The CD47 pathway is deregulated in human immune thrombocytopenia

Objective. A novel mechanism of platelet destruction involving the CD47/ signal regulatory protein - alpha (SIRP alpha) system has recently been suggested in a mouse model of immune thrombocytopenia (ITP). The CD47 molecule serves as ligand for SIRP alpha receptor and as receptor for thrombospondin acting as antagonistic to phagocyte activity and a regulator of apoptosis, respectively. In this study, we evaluated if the CD47/SIRP alpha axis may be involved in the apoptosis and clearance of platelets in human ITP. Materials and Methods. Using flow cytometry, we characterized whether expression of CD47 on fresh and in vitro aged platelets- and of SIRP alpha receptor on CD14-derived dendritic cells (DCs), macrophages, circulating DCs, and monocytes is reduced is ITP; whether the in vitro platelet phagocytic capacity of CD14-derived DCs and macrophages is differentially modulated in the presence or absence of antibodies against CD47 and SIRPa in ITP; and whether platelets are more susceptible to the CD47-induced death signal in ITP. Results. We demonstrated that low platelet count in ITP is not due to increased phagocytosis associated with decreased expression of CD47 on the platelet surface and, despite reduced SIRPa expression, blockage of SIRPa on immature CD14-derived DCs or CD47 on platelets by specific antibodies failed to modify platelet uptake/phagocytosis of DCs. In contrast, targeting platelet CD47 with specific antibody significantly increases platelet phagocytosis of CD14-derived macrophages, and platelets are not healthy because they show increased apoptosis and are resistant to CD47-induced death signal. Conclusions. Our results demonstrate that the CD47 pathway in ITP patients abnormally modulates platelet homeostasis. (C) 2011 ISEH - Society for Hematology and Stem Cells. Published by Elsevier Inc.