Which cytokine acts to promote differentiation of T cells to the Th1 subclass?

• A. IL-4
• B. IFN-α
• C. IL-12
• D. IL-10

Answer: (C)

Naive CD4+ T cells become Th1 cells when the environment signals promote cell-mediated immunity; the primary driver of this Th1 fate is IL-12, produced by macrophages and dendritic cells in response to intracellular pathogens. IL-12 engages its receptor on developing T cells, activating the STAT4 pathway and inducing the transcription factor T-bet, which steers differentiation toward the Th1 program and supports IFN-γ production. This makes IL-12 the best answer for promoting Th1 differentiation. By contrast, IL-4 promotes Th2 differentiation and antibody responses, IL-10 dampens inflammatory responses and can limit Th1 development, and IFN-α is antiviral but not the main driver of Th1 differentiation.