagnostic value of anti-β2GPI-DI IgG in APS were largely similar to those reported in literature. A recent meta-analysis was performed including 767 patients with APS from 9 studies, and reported an overall median prevalence of 35.7%.32 In our APS cohort, the general positivity rate was 32.5%, and comparable between PAPS (32.8%) and SAPS (32.0%). It has also been noted that the prevalence of anti-β2GPI-DI IgG varies significantly across different populations, which tends to be higher in aPLs carriers (41.9%)24 or patients with persistent anti-β2GPI IgG (57.8%),23 and lower in patients with SLE without APS (7%).33 In this study, 50.9% of the patients with APS with anti-β2GPI IgG/M antibodies and 8.1% of the non-APS SLE controls were positive for anti-β2GPI-DI IgG. Considering this, it is critical to evaluate the clinical profiles of the control subjects and criteria antibody profiles of patients when calculating the diagnostic performance of anti-β2GPI-DI. Moreover, the methods of assays and choice of cut-off values may potentially affect the titres and positivity of results. Reports from literature, as well as our report, generally showed its high specificities (over 95%) and low sensitivities (ranging from 20% to 60%) for differentiating APS from various controls, including health subjects, patients with SLE or other autoimmune diseases, patients without APS suffering from thrombosis or obstetric events.18 21 33–36 Compared with anti-β2GPI antibodies, the sensitivity of anti-β2GPI-DI was lower in diagnosing APS. This could be explained by the cross-positivity between these two antibodies, which showed that only 75.3% of those with anti-β2GPI-IgG positivity were positive with anti-β2GPI-DI (table 2). Moreover, this suggested the presence of antibodies against other epitopes of β2GPI in those with anti-β2GPI-IgG positivity, such as domain 4/5.23 In anti-β2GPI antibodies carriers, the positivity rates and titres of anti-β2GPI-DI antibodies were significantly higher in those with classical triple aPLs positivity,20 37 which was also confirmed by multiple studies in patients with APS.18 38 39 The presence of anti-β2GPI-DI correlated poorly with single aPL positivity (table 2). For example, a total of 48 (38.4%) patients had isolated LAC in our cohort, and none of them was positive with anti-β2GPI-DI. Addition of anti-β2GPI-DI antibodies to classic aPLs could hardly provide extra-diagnostic value of APS, this could be partially explained by its high level of agreement with triple positivity of classic aPLs (figure 1A).
The presence of anti-β2GPI-DI IgG was associated with increased risk of thrombosis and adverse pregnancy outcome in patients with LAC,15 persistent anti-β2GPI-DI IgG23 or SLE.16 However, the interpretations of these findings are limited if these patients with events could have been diagnosed as APS by classic aPLs. Accordingly, the potential role of this antibody in routine practice depends on the its clinical associations within patients with APS. While some early studies demonstrated higher titres of this antibody in thrombotic APS than in obstetric APS,18 19 more recent studies failed to show the same trend.20 21 The observed discrepancies could be partially explained by their differences in study design, study populations and methodology of antibody measurements. Recently, it has been suggested that the titres and positivity of anti-β2GPI-DI IgG were associated with late PM, defined as late fetal loss (>10 weeks) or premature delivery.21 23 We performed similar analysis and found no significant differences in either anti-β2GPI-DI IgG positivity (early PM 41.2% vs late PM 31.4%, p=0.46) or titres (online supplemental figure 1F). It is possible that the sample size of our study is insufficient and selection bias has led to inconsistent conclusions, and further studies with larger sample size are expected. More detailed analyses suggested the association of anti-β2GPI-DI IgG with recurrent thrombosis20 or APS nephropathy,40 which were also validated by data from our cohort. Prospective research is lacking concerning the predictive value of anti-β2GPI-DI for thrombosis or PM. In aPLs carriers, the presence of anti-β2GPI-DI was associated with thrombotic events during the follow-up.24 A recent prospective study followed 44 patients with thrombotic APS for a median of 39 months, and found 4 new thromboses. All of these recurrent patients presented anti-β2GPI-DI positivity.39 In our study, five out of the eight patients with thrombotic recurrence were positive for anti-β2GPI-DI, and further log-rank test failed to reveal significant predictive associations (figure 2A). Taken together, these results limit the use of anti-β2GPI-DI IgG in predicting thrombosis or PM among patients with APS.
In the recent decade, a large body of clinical studies emerged and suggested that extra-criteria clinical features were frequently associated with aPLs.2 In the APS ACTION registry, 58% of the 642 patients with APS and 47% of the 162 aPL carriers reported at least one extra-criteria manifestations.41 We have recently shown that thrombocytopenia could identify patients with PAPS at high risk of developing thrombotic events, PM and other severe extra-criteria events.26 Notably, patients with triple aPLs positivity experienced the most thrombocytopenia, aPL nephropathy and cardiac valve disease.41 42 It is also reported that aPL-positive patients with SLE demonstrated as higher frequency of thrombocytopenia, haemolytic anaemia and less IgG anti-β2GPI.43 While several scoring systems for predicting APS thrombosis have been developed and validated,8 44 reliable markers of extra-criteria features were lacking. In this study, we proposed that anti-β2GPI-DI IgG was a potential prediction marker of extra-criteria features in patients with APS. It was significantly associated with multiple extra-criteria features at the time of APS diagnosis (table 1). Meanwhile, extra-criteria manifestations were also more common in patients with SAPS compared with PAPS (online supplemental table 2). But the proportions of anti-β2GPI-DI IgG were the same between these two groups. Further multivariate regression analysis also proved that anti-β2GPI-DI IgG and SAPS were two independent factors associated with extra-criteria manifestations (table 3). In details, thrombocytopenia and APS nephropathy were more common in patients with anti-β2GPI-DI IgG, while those with SAPS experienced more thrombocytopenia, autoimmune haemolytic anaemia, valvular lesions and non-vascular neurological manifestations. This might implicate the different underlying pathogenesis among various extra-criteria manifestations. The two cases with APS nephropathy during the follow-up were positive with anti-β2GPI-DI IgG, but statistical analysis was limited by the sample size and follow-up duration.
Strengths of our study are its prospective design, relatively large sample size and integrated clinical profiling including extra-criteria features. There are several limitations to be noted. The study was based on data from a single centre, and the follow-up duration was relatively short. Future analysis based on the ongoing Chinese AntiphosPholipid Syndrome cohorT cOllaborative NEtworks (NCT0523001) registry would be a complement to this, with participants from multiple centres and extended follow-up period. All the included subjects were Asians, and the conclusions should be validated in patients with other races/ethnicities before translating into clinical practice in different scenarios. All the subjects were diagnosed based on the positivity of aPLs measured by chemiluminescence technique (CLIA), and potential bias should be noted considering the discrepancies in detection aPLs between CLIA and ELISA.45 Some studies have suggested that there are differences in the sensitivity and specificity of CLIA and ELISA methods, which are closely related to the choice of cut-off values.46 We did not validate the manufacturer’s cut-off value in local Chinese population. However, we have previously validated the diagnostic power of this CLIA system in Chinese patients with APS.30 Limited data from a previous study also suggested a cut-off of 20 CU was appropriate in Chinese.21 The time duration for the follow-up of patients with APS was relatively short, thus the event rate for extra-criteria manifestations was not enough for further statistical analysis. Also, treatment details were not included in the present study, which could limit our interpretations of the association between anti-β2GPI-DI IgG and thrombosis recurrence. Future studies with longer follow-up durations should be performed to validate the prediction value of anti-β2GPI-DI IgG for extra-criteria features and thrombosis recurrence, and to develop related risk prediction scores.