New insights on the role of costimulatory molecules in T helper cell function have yielded exciting alternatives to the development of therapeutic strategies that target T cell costimulatory pathways. Inducible costimulatory molecule (ICOS) signaling is now shown by Paulos and colleagues to support expansion of human T helper 17 (T(H)17) cells that could exert antitumor activity. Here we discuss (i) how these findings aid in our understanding of mechanisms that govern T(H)17 cell functions and (ii) the potential application of these new insights to the development of immunotherapies.
