Although there is a large body of evidence that immune mechanisms play an essential role in the development of acquired aplastic anemia (AA), knowledge of autoantigens eliciting the immune system attack is limited. In an immune-mediated subset of AA patients characterized by the presence of HLA-DRB1*1501 and an increase in the proportion of paroxysmal nocturnal hemoglobinuria (PNH)-type blood cells, not only T-cell response but also B-cell response to antigens derived from hematopoietic stem cells may take place. To test this hypothesis, we screened sera of such immune-mediated AA patients for the presence of IgG antibodies recognizing a megakaryoblastic leukemia cell line, UT-7, using immunofluorescence staining. Since the sera from several patients stained cytoplasm of UT-7 cells, lysates of the cell line were subjected to Western blotting using the same sera. A clear band of 80 kDa protein was revealed by the sera from several patients. Peptide mass finger printing of the protein extracted from the single band identified this protein as moesin. Moesin is a membrane-cytoskeleton linker protein in cytoplasm and involved in the formation of microvilli, cell adhesion sites, and ruffling membranes. When sera of a large number of AA patients were screened for the presence of antibodies specific to recombinant moesin protein with Western blotting and ELISA, significantly high titer of the antibodies was detected in 35 of 110 (32%) AA patients. The prevalence of anti-moesin antibodies in AA patients showing increased PNH-type cells (41%) was significantly higher than those (12%) without PNH-type cells ( p =0.002). Anti-moesin antibodies were detectable in 15 of 39 (38%) recently diagnosed AA patients before therapy and in 20 of 71 (28%) patients who had a long history of AA. Four of 6 newly diagnosed patients showing anti-moesin antibodies responded to ATG+cyclosporine therapy and restored good hematopoietic function. Since B lymphocytes reportedly secrete moesin as a form of exosome (J Biol Chem, 2003), we examined supernatants of various leukemia cell lines incubated for 1 hour for the presence of moesin. The membrane-linker protein was detectable in the culture supernatant of 4 (UT-7, K562, OUN-1 and NB4) of 10 myeloid leukemia cell lines. These findings indicate that B-cell response to moesin may represent immune pathophysiology of AA and that moesin secreted from hematopoietic progenitor cells as a form of exosome may be efficiently processed by antigen-presenting cells, leading to induction of antibodies and possibly T cells specific to moesin in immune-mediated AA.