Aberrant Expression of a Proliferation-Inducing Ligand Underlies Autoimmune Mechanisms in Immune Thrombocytopenia
Authors: Hao Yunfeng , Yang Renchi , Zhou Zeping 
Published: October 2019

Abstract
Background: Immunological thrombocytopenia (ITP) is an antibody-mediated autoimmune disease characterized by accelerated platelet destruction and suboptimal platelet production. The proliferation-inducing ligand (APRIL or TNFSF13), a member of the TNF superfamily, is structurally and functionally related to the TNF family of B cell activating factors (BAFF, TNFSF13b) and has been shown to regulate lymphocyte survival by interacting with its receptors. And activation. Transmembrane activators and calcium regulate cyclophilin ligand interactors (TACI) and B cell mature antigens (BCMA). APRIL is secreted by various cells as soluble factors, including inactive B cells, T cells, monocytes, neutrophils, macrophages and dendritic cells, as well as epithelial cells, osteoclasts and megakaryocytes. Recent studies have shown that APRIL not only participates in normal immune responses, but also plays an important role in the establishment and/or maintenance of autoimmune and inflammatory diseases.
Aims: Based on the relationship between APRIL, which promotes proliferation and regulates immunity, and the development of autoimmunity, we hypothesize that APRIL may play a role in the pathogenesis of ITP.
Methods:1. The EDTA anticoagulated whole blood was collected, and peripheral blood mononuclear cells (PBMC) were separated by Ficoll density gradient centrifugation. The APRIL levels on the surface of T cells, B cells, DC cells and platelets were detected by flow cytometry.Detection of plasma APRIL levels in patients with ITP by ELISA.Real time quantitative PCR were used for detecting the level of APRIL and its receptors BCMA and TACI from PBMC of healthy controls and ITP patients.Use soluble APRIL or BLyS protein and APRIL inhibitors to examine the effect of APRIL inhibition on IL-10 secretion by B cells. Flow cytometry and intracellular staining were used to evaluate B10 cells.
Resoult: 1. The APRIL on the platelet surface of patients with ITP was significantly lower than that of the normal control group (p<0.01). In the ITP patients of 10 patients with complete remission, the content of APRIL on the platelet surface was significantly increased after treatment (p=0.02), and there was no significant change in the treatment-ineffective group. .
The levels of APRIL and its receptors BCMA and TACI on B cells and DC cells in ITP patients were higher than those in normal controls, and the difference was statistically significant. APRIL is not expressed on CD4 + T cells, CD8 + T cells.The expression of APRIL mRNA in PBMNCs was significantly higher in ITP patients than in the normal control group (p <0.01). There was no difference in BCMA and TACI expression in PBMNC of ITP patients compared to normal controls.Plasma APRIL levels were significantly higher in ITP patients than in the normal control group, p = 0.04, and negatively correlated with platelet count, p = 0.029.In 10 patients with ITP, the percentage of CD19 + B cells remained similar between patients, and the results showed that the amount of B10 cells in the medium supplemented with APRIL was greater than that of B10 cells containing BLyS and control medium (p<0.01; p= 0.01), and the use of APRIL inhibitors resulted in a decrease in B10 cells.
Conclusion: Our study shows that aberrant expression of APRIL is involved in the autoimmune response of ITP, and the effect of treatment can be assessed by measuring changes in the level of APRIL on the platelet surface. We also speculate that APRIL inhibits, rather than promotes, an immune-mediated inflammatory response in the pathogenesis of ITP. Our current observations support that the immunomodulatory effects of APRIL may be due, at least in part, to stimulation of IL-10 producing B cells.

Disclosures
No relevant conflicts of interest to declare.