Activin in humoral immune responses

The synthesis and secretion of activin A are stimulated in immune cells, including Th2 cells, B cells, peritoneal macrophages, dendritic cells, and mast cells, in response to their activation. The increased expression of activin A is regulated at the level of transcription. Activin A increases antigen-specific IgE production, promotes the differentiation of macrophages into the M2 phenotype, and stimulates the maturation of mast cells in an autocrine/paracrine fashion. The roles of activin A in Th2 cells, B cells, and peritoneal macrophages are distinct from those of TGF-β, which acts primarily as an immune suppressor. In contrast, the activities of activin A and TGF-β are similar in mast cells. These findings suggest the discrete regulation of immune responses by activin and TGF-β, two structurally related growth/differentiation factors.