FIGURE 1.

Model of T cell differentiation, after contact with antigen-activated DCs, from uncommitted naive T cells into different T cell subsets that produce different cytokines and thus promote different functional activities. Black arrows indicate the cytokines that mainly drive the differentiation to specific subsets. RA together with TGF-β promotes the differentiation of Treg cells, which generally are antiinflammatory, whereas RA also decreases Th1 activity and often promotes Th2 functions (see text). Upward arrows show proposed “plasticity” or interconversion among some of the T cell subsets (see references 26, 37, and 39 for detailed reviews). The signature cytokines and potential outcomes, which also are determined by the environment and type of pathogen or antigen encountered, are listed. Effects of vitamin A or RA differ contextually but generally limit Th1 and Th17 processes while increasing Th2 and Treg-mediated processes. Ab, antibody; DC, dendritic cells; Fox, forkhead; IFNγ, interferon γ; NFκB, nuclear factor kappa-light-chain-enhancer of activated B cells; RA, retinoic acid; RORγt, retinoid orphan receptor γt; TGFβ, transforming growth factor β; Th, T helper; Treg, T-regulatory.