Quantification of the preexisting CD4 T-cell repertoire specific for human erythropoietin reveals its immunogenicity potential

Antibody-mediated pure red cell aplasia is a rare but serious event resulting from the induction of neutralizing erythropoietin (Epo)–specific antibodies provoked by treatment with recombinant Epo. Because of the crucial role of CD4 T cells in humoral response, we have quantified the number of Epo-specific CD4 T cells in the blood of normal donors by in vitro stimulation. An important repertoire of preexisting Epo-specific T cells was observed in almost half of the donors, comparable with that of non–self-proteins. This observation suggests that, at the steady state, endogenous Epo weakly contributes to tolerance induction and may be ignored by the immune system. As a result, circulating Epo-specific CD4 T cells could be prone to be activated by altered batches of Epo, providing them with costimulatory signals. Our data also highlight the relevance of T-cell assays performed with normal donors to evaluate the potential immunogenicity of therapeutic proteins.