Mothers Helper T Cell Clones from Alloimmunized HLA-DR 52 a of Diverse Antigen-Specific HPA-1 a Alloepitope and the Presenting Evidence for the Specificity for Platelet

Maternal alloantibodies against the human platelet Ag (HPA)-1a allotype of the platelet ␤ 3 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 Pro 33 3 Leu substitution (HPA-1b3 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 Leu 33 (but not Pro 33 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 ␣-amino acids; indeed, a recently identified variant with Val 33 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-␥-producing) profile and are suitable for testing selective immunotherapies that might be applicable in vivo. A lloimmune responses by mothers against fetal Ags have been studied much less than autoimmune reactions, although the original alloantigens are easier to identify. There have been some reports on maternal T cell responses to rhesus Ags (1), but few on fetomaternal alloimmune thrombocy-topenia (FMAIT) 3 (2), although much is known about its immu-nogenetics (2, 3). FMAIT causes considerable fetal and neonatal morbidity and mortality. The overall incidence of FMAIT is estimated to be 1 in 1163 live births (86 per 100,000), the incidence of severe throm-bocytopenia (platelet count Ͻ50 ϫ 109/L) is 1 in 1695 (or 59 per 100,000), and intracranial hemorrhage occurs in the range of 10 – 20%, most of which occur antenatally (4 – 6). Most of the deaths are associated with intracranial hemorrhage; the mortality has been estimated in some studies as Ͼ10% (7), although recent studies suggest this is an overestimate (5, 6). Unfortunately, unlike in he-molytic disease of the newborn, there is as yet no reliable laboratory method to predict pregnancies at high risk of severe FMAIT and intracranial hemorrhage (8, 9). FMAIT frequently affects first pregnancies (unlike hemolytic disease of the newborn), and …