TACI is required for the survival of short- and long-lived plasma cells

In a typical immune response, naïve B cells are activated, differentiated into short-lived plasma cells (SLPCs), and then matured into long-lived plasma cells (LLPCs) in the bone marrow. Among three TNF family receptors (BAFF-R, TACI, and BCMA), BAFF-R and BCMA are known to be crucial for the maintenance of naïve B cells and bone marrow LLPCs, respectively. In contrast, the function of TACI remains unclear, as analysis of global TACI knockout mice suggests the existence of two opposing roles in B and plasma cells. Here, to define the role of TACI during T cell-dependent (TD) immune responses in a B and plasma cell-intrinsic manner, we utilized adoptive transfer experiments employing B cells with a conditional TACI knockout. We show that B cell activation is apparently normal in the absence of TACI, while it is crucial for the expansion of IgM+ and IgG1+ SLPCs and maintenance of their resultant bone marrow LLPCs. Hence, our data suggest that the hyper-B cell activation seen in global TACI knockout mice could be due to an indirect control of BAFF levels or B cell extrinsic anomalies.