The mesenchymal stroma negatively regulates B cell lymphopoiesis through the expression of activin A

The negative control of B cell generation is only partially resolved. We assessed the role of activin A in regulation of B lymphopoiesis in view of its specific inhibitory effects on tumor B lineage cells. Activin A is constitutively expressed in mouse hemopoietic organs and in cultured mesenchymal cell lines. We observed an inverse relationship between activin A titer and B lineage cell production. In the spleen, the red pulp exhibited a relatively higher abundance of the protein as compared with the lymphoid follicles, wherein B cell accumulation occurs. Furthermore, a specific shut off in activin A expression was observed in bone marrow and spleen following in vivo induction of B lymphocyte polyclonal activation. We further substantiated these in vivo observations by in vitro studies of primary bone marrow cultures, in which the expression of functional activin A was found to be diminished prior to the onset of B lymphopoiesis. The reduction in functional activin A is shown to concomitantly occur with spontaneous induction of the expression of activin A specific inhibitors. We therefore propose that the mesenchymal organ stroma expresses activin A that negatively controls B cell lymphopoiesis.