Which cytokine is associated with promoting differentiation of T cells to the Th1 subset?

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

Answer: (C)

The cytokine that promotes differentiation of naive CD4+ T cells into the Th1 subset is IL-12. When antigen-presenting cells like macrophages and dendritic cells detect intracellular pathogens, they release IL-12. This signal acts on developing T cells through the IL-12 receptor and activates the STAT4 pathway, which drives expression of the transcription factor T-bet. T-bet then steers the cells toward the Th1 program and the production of IFN-γ, reinforcing a cell-mediated immune response that activates macrophages to kill intracellular pathogens.

This contrasts with IL-4, which pushes differentiation toward the Th2 lineage involved in humoral responses; IL-10, which is anti-inflammatory and can dampen Th1 responses; and IFN-α, which is a type I interferon primarily involved in antiviral states rather than the main driver of Th1 differentiation.