TACI Regulates Marginal Zone B Cell Development

The mature B cell compartment consists of follicular (FO) and marginal zone (MZ) B cells, which develop from transitional type 2 (T2) B cells and mount T-dependent and T-independent antibody responses, respectively. TACI, a member of the TNF receptor superfamily, is expressed on all mature B cells, with highest levels on MZ B cells and plasma cells. Previous studies reported that TACI is a negative regulator of B cell survival. However, this conclusion is confounded by elevated levels of BAFF, a cytokine that supports B cell survival, in TACI- deficient mice. We now show that TACI does not directly regulate B cell survival but rather has a cell-intrinsic role in MZ B cell development. Loss of TACI leads to reduced MZ B cell numbers and impaired T-independent antibody responses. Mechanistically, we show that TACI is required for MZ B cell development from T2 B cell precursors via activation of the PI3K-AKT pathway and subsequent inhibition of the FOXO1 transcription factor.