TACI is a TNFR homolog expressed by mature B lymphocytes that has been implicated in the positive regulation of B cell growth and antibody production, as well as in the development of autoimmune disease. Its biology is complex due to the existence of two ligands, BLyS and APRIL, and a homologous receptor, BCMA, that similarly binds both ligands. To determine its critical biological role, we generated TACI knockout mice. Surprisingly, these mice demonstrated a 2-fold increase in numbers of circulating and splenic B cells, apparently due to increased proliferation rate. Maturation of B cells and T-dependent antibody production was normal, but responses to T-independent type II antigens were almost completely abolished. It appears that TACI provides an essential costimulatory signal for the T-independent humoral response.
Von Bülow, G. U., Van Deursen, J. M., & Bram, R. J. (2001). Regulation of the T-independent humoral response by TACI. Immunity, 14(5), 573–582. https://doi.org/10.1016/S1074-7613(01)00130-3