The tumor microenvironment and its role in promoting tumor growth.

The tumor microenvironment is created by the tumor and dominated by tumor-induced interactions. Although various immune effector cells are recruited to the tumor site, their anti-tumor functions are downregulated, largely in response to tumor-derived signals. Infiltrates of inflammatory cells present in human tumors are chronic in nature and are enriched in regulatory T cells (T(reg)) as well as myeloid suppressor cells (MSC). Immune cells in the tumor microenvironment not only fail to exercise antitumor effector functions, but they are co-opted to promote tumor growth. Sustained activation of the NF-kappaB pathway in the tumor milieu represents one mechanism that appears to favor tumor survival and drive abortive activation of immune cells. The result is tumor escape from the host immune system. Tumor escape is accomplished through the activation of one or several molecular mechanisms that lead to inhibition of immune cell functions or to apoptosis of anti-tumor effector cells. The ability to block tumor escape depends on a better understanding of cellular and molecular pathways operating in the tumor microenvironment. Novel therapeutic strategies that emerge are designed to change the pro-tumor microenvironment to one favoring acute responses and potent anti-tumor activity.