TACI regulates marginal zone B cell development

The mature B cell compartment consists of follicular and marginal zone (MZ) B cells, which develop from transitional type 2 (T2) B cells. 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 in mice but rather has a cell-intrinsic role in MZ B cell development. Loss of TACI leads to reduced MZ B cell numbers and an impaired T-independent antibody response. 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.