Essential role for CAML in follicular B cell survival and homeostasis.

Calcium-modulating cyclophilin ligand (CAML) is a ubiquitously expressed protein that is important during thymopoiesis. However, whether it serves a function in mature lymphocytes is unknown. In this article, we show that CAML is essential for survival of peripheral follicular (Fo) B cells. Conditional deletion of CAML in CD19-Cre transgenic mice caused a significant reduction in Fo cell numbers and increased rates of homeostatic proliferation. CAML-deficient Fo cells showed increased cellular turnover and normal proliferative ability. Although CAML-deficient Fo cells responded to AgR stimulation and to B cell activating factor, they displayed decreased survival and increased apoptosis following stimulation with LPS and IL-4 in vitro. Failure to survive was not due to aberrant B cell development in the absence of CAML, because induced deletion of the gene in mature cells resulted in a similar phenotype. These data establish an essential and ongoing role for CAML in the long-term survival of mature B cells.