Background/Purpose: While classification criteria for pediatric APS are not yet available, recent research suggests pediatric APS patients are unique, and many present with extra-criteria manifestations as their primary clinical features. Criteria aPL may identify classical APS manifestations, but a clinically actionable biomarker for features unique to pediatric APS is not yet available. Here, we aimed to evaluate the presence, clinical associations, and potential mechanistic roles of non-criteria aPL and circulating NETs markers in pediatric APS patients.

Methods: Children with APS (n=19), venous thrombosis without APS (VT, n=20), systemic lupus erythematosus without aPL (SLE, n=11), and healthy controls (n=13) were evaluated for nine different types of aPL by Werfen assays (Table 1). Calprotectin, a well-known marker of circulating neutrophil extracellular traps (NETs), was measured in plasma using the Human S100A8/S100A9 heterodimer DuoSet ELISA (R&D). Univariate logistic regression was used to identify potential clinical associations. A platelet viability dye-based assay was performed to assess the potential impact of calprotectin on platelet survival (Fig 2A).

Results: Among the 19 pediatric APS patients (median age of 13 [range 3 to 18]), 74% were female, and 58% had primary APS. Most (89%) had experienced at least one venous thrombosis, and four had recurrent thrombotic events. The remaining two children with APS exhibited extra-criteria manifestations, persistent aPL, and a clinical diagnosis of APS. Almost half (47%) had at least one extra-criteria manifestation. Interestingly, 79% of pediatric APS patients had at least one non-criteria aPL at ≥40 units (Table 1). Aside from lupus anticoagulant, aPS/PT IgG and IgM were the most prevalent aPL followed by anti-D1 IgG. Univariate logistic regression demonstrated that positive anti-D1 (p=0.0109), positive aPS/PT IgG (p < 0.0001), and IgM (p < 0.0001) were significantly associated with venous thrombosis. Positive anti-D1 IgG (p=0.0116) and aPS/PT IgG (p=0.0116)/IgM (p=0.0321) were also associated with extra-criteria manifestations such as thrombocytopenia. Increased levels of calprotectin, which is a marker of NETs, were detected in children with APS (Fig 1A). Calprotectin levels correlated moderately with levels of aPS/PT IgM (r=0.49, p=0.0345) and absolute neutrophil count (r=0.63, p=0.0079), whereas they were negatively correlated with hemoglobin (r=-0.74, p=0.0189) and platelet count (r=-0.59, p=0.0150) (Fig 1B). Mechanistically, plasma from pediatric APS patients with high calprotectin levels impaired platelet survival in vitro in a dose-dependent manner (Fig 2A-B), and APS calprotectin levels correlated with decreased platelet viability (Fig 2C).

Conclusion: Our study suggests non-criteria aPL and circulating calprotectin were highly prevalent among pediatric APS patients and associated with extra-criteria manifestations more common in pediatric APS. The role of non-criteria aPL and calprotectin as clinically actionable biomarkers that might enable earlier diagnosis and inform targeted therapy for some pediatric aPL-positive patients warrants further evaluation.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/non-criteria-antiphospholipid-antibodies-and-calprotectin-as-potential-biomarkers-in-pediatric-antiphospholipid-syndrome/