Regulation of the T-independent humoral response by TACI

TACI is a TNFR homolog expressed by mature B lymphocytes that has been impl icated in the positive regulation of B cell growth and antibody production, as well as in the development of autoimmune disease. Its biology is comple x due to the existence of two ligands, BLyS and APRIL, and a homologous rec eptor, BCMA, that similarly binds both ligands. To determine its critical b iological role, we generated TACI knockout mice. Surprisingly, these mice d emonstrated 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 pr ovides an essential costimulatory signal for the T-independent humoral resp onse.