Th17 cells in cancer: help or hindrance?

The role of CD4+ T helper (Th) 17 cells in malignancy is currently under debate. However, upon closer scrutiny, it becomes apparent that this discussion includes not only evaluations of Th17 cells but also IL-17+ cells from other immune populations, the cytokine interleukin (IL)-17 itself (both endogenous and exogenous) and IL-23. Further complicating the matter are occasionally conflicting results of studies in humans versus those in mice and contradictory data from immunocompetent versus immunodeficient mice. To better understand the role of Th17 cells in the tumor-bearing host, we focus first upon those studies investigating Th17 cells in patients and then those in mice, all the while keeping in mind that variables such as tumor-initiating agents, a pre-existing inflammatory environment and the immune competence of the host may have direct effects upon this T-cell subset. In this review, we will describe the phenotype of tumor-associated Th17 cells, review those studies that have examined the population directly, and finally, briefly discuss the studies involving Th17-associated signature cytokines.