The present disclosure demonstrates for the first time that Th-17 cells are the product of a distinct CD4+ T cell lineage with unique developmental and functional characteristics. The developmental signals required for Th-17 commitment and development are also described. As a result, the teachings of the present disclosure provide a basis for understanding the developmental pathway of Th-17 cells and offer novel avenues to modulate (stimulate or inhibit) this developmental program and offer new methods for the treatment and prevention of disease states and conditions related to aberrant activity of Th-17 cells.