Immature B cells in bone marrow express Fas/FasL

Negative selection is a process by which autoreactive lymphocytes are elimi nated from the developing antigen receptor repertoire. The mechanisms regul ating negative selection of immature B lymphocytes in the bone marrow are p oorly elucidated. Human bone marrow cells were examined in order to investi gate the presence of the members of the Fas (APO-1/CD95) system. Here we de monstrate the expression of Fas in immature B lymphocytes (CD10/CD19(+)/CD4 0(+)/sIg(+)), and the presence of Fas natural ligand (FasL) in CD19(+) bone marrow cells. The observed expression of apoptosis-related molecules might indicate how negative selection of autoreactive B cells can occur in human bone marrow.