Th9 and Th17 cells: the controversial twins in cancer immunity.

Th17 cells (producing IL-17) and Th9 cells (producing IL-9) exhibit functional plasticity, and their role in tumorigenicity is controversial. Th17/IL-17 and Th9/IL-9 exhibit critical, but often opposing, roles in tumor progression. In this issue of the JCI, Salazar et al. show that while IL-17 and IL-9 induced distinct but complementary molecular pathways, both cytokines also induced epithelial-mesenchymal transition (EMT) in lung cancer cells and promoted metastatic spreading. A key question before us now is whether IL-9 and IL-17 contribute to tumor progression in a sequential and stage-specific manner within the tumor microenvironment.