PLATELETS DO EXPRESS HLA-C.
Stein S., G. Datema, A. Mulder, F.H.J. Claas, and I.I.N. Doxiadis. Department of Immunohaematology and Blood Bank, Leiden University Medical Center, Leiden, The Netherlands.

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Refractoriness to platelet transfusion can be induced by alloantibodies directed against polymorphic antigens expressed on the cell surface of platelets. The refractoriness is often associated with increased bleeding risk. It is generally accepted that platelets express blood group antigens of the ABH, Lewis, P, an I system, HLA-A and HLA-B, and the platelet specific antigens (HPA). Very little is known to date on the expression of HLA-C on the platelet membrane and its role in platelet refractoriness. The detection of HLA-C was hampered until now because of lack of specific reagents towards HLA-C. Furthermore, serological typing for HLA-C is not accurate enough for large scale studies on the clinical significance of HLA-C in platelet transfusion. We were able to produce a human monoclonal antibody (Mab) towards HLA-Cw1 (VP6G3, R=0.95 in 136 individuals tested) with excellent results in complement mediated cytotoxicity (CDC). This Mab was also able to block HLA-Cw1 directed cytotoxic T cell activity. In a flow cytometry assay we addressed the question whether platelets express gene products of the HLA-C locus or not. We have tested in parallel peripheral blood lymphocytes and platelets of HLA-Cw1 typed individuals (N=10), as well as individuals which were HLA-Cw1 negative (N=10). As additional controls we used other locally produced human Mab’s towards HLA-A and HLA-B antigens. In all cases where HLA-Cw1 was found on the surface of lymphocytes it was also observed on the platetes of the same individual. In conclusion, platelets do express HLA-Cw1 on their cell surface. This finding calls for reconsideration of the role of HLA-C in platelet transfusion.