IL-17 Signaling in the Tumor Microenvironment

Inflammation is recognized as representing a double-edged sword in terms of tumor growth, in some instances contributing to attenuation of growth and in others to enhanced progression and metastasis. Extracellular signals, released by cells within the tumor microenvironment (TME), including cancer cells themselves, as well as infiltrating immune cells, stromal cells, and other components of the extracellular matrix, all can contribute to reshaping the tumor microenvironment (TME) and tumor growth/survival. Most recently, attention has centered on contributions in the TME made by the pro-inflammatory interleukin 17 (IL-17) and the T cells (Th17) and non-T cells which produce this cytokine, as well as the target cells (IL-17 receptor positive, IL-17R⁺) signaled by IL-17. The IL-17 family itself comprises at least six members, IL-17A, IL-17B, IL-17C, IL-17D, IL-17E (also called IL-25), and IL-17F, all of which are known to be secreted as disulfide-linked homo- or heterodimers. These in turn bind to IL-17R, a type I cell surface receptor, of which at least five variants have been described to date, IL-17RA to IL-17RE. The discussion below focuses on what we know to date about the role of IL-17/IL-17R interactions in the tumor microenvironment in regulation of tumor growth and metastasis and highlights recent ideas concerning the possible utility of this knowledge in the clinic.