The objective of this proposal is to test a potential mechanism for the thrombocytopenia of the Wiskott- Aldrich Syndrome (WAS). There is ample evidence in the literature that rapid platelet consumption plays a central role in this condition, which has multiple similarities to immune thrombocytopenic purpura (ITP). Our preliminary results in a murine model of WAS demonstrate both rapid in vivo platelet consumption and enhanced ex-vivo phagocytosis of opsonized WASP(-) platelets by macrophages. In a related animal model (CD47 deficiency), rapid phagocytosis of platelets and red cells correlates with a proclivity toward ITP and autoimmune hemolytic anemia (AIHA). The latter is the most common of several autoimmune conditions frequently seen in WAS patients. We have also detected antiplatelet antibodies in a significant fraction of WASP(-) mice. Our hypothesis is that enhanced phagocytosis of platelets is central to the pathogenesis of the thrombocytopenia of clinical WAS, both because it directly enhances consumption of platelets and because of its association with an autoimmune response that further enhances consumption and/or inhibits platelet production. We propose to test this by developing an ex vivo assay of human platelet phagocytosis, using a human-derived macrophage cell line and appropriate anti-platelet antibodies. We will test multiple combinations of available cell lines, antibodies, and labeling techniques using opsonized normal human platelets. Those combinations that show detectable phagocytosis (comparable to what we have seen with WT murine platelets and murine macrophages) will then be used to test platelets prepared from WAS patients. We will scale down the assay to accommodate the small numbers of platelets available from WAS patients. We will also adapt our antiplatelet antibody assay to the human system, and use it to detect such antibodies in the sera of WAS patients. If we detect them, we will generate immortalized B-cell lines from these patients, screen them for antiplatelet antibodies, and characterize the latter in terms of antigen specificity, affinity, and class. These studies will determine whether the platelet abnormalities we have seen in murine WAS are clinically relevant. If that is the case, they will lead to further investigations of the molecular mechanism(s) responsible for increased platelet phagocytosis, and of mechanistic links between rapid platelet consumption, enhanced host antigen presentation, and autoimmunity. A better understanding of the pathogenesis of WAS will provide a better means of assessing the efficacy of existing therapeutic measures, allow us to determine why ineffective treatments fail, and motivate rational design of new therapies.
This project addresses the pathogenesis of a rare genetic disease, and as such it has public health significance only to the extent that there are many rare genetic diseases which taken together affect a large number of people. The mechanisms under investigation are also broadly relevant to thrombocytopenias and autoimmunity in general. If our hypothesis is correct, these studies will demonstrate a firm link between the thrombocytopenia of WAS and a much more common (and poorly understood) condition, immune thrombocytopenic purpura.
