Evidence for the Specificity for Platelet HPA-1a Alloepitope and the Presenting HLA-DR52a of Diverse Antigen-Specific Helper T Cell Clones from Alloimmunized Mothers

Abstract Maternal alloantibodies against the human platelet Ag (HPA)-1a allotype of the platelet β3 integrin GpIIb/IIIa can cause severe fetal or neonatal hemorrhage. Almost all anti-HPA-1a-immune mothers are homozygous for HPA-1b and carry HLA-DR52a (DRB3*0101). The single Pro33 →Leu substitution (HPA-1b→HPA-1a) was previously predicted to create a binding motif for HLA-DR52a that can lead to alloimmunization. We have isolated six CD4+ T cell clones from three such mothers, which all respond to intact HPA-1a+, but not HPA-1b+, platelets. We used them to define the “core” and “anchor” residues of this natural T cell epitope. Molecular modeling based on a recently published crystal structure can explain the preferential presentation of the Leu33 (but not Pro33 variant) by HLA-DR52a rather than the linked HLA-DR3 or the allelic DR52b. The modeling also predicts efficient anchoring at position 33 by several alternative hydrophobic α-amino acids; indeed, a recently identified variant with Val33 is presented well to two clones, and is therefore potentially alloimmunogenic. Finally, these HPA-1a-specific T cell clones use a variety of T cell receptors, but all have a “Th1” (IFN-γ-producing) profile and are suitable for testing selective immunotherapies that might be applicable in vivo.