TAM receptor signaling and immune regulation |
TAM receptors are fundamental inhibitors of the innate immune response in sentinel cells of the immune system. Pathogen encounter by dendritic cells (DCs) and macrophages triggers a rapid inflammatory response that is essential to combating infection. However, this response must be tightly regulated, since unrestrained Toll-like receptor (TLR) and cytokine receptor signaling generates a chronic inflammatory milieu that often leads to autoimmune disease. We have found that the TAM receptor tyrosine kinases - Tyro3, Axl, and Mer - broadly inhibit both TLR and TLR-induced cytokine receptor cascades1, 2. Remarkably, TAM inhibition of inflammation is transduced through an essential stimulator of inflammation - the type I interferon receptor (IFNAR) and its associated transcription factor STAT1. TLR induction of IFNAR-STAT1 signaling up-regulates components of the TAM system, which in turn usurp the IFNAR-STAT1 cassette to induce the cytokine and TLR suppressors SOCS1 and SOCS3. These events delineate a self-regulating cycle of inflammation, in which the obligatory, cytokine-dependent activation of TAM signaling hijacks a pro-inflammatory pathway to provide an intrinsic feedback inhibitor of both TLR- and cytokine-driven immune responses. We are currently studying how both inflammatory (e.g., LPS, poly I:C) and immunosuppressive (e.g., IL-10, glucocorticoids) stimuli differentially regulate TAM receptor expression in DCs, macrophages, and Langerhans cells3; and the consequences of this differential regulation for sentinel cell function. |