Interaction of regulatory T cells with antigen-presenting cells in health and disease.

Among antigen-presenting cells (APCs), dendritic cells as well as monocytes acquire immunostimulatory capacity only after appropriate maturation. Therefore, blockade of the maturation/activation results in a steady state or alternatively activated phenotype, which induces tolerance rather than immunity. Functional analyses revealed recently that steady-state dendritic cells and alternatively activated macrophages, respectively, actively induce regulatory T cells (Tregs) in the periphery of the body. Thus, production of Tregs does not rely exclusively on thymic development. Vice versa, Tregs respond to APCs by several means. Recent lines of evidence indicate that Tregs prevent terminal differentiation of subpopulations of APCs or lead to upregulation of surface expression of immunosuppressive molecules. Thus, Tregs foster an environment that further promotes their development. In conclusion, the mutual interaction of Tregs and APCs enables Tregs to sustain their immunosuppressive function(s), which in healthy individuals may be crucial for the maintenance of peripheral tolerance. Since macrophages bridge the innate and the acquired immune system, Tregs are able to gain influence on the innate immune system by interacting with macrophages beyond the mere interaction with effector T cells.