Which cytokine is the key growth factor driving T cell proliferation?

• A. IL-4
• B. IL-6
• C. IFN-γ
• D. IL-2

Answer: (D)

T cell expansion after antigen encounter is driven by interleukin-2, the primary growth factor for T cells. Activated T cells produce IL-2 and upregulate the high-affinity IL-2 receptor (CD25), creating an autocrine loop that promotes clonal proliferation. The signaling triggered by IL-2 involves pathways like JAK-STAT (notably STAT5), as well as PI3K-AKT and MAPK, which push cells from G1 into S phase and expand the T cell population. Without IL-2 signaling, T cells fail to proliferate effectively after activation. The other cytokines have different roles—IL-4 guides Th2 differentiation and humoral responses; IL-6 supports inflammation and B cell maturation; IFN-γ activates macrophages and promotes Th1 responses—so they are not the primary drivers of T cell proliferation. IL-2’s central role in T cell growth and in T cell homeostasis highlights why it’s the key growth factor for T cell expansion.