CD127⁺ Natural Killer Cells Represent a Distinct, Interleukin-15-Independent and Thymus-Independent Subset in Mice

Natural killer (NK) cells, key effectors of innate immunity, are classically categorized into CD56^dim and CD56^bright subsets in humans. While murine NK cell heterogeneity has become increasingly recognized, the classification of mature NK cell subsets remains incompletely defined. Here, we comprehensively characterized CD127⁺ NK cells in mice and identified them as a distinct, mature subset, developing independently of the thymus and interleukin (IL)-15 signaling. Flow cytometric analyses revealed that CD127⁺ NK cells are broadly distributed across lymphoid and non-lymphoid tissues-including in C57BL/6 wild-type and athymic Foxn1^-/- mice-and exhibit a surface phenotype distinct from CD127^- NK and thymus-derived CD127⁺ NK cells. Functional assays demonstrated that CD127⁺ NK cells produce interferon-γ and exert cytotoxic activity, despite expressing markers typically associated with immature NK cells. CD127⁺ NK cells were absent in IL-7Rα^-/- mice but present in IL-15^-/- and IL-15Rα^-/- mice, indicating a selective dependence on IL-7 signaling. IL-7 promoted their proliferation and activation both in vitro and in vivo. These findings revise current models of NK cell development by identifying a novel, IL-7-responsive, IL-15-independent, thymus-independent, and functionally competent CD127⁺ NK cell subset that is phenotypically distinct from helper-like innate lymphoid cells (ILCs). This study provides a framework for future investigations on NK cell heterogeneity, tissue specialization, and cytokine-mediated regulation.