Most members of the tumor-necrosis factor superfamily have been reported to mediate reverse signaling in T cells, macrophages, and/or dendritic cells. BAFF has been reported to have important functions in B-cell survival through forward signaling, but the presence of reverse signaling has not been explored. To investigate the possibility of BAFF-mediated reverse signaling, the expression patterns and functions of BAFF were analyzed in monocytic cell lines including the human macrophage-like cell line, THP-1. The expression of BAFF and its receptors was detected in monocytic cell lines, either before or after activation. The stimulation of BAFF induced the expression of matrix metalloproteinase (MMP)-9, interleukin -8, and transforming growth factor-beta-induced gene product (beta ig-h3) and the upregulation of intercellular adhesion molecule-1 in THP-1 cells. The activation of mitogen-activated protein kinase extracellular signal-regulated kinase1/2 and nuclear factor-kappaB was required for these responses. In addition to these stimulatory effects, BAFF-mediated signaling inhibited processes involving cytoskeletal movement such as phagocytosis and transmigration through blocking the activation of phosphatidylinositol 3-kinase/AKT and Rac-1. Furthermore, murine primary macrophage culture such as peritoneal macrophages expressed BAFF and stimulation of it induced the expression of MMP-9. These observations show that the reverse signaling initiated from BAFF induces the expression of inflammatory mediators while suppressing the cytoskeletal movements associated with phagocytosis and transmigration.