The Platelet Immunology Subcommittee addressed topics in two broad areas: autoimmune thrombocytopenia and alloimmune thrombocytopenia. New subcommittee activities were initiated in both areas this year.
The pathogenic mechanism of autoimmune thrombocytopenic purpura (ITP) involves immunoglobulin opsonization of platelets by autoantibodies and clearance of the platelets in the reticuloendothelial system. Triggers for production of the autoantibodies include immunodeficiency states, viral stimulation, and possible immunoglobulin germline genetic states. Mechanisms for the emergence of B cells that produce autoantibodies are under investigation. This topic will be included in the agenda for the next meeting. It was also suggested that platelet immunology laboratories and ITP treaters be polled as to whether alternative markers can be used to eliminate the need to do a bone marrow examination on the patient suspected to have ITP. In all cases, it was emphasized that the peripheral blood film must be examined to rule out any evidence for microangiopathy, abnormal leukocytes, giant platelets, or myelopthisis (tear drop forms or nucleated erythrocytes.)
Alloimmune thrombocytopenia (AIT) can affect both the neonate and fetus. Management of the first affected infant in a family may include transfusion with alloantigen negative or random donor platelets, treatment with IV IgG and/or steroids, or possibly exchange transfusion. Management of subsequent pregnancies affected by AIT varies. In the United States, weekly immunoglobulin is usual, while in Europe, practices vary in different countries. A multicenter study is in the planning phase in Europe. The subcommittee decided to undertake preparation of a review and analysis of the results of antenatal IV IgG treatment for pregnancies affected by AIT. This should be valuable to the clinician who has to make decisions in individual cases. In addition, it was suggested that the subcommittee publish a letter detailing considerations and recommendations for the preparation of antigen negative platelets for transfusion in utero at the time of percutaneous umbilical blood sampling. Features to be considered include filtration, irradiation, HLA compatibility, plasma depletion, and testing for transfusion transmitted diseases.
Standardization of nomenclature for the platelet alloantigens is an objective of this subcommittee. It was decided to cooperate with the International Society for Blood Transfusion (ISBT) platelet serology group in an attempt to come to a consensus regarding nomenclature for the platelet alloantigens.