A large body of evidence has suggested that acquired aplastic anemia (AA) of patients carrying HLA-DR15 is a kind of organ-specific autoimmune disease where hematopoietic progenitor cells in bone marrow are attacked by CD4+ T cells recognizing endogenous antigens. We recently identified diazepam-binding inhibitor-related protein 1 (DRS-1) as a candidate autoantigen capable of provoking immune system attack against hematopoietic progenitor cells in AA (Blood, 2004). Although in other organ-specific autoimmune diseases such as insulin-dependent diabetes mellitus and primary biliary cirrhosis, cytoplasmic proteins including glutamic acid decarboxylase 65 and pyruvate dehydrogenase complex have been shown to serve as autoantigens and mediate organ damages by CD4+ T cells, it remains unclear whether a peroxisomal protein like DRS-1 can be processed in hematopoietic progenitor cells, presented by HLA-DR15, and eventually serve as a target antigen of specific CD4+ T cells, leading to killing of hematopoietic progenitor cells themselves. To clarify these issues, we established a CD4+ T-cell line specific to a DRS-1 peptide (amino acid residues 191–204) from an AA patient carrying HLA-DR15 who had exhibited a high titer of anti-DRS-1 antibody as well as a high frequency of T-cell precursors specific to DRS-1, and then examined the cytotoxicity of the DRS-1-specific T-cell line against (1) autologous lymphoblastoid cell line (LCL) cells transfected with full length DRS-1 cDNA using a lentiviral vector, (2) myeloid leukemia cell lines carrying HLA-DR15 (KH88 and SAS413) and a leukemia cell line not carrying HLA-DR15 (K562), and (3) CD34+ progenitor cells from normal individuals. When all leukemia cell lines and LCL cells were examined for DRS-1 expression using Western blotting with specific monoclonal antibodies, DRS-1 protein was detected in DRS-1-transfected LCL cells, KH88 and K562, but not in nontransfected LCL cells and SAS413. Overexpression of DRS-1 gene by the CD34+ cells from normal individuals was ascertained by real-time PCR. In the 51Cr release assay, DRS-1-specific T cells showed cytotoxicity against only DRS-1-transfected LCL cells and KH88 in a dose-dependent manner (Figure), indicating that the T cell line requires presence of both DRS-1 and HLA-DR15 on target cells to exert cytotoxicity. When the DRS-1-specific T cells were incubated with CD34+ cells isolated from normal individuals with or without HLA-DR15 at an 10:1 ratio for 4 hours and cultured in a methylcellulose medium supplemented with colony-stimulating factors, the numbers of CFU-GM and BFU-E colonies derived from an HLA-DR15+ individual were 60.0% and 52.9% of a control whereas those derived from an HLA-DR15− individual were 90.1% and 88.2%. These findings indicate that hematopoietic progenitor cells in individuals with HLA-DR15 can present DRS-1 through the DR molecule and a breakdown of immune tolerance to DRS-1 may lead to development of AA.