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- PATIENTS AND METHODS
Antiphospholipid antibodies (aPL) are a heterogeneous group of autoantibodies reactive to phospholipid or phospholipid-binding protein complexes that can be detected by lupus anticoagulant (LAC) assays, anticardiolipin (aCL) enzyme-linked immunosorbent assays (ELISA), or screening tests for syphilis. Recent efforts to better characterize the specificity of aPL have resulted in the identification of autoantibodies directed against β2-glycoprotein I (anti-β2GPI), a phospholipid-binding protein. Evidence suggests that β2GPI may be necessary for the binding of particular aPL to phospholipids and may actually represent the true target of aPL. The physiologic function of β2GPI is not known; however, in vitro studies have demonstrated a potential role in anti-coagulation (1, 2).
Antiphospholipid syndrome (APS) has been defined as the association of aPL with arterial and venous thrombosis, fetal loss, thrombocytopenia, and other hematologic and neurologic complications. Recent stringent criteria for APS have been developed in adult populations (3); however, similar criteria have not been developed for pediatric populations. As in adults, APS can be seen in children with systemic lupus erythematosus (SLE) (secondary APS) or in children without evidence of an underlying autoimmune disorder (primary APS). In general, both autoimmunity and thrombosis occur less frequently in children than adults, suggesting that children may exhibit a different relationship between aPL and clinical manifestations of APS.
Many adult and pediatric patients with SLE have detectable aPL in their serum, yet never develop the clinical complications characteristic of APS. Additionally, patients may develop clinical complications of APS without testing positive for aPL by standard assays. This inconsistency likely reflects the underlying heterogeneity of aPL, the indirect nature of many of the aPL assays, in particular LAC, and the currently unexplained role of aPL in pathophysiology of APS (4). The standard methods for detection of aPL include coagulation assays for LAC (partial thromboplastin time [PTT], and dilute Russell's viper venom time [dRVVT]), detection by ELISA of aCL, and screening tests for syphilis (VDRL and rapid plasma reagin [RPR]). In general, there is good correlation across assays for aPL detection. However, the discordant behavior of some patient sera suggests that there may be a number of aPL antibody subsets that recognize distinct phospholipid or phospholipid-binding protein antigens.
Previous observations in adult cohorts suggest that anti-β2GPI are frequently seen in SLE and APS, and may demonstrate greater specificity for APS than standard aPL assays (5–7). However, this has not been assessed in a pediatric cohort. Although earlier studies in children have evaluated the prevalence of aPL and clinical characteristics of APS in children, none have included detection of anti-β2GPI. This investigation was undertaken to determine if measurement of serum anti-β2GPI improves the clinical characterization of pediatric patients with aPL. Children and adolescents with primary APS, SLE alone, or secondary APS were studied. The isotype and level of anti-β2GPI were correlated with clinical manifestations of hypercoagulability and results of other aPL assays. The goals of this study were to determine whether a particular aPL assay correlated best with specific clinical manifestations and whether β2GPI screening improved identification of children at risk for APS.
- Top of page
- PATIENTS AND METHODS
The association between aPL and thrombosis has been well established in both adults and children with SLE and primary APS. However, many questions remain regarding the antigen specificity of aPL, the role of aPL in APS pathogenesis, and appropriate aPL assays for the clinical setting. Screening for anti-β2GPI has generated enthusiasm for better characterization of this condition. The aims of this study were to characterize the clinical and antibody features of APS in a pediatric population and to determine if anti-β2GPI detection improves identification of subjects at risk for thrombosis or other manifestations of APS.
Although all APS subjects in this cohort do not meet the recently described stringent criteria developed for diagnosis of APS in adults (3), they do demonstrate accepted clinical manifestations of the syndrome (15–17). The most frequent APS manifestation seen in this cohort was chronic thrombocytopenia. Although not specific for APS, chronic thrombocytopenia is a well-recognized complication in the spectrum of APS. The prevalence of chronic thrombocytopenia in pediatric APS has not previously been reported, however the prevalence of 29% in our cohort is similar to that previously reported in adult series (18). The positive association of APS with Caucasian and Hispanic ethnicity and negative association with Asian ethnicity, may reflect referral bias or may suggest the presence of underlying genetic factors. Many of the subjects with primary APS were Caucasian; therefore, the large number of subjects with primary APS in this cohort may have biased the association of APS with Caucasian ethnicity.
Although the literature contains numerous case reports of children with primary APS, there are no published series, and the prevalence of primary APS in childhood is not known. This report adds 25 subjects to the literature on primary APS in children. We observed anti-β2GPI antibodies in only 17% (4/24) of these subjects with childhood-onset primary APS, in contrast to the 80% previously reported for adult-onset primary APS (5). This may reflect the influence of age or differences in clinical manifestations.
Most published series on the prevalence of aPL in pediatric SLE are small, ranging from 18–59 subjects (16, 19–22). Our finding of 53% positivity for aCL, 23% positivity for LAC, and 67% positivity for aPL in general in childhood SLE is similar to previous reports (16, 19, 20, 22, 23). No published series to date has reported the prevalence of anti-β2GPI in pediatric SLE, however our finding of anti-β2GPI in 48% of pediatric SLE subjects is consistent with the observed prevalence of anti-β2GPI in adult SLE cohorts, which ranges from 4%–89% (6, 7, 24) depending on whether or not the patients met criteria for APS. As expected, many patient sera were positive by only 1 or a limited number of aPL assays. Our findings confirm earlier observations that if any 1 of the standard assays are not performed, a subset of aPL may be missed. This is most notable for aCL detection, which we conclude should be included in all evaluations for aPL; and least notable for anti-β2GPI detection.
Earlier reports have suggested a greater specificity of anti-β2GPI for predicting APS than other aPL assays (24), however in this cohort, anti-β2GPI were not seen as frequently as aCL or LAC and demonstrated weaker correlation with APS than other aPL assays. Published reports suggest that LAC is a better predictor of thrombosis than other aPL assays (25, 26); however, our findings did not support this observation. Although screening for anti-β2GPI was determined to be insensitive for prediction of APS risk in general, there was a strong association between the presence of anti-β2GPI and occurrence of stroke. Larger studies are needed to confirm this association and to evaluate a potential role for anti-β2GPI in the pathogenesis of CNS thrombosis.
Separate analysis of SLE subjects did demonstrate a significant association of aCL and LAC with APS. However, in contrast with Cabiedes et al (24), who demonstrated an association between anti-β2 GPI and APS in adult SLE, we found no significant difference in prevalence of anti-β2GPI among these groups in children (45% in SLE with no APS versus 62% in SLE with APS). One of the most interesting associations identified in this study was the strong association of anti-β2GPI with SLE, and its ability to discriminate between primary and secondary APS in pediatric subjects with chronic thrombocytopenia. Chronic ITP, a common pediatric condition, is a reason for concern because of its potential evolution to SLE. Our observations suggest that there may be a role for anti-β2GPI detection in identification of the subset of children with ITP who are at risk for future development of SLE. This needs to be confirmed in a prospective study. If confirmed, the presence of anti-β2GPI may serve as a useful prognostic marker and indicator for more aggressive immunosuppressive therapy.
One limitation of this study is potential referral bias. Although subjects were not recruited systematically, referral patterns in northern California, combined with efforts to recruit from the only other 2 pediatric rheumatology centers in the area, support our belief that a broad spectrum of subjects with rheumatic diseases has been included in this cohort. This study was neither designed to address the correlation of aPL levels with disease activity nor response to therapy, and prospective longitudinal studies are needed to answer those questions. Finally, background positivity for anti-β2GPI in healthy children was not determined, but is assumed to be quite low, as previously reported for healthy adults (7).
In summary, this study extends the results of previous reports in several ways. First, in addition to SLE, this cohort includes children with either SLE-like or primary APS and therefore adds significantly to the limited literature available on primary APS in children and adolescents. Second, this is the first report on the prevalence of anti-β2GPI in a pediatric cohort and suggests that β2GPI screening is less sensitive than aCL or LAC assays for detection of APS. Furthermore, among subjects with anti-β2GPI, neither isotype nor antibody level was clinically significant. Third, our data suggests a role for β2GPI detection in predicting evolution to SLE in children with chronic ITP, a common pediatric condition.