IL-18 primes T cells with an antigen-inexperienced memory phenotype for proliferation and differentiation into effector cells through Notch signaling.

Recent studies have revealed that a subset of CD8+ T cells exhibit innate features and can be activated by cytokines. However, the precise mechanisms underlying the proliferation and differentiation of these cells remain unclear. Here, we demonstrated that CD44highCD8+ T cells in the mouse spleen express functional interleukin-18 (IL-18) receptors, whereas CD44lowCD8+ T cells do not. In response to IL-18 stimulation, these cells activated various metabolic pathways, upregulated the expression of surface molecules, such as c-Kit (CD117), CD25, and PD-1, and induced progression through the G1/S phase in the cell cycle. IL-18-primed cells, expressing a high-affinity receptor for IL-2, exhibited robust proliferation in response to IL-2 and underwent differentiation into effector cells. The splenic CD44highCD8+ T cells exhibited high expression levels of CD122, CD62L, CCR7, and CXCR3, along with CD5, indicating their potential for migration to the lymph nodes, where they could undergo expansion and terminal differentiation into effector cells. Additionally, in a tumor model, administration of IL-18 increased the accumulation of CD8+ T cells in both the lymph nodes and tumors. It is noteworthy that stimulation of CD44highCD8+ T cells with IL-18 upregulated the Notch-1 receptor and c-Myc. Moreover, inclusion of γ-secretase inhibitors attenuated the effect of IL-18 on both proliferation and interferon-γ production in the cells. These results demonstrate that IL-18 primes CD44highCD122highCXCR3highCD62LhighCD8+ T cells for expansion and differentiation into effector cells in a Notch signaling-dependent manner.