Combination peptide immunotherapy suppresses antibody and helper T cell responses to the major human platelet autoantigen GPIIb/IIIa in HLA-transgenic mice.

Platelet destruction in immune thrombocytopenia is caused by autoreactive antibody and T-cell responses, most commonly direct-ed against platelet glycoprotein IIb/IIIa. Loss of self-tolerance in the disease is also associated with deficient activity of regulatory T cells. Having previously mapped seven major epitopes on platelet glycopro-tein IIIa that are recognized by helper T cells from patients with immune thrombocytopenia, the aim was to test whether peptide therapy with any of these sequences, alone or in combination, could inhibit responses to the antigen in humanized mice expressing HLA-DR15. None of the individual peptides, delivered by a putative tolerogenic regimen, consis-tently suppressed the antibody response to subsequent immunization with human platelet glycoprotein IIb/IIIa. However, the combination of glycoprotein IIIa peptides aa6-20 and aa711-725, which contain the pre-dominant helper epitopes in patients and elicited the strongest trends to suppress when used individually, did abrogate this response. The pep-tide combination also blunted, but did not reverse, the ongoing antibody response when given after immunization. Suppression of antibody was associated with reduced splenocyte T-cell responsiveness to the antigen, and with the induction of a regulatory T-cell population that is more responsive to the peptides than to purified platelet glycoprotein IIb/IIIa. Overall, these data demonstrate that combinations of peptides contain-ing helper epitopes, such as platelet glycoprotein IIIa aa6-20 and aa711725, can promote in vivo suppression of responses to the major antigen implicated in immune thrombocytopenia. The approach offers a prom-ising therapeutic option to boost T-cell regulation, which should be taken forward to clinical trials.