ICOS and neoplasm: In contrast to long-term expanded cells, day-4 Th17 cells 1) express peak levels of IL-2Ralphaα and costimulatory molecules (CD28, OX40, ICOS), 2) persist at greater fold once infused in the animal, 3) induce significantly increased production of IL-6, IL-17, and GM-CSF within the tumor- bearing host, and 4) provide long-lived protection against tumor recurrence.