Tumor-host interactions: a far-reaching relationship.

Carcinomas are composed of neoplastic epithelial cells, which form the heart of the tumor, as well as a variety of mesenchymal cell types and extracellular matrix components that comprise the tumor stroma, often termed its microenvironment. The normal counterparts of some stromal cells are thought to limit tumor growth, while tumor-associated stromal cells have been convincingly shown to actively promote tumor progression via complex heterotypic interactions with the nearby carcinoma cells. More recent advances have revealed that tumor-host interactions extend well beyond the local tissue microenvironment (ie, interactions between the neoplastic cells and the nearby stroma) and that tumors not only respond to, but actively perturb host organs at distant anatomic sites. This indicates that many aspects of tumor biology can only be explained by a detailed understanding of both local and systemic interactions, yet we currently have only a fragmentary understanding of both processes. In this review, we address the recent advances in our understanding of the contributions of local and systemic environments to cancer progression, the ability of tumors to actively perturb the host environment, and current therapeutic approaches that are designed to disrupt tumor-host relationships.