Approximately 20% of patients receiving multiple platelet transfusions develop platelet alloantibodies, which can be directed against Human Leukocyte Antigens (HLA) and to a lesser extent to Human Platelet Antigens (HPA). These antibodies can lead to the rapid clearance of donor platelets, presumably through IgG-Fc Receptor (FcγR)-mediated phagocytosis or via complement activation, resulting in platelet refractoriness (PR). Strikingly, not all patients with anti-HLA or -HPA antibodies develop PR upon unmatched platelet transfusions. Previously, we found IgG Fc glycosylation of anti-HLA antibodies to be highly variable between patients with PR, especially with respect to galactosylation and sialylation of the Fc-bound sugar moiety. Here, we produced recombinant glycoengineered anti-HLA and -HPA-1a monoclonal antibodies (mAbs) with varying Fc galactosylation and sialylation levels and studied their ability to activate the classical complement pathway. We observed that anti-HLA mAbs with different specificities, binding simultaneously to the same HLA-molecules, or anti-HLA in combination with anti-HPA-1a mAbs interact synergistically with C1q, the first component of the classical pathway. Elevated Fc galactosylation and to a lesser extent sialylation significantly increased the complement activating properties of anti- HLA and anti-HPA-1a mAbs. We propose that both the breadth of the polyclonal immune response, with recognition of different HLA epitopes and in some cases HPA antigens and the type of Fc glycosylation, can provide an optimal stoichiometry for C1q binding and subsequent complement activation. These factors can shift the effect of a platelet alloimmune response to a clinically relevant response, leading to complement mediated clearance of donor platelets as observed in PR.
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