The study explores the gene regulatory basis of bystander activation in neonatal CD8+ T cells, revealing their enhanced innate-like functions compared to adult CD8+ T cells. Neonatal CD8+ T cells exhibit a robust and diverse bystander activation program, enabling them to respond to innate cytokines without TCR stimulation. This is attributed to their ability to undergo rapid chromatin remodeling, leading to the use of distinct enhancers and transcription factors typically found in innate-like T cells. The switch between innate and adaptive functions in CD8+ T cells is mediated by changes in the abundance of distinct cell subsets. Neonatal CD8+ T cells, which are more innately programmed, persist into adulthood and retain their innate-like properties, including enhanced IFN-γ production in response to innate cytokines. These findings support the layered immune hypothesis, indicating that the CD8+ T cell compartment is more functionally diverse than previously thought. Neonatal CD8+ T cells provide innate protection against diverse pathogens, as demonstrated by their ability to produce unexpected cytokines like IL-13, IL-10, and GM-CSF in response to innate cytokines. This distinct program of bystander activation is conserved in both mice and humans. Neonatal CD8+ T cells are composed of multiple innate subsets, with fetal-derived virtual memory cells showing higher innateness scores and responsiveness to innate cytokines. The ability of neonatal CD8+ T cells to respond rapidly to innate cytokines is linked to extensive chromatin remodeling, which allows for the accessibility of a multitude of AP-1 binding sites. This enables the production of a broad spectrum of cytokines, contributing to enhanced innate immune protection. The Bach2/AP-1 axis specifies CD8+ T cell innateness, with lower Bach2 levels in neonatal cells increasing AP-1 binding and enhancing innate responsiveness. In contrast, adult CD8+ T cells have higher Bach2 levels, which suppress their ability to respond to innate cytokines. These findings highlight the functional diversity of CD8+ T cells and the importance of the Bach2/AP-1 axis in regulating innate and adaptive responses. The study also shows that human CD8+ T cells exhibit similar patterns of innateness, with neonatal cells showing greater responsiveness to inflammation and producing a variety of cytokines. These results support the layered immune model of development, indicating that the CD8+ T cell compartment is more diverse and functionally complex than previously recognized.The study explores the gene regulatory basis of bystander activation in neonatal CD8+ T cells, revealing their enhanced innate-like functions compared to adult CD8+ T cells. Neonatal CD8+ T cells exhibit a robust and diverse bystander activation program, enabling them to respond to innate cytokines without TCR stimulation. This is attributed to their ability to undergo rapid chromatin remodeling, leading to the use of distinct enhancers and transcription factors typically found in innate-like T cells. The switch between innate and adaptive functions in CD8+ T cells is mediated by changes in the abundance of distinct cell subsets. Neonatal CD8+ T cells, which are more innately programmed, persist into adulthood and retain their innate-like properties, including enhanced IFN-γ production in response to innate cytokines. These findings support the layered immune hypothesis, indicating that the CD8+ T cell compartment is more functionally diverse than previously thought. Neonatal CD8+ T cells provide innate protection against diverse pathogens, as demonstrated by their ability to produce unexpected cytokines like IL-13, IL-10, and GM-CSF in response to innate cytokines. This distinct program of bystander activation is conserved in both mice and humans. Neonatal CD8+ T cells are composed of multiple innate subsets, with fetal-derived virtual memory cells showing higher innateness scores and responsiveness to innate cytokines. The ability of neonatal CD8+ T cells to respond rapidly to innate cytokines is linked to extensive chromatin remodeling, which allows for the accessibility of a multitude of AP-1 binding sites. This enables the production of a broad spectrum of cytokines, contributing to enhanced innate immune protection. The Bach2/AP-1 axis specifies CD8+ T cell innateness, with lower Bach2 levels in neonatal cells increasing AP-1 binding and enhancing innate responsiveness. In contrast, adult CD8+ T cells have higher Bach2 levels, which suppress their ability to respond to innate cytokines. These findings highlight the functional diversity of CD8+ T cells and the importance of the Bach2/AP-1 axis in regulating innate and adaptive responses. The study also shows that human CD8+ T cells exhibit similar patterns of innateness, with neonatal cells showing greater responsiveness to inflammation and producing a variety of cytokines. These results support the layered immune model of development, indicating that the CD8+ T cell compartment is more diverse and functionally complex than previously recognized.