Reactivity of T cells from women with antibodies to the human platelet antigen (HPA)-1a to peptides encompassing the HPA-1 polymorphism

Abstract

The human platelet antigen-1a (HPA-1a) is the most common alloantigenic target in fetal and neonatal alloimmune thrombocytopenia (NAIT). Treatment currently depends on the outcome in previous pregnancies. HPA-1 specific T cell responses were determined in 14 HPA-1a alloimmunized women during or after pregnancies affected by NAIT. Peripheral blood mononuclear cells were incubated with peptides encompassing the Leu33Pro polymorphism (residues 20-39 and 24-45 in both Leu33 (HPA-1a) and Pro33 (HPA-1b) forms) or control recall antigens in the presence of autologous sera and T cell proliferation was measured by (3)H-thymidine incorporation. Control antenatal and postpartum sera suppressed T cell proliferation and use of such sera was avoided. Most patients (86%) responded to the HPA-1a peptides with 64% also having weaker T cell proliferation to the HPA-1b peptides; 14% had no activity towards any peptide despite responding to control antigens. Administration of IVIG during pregnancy appeared to reduce T cell reactivity to HPA-1 peptides. Postnatal anti-HPA-1a T cell responses from women who had a severe history of NAIT (an intracranial haemorrhage in a previous fetus) were greater than those from women with a mild history. This assay may have the potential to predict disease severity if performed prior to or early in pregnancy.

Fig. 1

Proliferative responses to PPD (♦,◊) and TT (▴,Δ) of T cells from three patients, (a) no. 6i, 20 weeks GA; (b) no. 3iii, 6 weeks PP; (c) no. 5ii, 21 weeks, PP cultured in the presence of autologous serum (♦,▴) or AB serum (◊,Δ). Mean ± SEM.

Fig. 2

Proliferative responses to PPD of T cells from two nonpregnant donors (a) male (squares) and (b) female (circles) cultured in the presence of autologous serum (grey symbols), AB serum (open symbols) or seven weeks postpartum serum (filled symbols). Mean ± SEM.

Fig. 3

T cell proliferative responses of 33 samples from 14 HPA-1a alloimmunized patients to HPA-1a and HPA-1b peptides. The combined cumulative SI of values were obtained by summation of the individual daily responses at days 4–7 to all concentrations of the HPA-1ai plus HPA-1aii peptides (□) or the HPA-1bi plus HPA-1bii peptides (▪).