Anti-platelet antibodies in childhood immune thrombocytopenia: Prevalence and prognostic implications

Abstract

Background: Anti-platelet antibody testing may be useful for the diagnosis and management of childhood immune thrombocytopenia (ITP).

Objectives: Here we aimed to assess the prevalence and prognostic significance of anti-platelet glycoprotein-specific IgM and IgG antibodies.

Methods: Children with newly diagnosed ITP were included at diagnosis and randomized to an intravenous immunoglobulins (IVIg) or careful observation group (TIKI trial). In this well-defined and longitudinally followed cohort (N = 179), anti-platelet glycoprotein-specific IgM and IgG antibodies were determined by monoclonal antibody-immobilization of platelet antigens.

Results: The dominant circulating anti-platelet antibody class in childhood ITP was IgM (62% of patients); but IgG antibodies were also found (10%). Children without IgM platelet antibodies were older and more often female. There was weak evidence for an association between IgM anti-GP IIb/IIIa antibodies and an increased bleeding severity (P = .03). The IgM and IgG anti-platelet responses partially overlapped, and reactivity was frequently directed against multiple glycoproteins. During 1-year follow-up, children with IgM antibodies in the observation group displayed a faster platelet recovery compared to children without, also after adjustment for age and preceding infections (P = 7.1 × 10^-5 ). The small group of patients with detectable IgG anti-platelet antibodies exhibited an almost complete response to IVIg treatment (N = 12; P = .02), suggesting that IVIg was particularly efficacious in these children.

Conclusions: Testing for circulating anti-platelet antibodies may be helpful for the clinical prognostication and the guidance of treatment decisions in newly diagnosed childhood ITP. Our data suggest that the development of even more sensitive tests may further improve the clinical value of antibody testing.

Keywords: autoantibodies; immune thrombocytopenia; intravenous immunoglobulins; pediatrics; platelets.

Figure 1

Glycoprotein‐specific anti‐platelet antibodies as assessed by monoclonal antibody‐specific immobilization of platelet antigens assay (MAIPA). A, Overview of anti‐glycoprotein (GP) IIb/IIIa specific IgM antibodies in various cohorts. Exact P‐values for a post‐hoc Nemenyi test in comparison to newly diagnosed childhood ITP (Therapy with or without Intravenous Immunoglobulins for Newly Diagnosed Immune Thrombocytopenia in Kids [TIKI] trial; N = 167) are: Healthy controls (N = 65; P = 3.2 × 10⁻⁹), Adult ITP (N = 42; P = 5.4 × 10⁻¹⁴), juvenile idiopathic arthritis (JIA; N = 22; P = 5.5 × 10⁻⁴); autoimmune neutropenia (AIN; N = 48; P = 3.9 × 10⁻⁴); Chronic childhood ITP (CINKID; N = 33; P = 5.4 × 10⁻¹⁴). B, Overview of anti‐GP IIb/IIIa specific IgG antibodies in various cohorts. Exact P‐values for a post‐hoc Nemenyi test in comparison to newly diagnosed childhood ITP (TIKI; N = 176) are: Healthy controls (N = 68; P = 3.8 × 10⁻⁴), JIA (N = 11; P = 5.9 × 10⁻³); Chronic childhood ITP (CINKID; N = 33; P = .30). C, IgM anti‐platelet glycoprotein‐specific antibody levels in newly diagnosed childhood ITP (TIKI). D, IgG anti‐platelet glycoprotein‐specific antibody levels in newly diagnosed childhood ITP (TIKI). E, Venn diagrams of overlap of positive anti‐platelet glycoprotein antibodies within single patients for IgM (left panel) and IgG (right panel). F, Principal component analysis of all glycoprotein‐specific antibody levels reveals reduction of IgM and IgG antibody levels in two distinct vectors. PC1 and PC2 described ~80% of the total variance. Values were log₁₀ transformed before calculating eigenvalues to account for skewedness of the data. G, Correlation between glycoprotein‐specific antibody levels. In panels A‐C, the dashed line (optical density [OD] 0.130) is showing the cut‐off of positive and negative results

Figure 2

Clinical characteristics of patients with positive IgM anti‐platelet antibodies. A, Children with circulating IgM antibodies were younger (P < .001); B, showed longer duration of symptoms (P = .009); C, and were more often male (P = .006)

Figure 3

Glycoprotein‐specific anti‐platelet antibody measurements during 1‐year follow‐up. A, Longitudinal measurement of IgM anti‐GP IIb/IIIa antibody levels revealed disappearance of the antibodies already 1 week after diagnosis and persistent negative results 1 year thereafter. B, Patients who showed IgM anti‐GP IIb/IIIa antibodies did not class‐switch to IgG anti‐GP IIb/IIIa antibodies during follow‐up. C, The majority of patients with positive IgG antibody tests kept an anti‐GP IIb/IIIa antibody during 1‐year follow‐up. In all panels the dashed line indicates the optical density 0.130, the cut‐off of positive and negative results according to technical and healthy controls

Figure 4

Prognostic significance of IgM and IgG anti‐platelet antibodies. A, Patients with IgM anti‐platelet antibodies against GP IIb/IIIa showed higher platelet counts during spontaneous recovery after 1‐month follow‐up. P‐value for mixed effects model with random effects for patient and fixed effects interaction between time and IgM antibody status. B, Complete recovery (CR) rate based on IgM antibody status. No effect was observed for IVIg‐treated patients, as shown in (A). No significance test was performed due to the crossing survival curves at week 1 (nd, not determined). C, Patients with anti‐platelet IgG antibodies showed higher platelet counts after IVIg. P‐value for mixed effects model with random effects for patient and fixed effects interaction between time and IgM antibody status. D, CR rate based on IgG antibody status. Patients with IgG anti‐platelet antibodies showed complete recovery after IVIg treatment. P‐value for a log‐rank test. For the observation group no significance test was performed due to the crossing survival curves (nd, not determined)