HLA
antibodies are associated with refractoriness to
platelet transfusion, leading to rapid platelet clearance, sometimes coinciding with clinical side effects such as
fever and
chills. The presence of HLA
antibodies is not always manifested by clinical symptoms. It is currently unclear why refractoriness to
platelet transfusion is only observed in a subset of patients. Here, we utilized the availability of a unique panel of human
monoclonal antibodies to study whether these were capable of activating platelets. Three out of eight human HLA-specific
monoclonal antibodies induced activation of HLA-matched platelets from healthy donors as evidenced by enhanced α-granule release, aggregation, and αIIbb3 activation. The propensity of HLA
monoclonal antibodies to activate platelets was independent of the HLA subtype to which they were directed, but was dependent on the recognized
epitope. Activation was fully inhibited either by blocking FcγRIIa, or by blocking FcγRIIa-dependent signaling with Syk inhibitor IV. Furthermore, activation required the presence of the
IgG-Fc part, as F(ab')2 fragments of HLA
monoclonal antibodies were unable to induce platelet activation. Mixing experiments revealed that activation of platelets occurred in an intra-platelet dependent manner. Accordingly, a proportion of sera from refractory patients with HLA
antibodies induced FcγRIIa-dependent platelet activation. Our data show that a subset of HLA
antibodies is capable of crosslinking HLA and FcγRIIa thereby promoting platelet activation and enhancing these cells' phagocytosis by macrophages. Based on these findings we suggest that FcγRIIa-dependent platelet activation may contribute to the decreased platelet survival in
platelet-transfusion-dependent patients with HLA
antibodies.