B lymphocyte chemoattractant (BLC/CXCL13) is ectopically and highly expressed in the target organs such as the thymus and kidney in aged (NZB x NZW)F1 (BWF1) mice, a murine model for SLE. Ectopic expression of BLC/CXCL13 was attributed to mature myeloid DCs infiltrating in the target organs. DCs were also increased in the peripheral blood in aged BWF1 mice and differentiated into BLC/CXCL13-producing DCs in the presence of TNF-alpha or IL-1beta, but not IFN-alpha or IFN-gamma. BLC/CXCL13 expression in mature myeloid DCs was confirmed in bone marrow derived-DCs generated in vitro in the presence of GM-CSF and TNF-alpha. B1 cells expressed higher level of CXCR5 and migrated towards BLC/CXCL13 much better than B2 cells. B1 cells failed to home to the peritoneal cavity and preferentially recruited to the target organs in aged BWF1 mice developing lupus nephritis. B1, but not B2 cells possessed a potent antigen presenting activity in allo-MLR and activated autologous thymic CD4 T cells in the presence of IL-2. CXCR5+ CD4 T cells were also increased in aged BWF1 mice and they enhanced IgG production by B1 cells in vitro. These results suggest a possible involvement of aberrant B1 cell trafficking in activation of autoreactive CD4 T cells and autoantibody production in the target organs during the development of lupus, providing a new insight for the pathogenesis of B1 cells in lupus.