The role of cytokines in T-cell memory in health and disease

Upon stimulation with their cognate antigen, naive T cells undergo proliferation and differentiation into effector cells, followed by apoptosis or survival as precursors of long-lived memory cells. These phases of a T-cell response and the ensuing maintenance of memory T cells are shaped by cytokines, most notably interleukin-2 (IL-2), IL-7, and IL-15 that share the common γ chain (γ_c) cytokine receptor. Steady-state production of IL-7 and IL-15 is necessary for background proliferation and homeostatic survival of CD4⁺ and CD8⁺ memory T cells. During immune responses, augmented levels of IL-2, IL-15, IL-21, IL-12, IL-18, and type-I interferons determine the memory potential of antigen-specific effector CD8⁺ cells, while increased IL-2 and IL-15 cause bystander proliferation of heterologous CD4⁺ and CD8⁺ memory T cells. Limiting availability of γ_c cytokines, reduction in regulatory T cells or IL-10, and persistence of inflammation or cognate antigen can result in memory T cells, which fail to become cytokine-dependent long-lived cells. Conversely, increased IL-7 and IL-15 can expand memory T cells, including pathogenic tissue-resident memory T cells, as seen in lymphopenia and certain chronic-inflammatory disorders and malignancies. These abovementioned factors impact immunotherapy and vaccines directed at memory T cells in cancer and chronic infection.