Genetic Removal of Terminal Sialic Acid Residues from the O-Linked Glycans Adjacent to the HPA-9b Polymorphism of Platelet Membrane Glycoprotein IIb Improves the Binding and Detection of HPA-9b Patient Alloantibodies

Sialic acids occupy the terminal position of glycan chains, and have the potential to influence the antigenicity of glycoproteins. Antibody binding sites on a glycoprotein can be solely protein in nature, or can include or be affected by nearby glycan chains, which may either mask the epitope, or conversely comprise part of the antibody binding site. The polymorphisms responsible for formation of the Human Platelet Alloantigens (HPA)-3 (Ile843Ser) and HPA-9 (Val837Met) are next to each other near the C-terminus of the extracellular domain of platelet membrane glycoprotein (GP)IIb, and are adjacent to sialyl-Core 1 O-glycans that emanate from serines 845 and 847. Previous studies have shown that these O-linked glycans are required to support the binding of some of HPA-3a alloantibodies. Loss of these glycans, especially terminal sialic acid residues, during platelet storage or preparation, can present major difficulties in detecting clinically important anti-HPA-3a alloantibodies in suspected cases of fetal/neonatal alloimmune thrombocytopenia (FNAIT). Similarly, detection and identification of anti-HPA-9b alloantibodies from FNAIT patient sera can also be extremely challenging, resulting in the inability to resolve clinical cases of this bleeding disorder. Whether the nearby O-glycans on serines 845 and 847 of GPIIb affect the antigenicity of HPA-9b, and/or influence the binding of anti-HPA-9b alloantibodies in clinically significant cases of FNAIT is unknown.

We previously reported the generation of bioengineered, HLA class I-negative, HPA-9a or -9b allele-specific megakaryocytes (MKs) from human induced pluripotent stem cells (iPSCs) that are suitable for whole-cell flow cytometric detection of anti-HPA-9b alloantibodies (Blood 2019;134(22):e1-e8). Unexpectedly, treatment of these allele-specific MKs with neuraminidase actually enhanced the binding of anti-HPA-9b alloantibodies, suggesting that terminal sialic acids on GPIIb partially mask the HPA-9b epitope. To test the hypothesis that removal of terminal sialic acids on nearby O-glycans, or removal of the entire O-glycan chains emanating from Ser845/847 of GPIIb, might enhance the detection of anti-HPA-9b patient alloantibodies, we created a series of deletion mutants in two major sialidases, ST3GAL1 and ST3GAL2, known to be responsible for transferring terminal sialic acid residues to Core 1 O-glycans, in our HPA-9a and -9b allele-specific iPSCs. Immunoprecipitation/western blot analysis confirmed the complete removal of terminal sialic acids on the O-glycan chains of GPIIb in ST3GAL1/2 knockout (KO) iPSC-derived MKs, as reported by the binding of the lectin PNA to the exposed Core 1 structure on GPIIb. These sialylation-deficient ST3GAL1/2 KO HPA-9b MKs exhibited dramatically increased anti-HPA-9b alloantibody binding, further confirming the notion that HPA-9b epitopes are partially masked by terminal sialic acids on nearby GPIIb O-glycan chains. Finally, allele-specific iPSCs lacking the complete O-glycan chains attached to serines 845 and 847 of GPIIb were generated by mutating those residues to alanines using a similar CRISPR/Cas9 gene editing approach. Interestingly, O-glycan chain-deficient Ala845/847 mutant MKs carrying the HPA-9b polymorphism exhibited slightly to moderately reduced binding of anti-HPA-9b alloantibodies, indicating that the presence of the Core 1 O-glycan chains attached to GPIIb serine residues 845 and 847 contribute to the presentation of the HPA-9b epitope - perhaps by stabilizing the conformation of the glycoprotein in this region. Taken together, these data suggest that detection of anti-HPA-9b alloantibodies may be enhanced through the use of iPSC-derived HPA-9b-specific MKs that have been genetically altered to lack nearby terminal sialic acid residues, but retain the glycan chains to which they are attached.