Characterization of the alloreactive helper T-cell response to the platelet membrane glycoprotein IIIa (integrin-beta3) in human platelet antigen-1a alloimmunized human platelet antigen-1b1b women.

Background: The aims were to characterize the helper T-cell response to platelet (PLT) glycoprotein (GP) IIIa, which stimulates the alloimmune antibody response to human PLT antigen (HPA)-1a, to identify immunodominant epitopes and to examine the HLA Class II associations.
Study Design and Methods: Peripheral blood mononuclear cells (PBMNCs) were obtained from 21 HPA-1b1b women who had an HPA-1a-mismatched pregnancy, 14 of whom developed anti-HPA-1a, and 11 control donors. PBMNCs were stimulated with two panels of 15-mer peptides corresponding to the HPA-1a/1b polymorphic region, with either Leu33 (-1a) or Pro33 (-1b) at each possible position, and the proliferative responses were measured. HLA Class II and HPA genotyping was by conventional polymerase chain reaction-sequence-specific priming.
Results: Peptides with Leu33 at, or near, the C-terminus contained an immunodominant epitope, stimulating proliferation by helper T cells from all nine women who had anti-HPA-1a at the time of testing; peptide L1 (Val19-Leu33) stimulated a response in 50 percent of these women. Their T cells did not respond to the corresponding HPA-1b Pro33 peptides, and responses to either peptide panel were rare in unimmunized women and controls. HLA-DRB3*01+ was significantly overrepresented (p = 0.014) in alloimmunized women whose T cells responded to the major HPA-1a Leu33-containing epitope. Conversely, HLA-DRB1*15 was negatively associated (p = 0.014) with this response.
Conclusions: The HPA-1a polymorphic region of GPIIIa contains both the linear T-cell and the conformational B-cell epitopes. The immunodominant T-cell epitope is constrained by HLA-DRB3*01+, and if presented by a tolerogenic route, a peptide containing this epitope may form the basis for the prevention or reversal of the alloimmune response to HPA-1a.