Proposed model for Th1 and Th1β development. CD4⁺ T-helper cells (Th) in the presence of IL-12 or IL-23 develop into Th1 or Th1β phenotypes, respectively. Although T-bet induces Th1 development, it negatively regulates IL-23-mediated T-cell development. Antigen-receptor engagement of differentiated IL-17-secreting Th1β cells induces T-bet expression and promotes a switch to a Th1 cytokine-secreting phenotype.