Altered Fc glycosylation of anti‐HLA alloantibodies in hemato‐oncological patients receiving platelet transfusions

Abstract

Background The formation of alloantibodies directed against class I human leukocyte antigens (HLA) continues to be a clinically challenging complication after platelet transfusions, which can lead to platelet refractoriness (PR) and occurs in approximately 5%–15% of patients with chronic platelet support. Interestingly, anti‐HLA IgG levels in alloimmunized patients do not seem to predict PR, suggesting functional or qualitative differences among anti‐HLA IgG. The binding of these alloantibodies to donor platelets can result in rapid clearance after transfusion, presumably via FcγR‐mediated phagocytosis and/or complement activation, which both are affected by the IgG‐Fc glycosylation. Objectives To characterize the Fc glycosylation profile of anti‐HLA class I antibodies formed after platelet transfusion and to investigate its effect on clinical outcome. Patients/Methods We screened and captured anti‐HLA class I antibodies (anti‐HLA A2, anti‐HLA A24, and anti‐HLA B7) developed after platelet transfusions in hemato‐oncology patients, who were included in the PREPAReS Trial. Using liquid chromatography‐mass spectrometry, we analyzed the glycosylation profiles of total and anti‐HLA IgG1 developed over time. Subsequently, the glycosylation data was linked to the patients' clinical information and posttransfusion increments. Results The glycosylation profile of anti‐HLA antibodies was highly variable between patients. In general, Fc galactosylation and sialylation levels were elevated compared to total plasma IgG, which correlated negatively with the platelet count increment. Furthermore, high levels of afucosylation were observed for two patients. Conclusions These differences in composition of anti‐HLA Fc‐glycosylation profiles could potentially explain the variation in clinical severity between patients.

In vitro characterization of rare anti-αIIbβ3 isoantibodies produced by patients with Glanzmann thrombasthenia that severely block fibrinogen binding and generate procoagulant platelets via complement activation

“…This suggests that a specific structural organization of platelet-bound IgG is crucial for initiation of the complement cascade. Moreover, additional properties of anti-α IIb β 3 isoantibodies such as specific subclass or composition of the glycan in the Fc part of IgGs may modulate complement activation [ 34 , 35 ]. Regarding the binding site, data suggest that the activating antibodies block access of binding site of the CD41a antibody clone.…”

Cellular surface plasmon resonance-based detection of anti-HPA-1a antibody glycosylation in fetal and neonatal alloimmune thrombocytopenia

“…Moreover, anti-HLA antibodies may play a possible accessory role (which seems to be minor) in combination with anti-HPA antibodies ( 46 , 47 ). Nevertheless, the available data seem to suggest a strong role for FcγRs that is influenced by IgG glycosylation, which can be fundamentally different for IgG targeting alloantigens, including HPA antigens and in some cases also anti-HLA IgG ( 8 , 9 ). These changes in IgG glycosylation consist first and foremost of afucosylated IgG that has up to 40-fold elevated affinity for FcγRIII, which may have an even greater impact on functional activities through even stronger avidity effects ( 14 , 48 ).…”

“…The effect of galactose may be because its presence further enhances the binding affinity of afucosylated IgG to FcγRIII (by a factor of 2) ( 14 ), or because of its role in enhancing IgG hexamerization and thereby complement activation ( 53 , 54 ). IgG galactosylation also seems to affect platelet-mediated clearance in vivo ( 8 , 41 ), but the relative importance of these two mechanistic effects of IgG galactosylation on either FcγR or complement is as of yet unknown.…”

“…Importantly, this glycan sensitivity is explained by the carbohydrate-carbohydrate interactions at the IgG-Fc N297 - FcγRIIIa-N162 interface ( 17 – 19 ). The appearance of afucosylated IgG responses in alloimmunization during platelet transfusion ( 8 ) and also in pregnancy ( 3 , 7 , 20 ) and the enhanced FcγR-mediated functional capacity make this class of responses highly relevant in transfusion medicine and alloimmune responses in pregnancy.…”