Platelet Desialylation As a Predictive Marker in Childhood Immune Thrombocytopenia (ITP)

Background and aim: Immune thrombocytopenia (ITP) is the most common bleeding condition in children. Its prognosis is mostly superior, however, severe refractory disease remains diagnostic and therapeutic challenge. Low platelet counts (<100× 10⁹/L) are associated with increased platelet clearance by two parallel mechanisms: classical antibody-mediated pathway and a novel lectin-carbohydrate mediated pathway. The latter is based on platelet desialylation, where terminal sialic acids are cleaved from glycoconjugates, mainly glycoproteins (GPs), on the platelet surface. The loss of sialic acid enhances bond of the penultimate β-galactose to asialoglycoprotein receptors (ASGPRs, also called Ashwell-Morell receptors) on hepatocytes. Desialylated platelets are then captured and phagocytosed by ASGPR-expressing hepatocytes. Desialylation has been shown to be responsible for platelet destruction in many contexts, e.g., infection-related thrombocytopenia or clearance of senescent platelets. Loss of T-cell tolerance is another underlying mechanism in ITP; CD8⁺ regulatory T cells (Tregs) are able to inhibit overactive immune response and maintain immune homeostasis. Forkhead box P3 (FOXP3) and GATA3 are transcription factors crucial for development and proper function of Tregs limiting the Th2-type inflammatory response. Our aims were to distinguish contribution of the mentioned processes to ITP development and to characterize immune response in children with ITP during the course of disease (diagnosis, ongoing therapy, remission, refractory/persistent ITP).

Patients and Methods: We examined 30 samples from 20 children with ITP (12 males, 8 females, age 3-17 years; 3 acute ITP, 17 chronic ITP) and 10 healthy controls (age 4-15). The degree of desialylation was determined by flow cytometry using FITC-labeled Ricinus communis agglutinin (RCA-I) specific for terminal galactose or N-acetylgalactosamine. Expression of platelet surface markers was given quantitatively as mean fluorescence intensity (MFI). Presence of platelet surface-bond antibodies (IgG, IgA and IgM) was examined by flow cytometry. Subpopulations of CD4+ and CD8+ T-cells were characterized based on intracellular expression of transcription factors T-bet (Th1 cells), GATA3 (Th2 cells), ROR gamma T (Th17 cells) and FOXP3 (for Tregs) using multicolor flow cytometry.

Results: Patients with ITP showed significant increase in RCA-I reactivity in comparison with healthy controls (p<0.001). Patients with newly diagnosed ITP showed the most aberrant sialylation (i.e., maximum desialylation) of platelet surface proteins. A decrease in desialylation intensity was noticeable as soon as at three days after therapy initiation. Sialylation levels returned to normal after one month of successful treatment and were similar to healthy controls in children with ITP remission. Platelet surface-bond immunoglobulins were increased in 10 (50%) patients independently on their sialylation level. We observed significant changes in T-cell subpopulations in ITP: T lymphocytes producing T-bet were decreased within both CD4+ and CD8+ populations. Percentage of CD4+ cells expressing ROR gamma T was also reduced. Proportions of cells expressing FOXP3 and GATA3 were decreased within the CD8+ but not within the CD4+ population.

Conclusion: Our results highlight the importance of Fc-independent hepatic platelet clearance in ITP. Interindividual differences in ITP pathophysiology are reflected by treatment response and may improve therapeutic management and prognostication. E.g., intravenous immunoglobulins or splenectomy will be ineffective in patients with prevalent Fc-independent mechanisms, and contrarily, possibilities for novel targeted treatment (neuraminidase inhibitors) arise. Better understanding of immune-mediated processes involved in ITP pathogenesis may reduce adverse effects of immunosuppressive therapy and considerably improve quality of life in patients with ITP.

Supported by: MH CZ - DRO (FNOl, 00098892), Project ENOCH (No. CZ.02.1.01/0.0/0.0/16_019/0000868) and Ministry of Education, Youth and Sports OPVVV CEREBIT CZ.02.1.01/0.0/0.0/16_025/0007397.