Harnessing T9 cells in cancer immunotherapy.

CD4 T cell effector subsets not only profoundly affect cancer progression, but recent evidence also underscores their critical contribution to the anticancer efficacy of immune checkpoint inhibitors. In 2012, the two seminal studies suggested the superior antimelanoma activity of T9 cells over other T cell subsets upon adoptive T cell transfer. While these findings provided great impetus to investigate further the unique functions of T9 cells and explore their relevance in cancer immunotherapy, the following questions still remain outstanding: are T9 cell anticancer functions restricted to melanoma? What are the factors favouring T9 cell effector functions? What is the contribution of T9 cells to cancer immunotherapy treatments? Can T9 cells be identified in humans and, if so, what is their clinical relevance? By reviewing the studies addressing these questions, we will discuss how T9 cells could be therapeutically harnessed for cancer immunotherapy strategies.