Characterization of the CD47/SIRP-Alpha System in Patients with Immune Thrombocytopenia.

The CD47 antigen is a transmembrane glycoprotein ubiquitously expressed on hematopoietic and non-hematopoietic cells. It serves as a ligand for SIRP-alpha (signal regulatory protein-alpha) receptor and as a receptor for Thrombospondin, acting, respectively, as antagonistic to phagocyte activity and as regulator of apoptosis. Based on mouse studies, a novel mechanism of platelet destruction involving the CD47/SIRP-alpha system has been recently proposed in immune thrombocytopenia. This mechanism suggests that platelet homeostasis is regulated by platelet expression of CD47 and that interaction between platelet CD47 and macrophage SIRP-alpha receptor is important in regulating platelet macrophage phagocytosis. However, the role of this system in platelet uptake/phagocytosis by dendritic cells (DCs) has never been investigated in immune thrombocytopenia (ITP) in humans. Therefore, our purpose was to evaluate whether alterations of the CD47/SIRP-alpha system may have a role in the pathogenesis of ITP in humans.

Twenty five ITP patients were studied. We phenotypically characterized apoptosis (Annexin-V FITC staining) and the expression of CD47 on platelets and SIRP-alpha on CD14-derived and circulating DCs by flow cytometry. To determine whether platelet apoptosis was due to activation of CD47-cell death pathway, in parallel experiments, we assessed the in vitro sensitivity of platelets to antibody CD47 ligation. In addition, to investigate the role of CD47/SIRP-alpha system on platelet phagocytic capacity of CD14-derived DCs, immature DCs were coincubated with PKH26-labelled platelets in the presence or absence of antibodies against CD47 and SIRP-alpha. The percentage of ingested platelets was then evaluated by flow cytometry.

We demonstrate that in ITP: 1) CD47 expression is not altered in freshly isolated platelets; 2) after in vitro aging, platelet apoptosis is increased as compared with the normal counterparts; 3) CD47 expression is unchanged in apoptotic platelets; by contrast, it increases in normal platelets; 4) the increased platelet apoptosis is not due to the activation of the CD47-induced cell death pathway; 5) despite low level expression of SIRP-alpha in CD14-derived DCs and in circulating DCs, the CD47/ SIRP-alpha system does not play a central role for in vitro platelet phagocytosis of DCs, since blockage of SIRP-alpha on DCs or CD47 on platelets by specific antibodies failed to modify phagocytosis.

In conclusion, we demonstrate that in ITP platelet CD47 expression does not play a role in the pathogenesis of the disease. We also show that, in ITP patients, higher platelet apoptosis is not due to different CD47-induced cell death susceptibility. Whether this is due to the fact that CD47 expressed on apoptotic platelets from ITP patients may have a peculiar conformation, avoiding the delivery of cell death signal, remains open question. Furthermore, the platelet uptake/phagocytosis by DCs is not significantly influenced by the CD47/ SIRP-alpha system. Supported in part by BolognaAIL (Italian association against Leukemia, Bologna section)

No relevant conflicts of interest to declare.