Regulatory B cells: evidence, developmental origin and population diversity.

The adaptive immune system has developed several highly effective mechanisms in order to avoid excessive or unwanted reactions and promote resolution of immune activation. An emerging, significant body of evidence indicates that B cells can actively modulate immune responses by mechanisms that do not directly involve the production of antibodies. B cells appear to have the capacity to both induce and suppress immune effector mechanisms and they exert these functions both by contact-dependent interactions and through the secretion of cytokines. In this review we will focus on the regulatory suppressive function of several recently described B cell populations, functionally defined "regulatory B cells" or Breg cells. We will first outline the evidence that has led to their identification and then we will summarize current hypotheses on their ontogeny and possible lineage relationship.