Lineage Relationships and Differentiation of Natural Killer (NK) T Cells: Intrathymic Selection and Interleukin (IL)-4 Production in the Absence of NKR-P1 and Ly49 Molecules

In this report, we have assessed the lineage relationships and cytokine dependency of natural killer (NK) T cells compared with mainstream TCR-αβ T cells and NK cells. For this purpose, we studied common γ chain (γc)-deficient mice, which demonstrate a selective defect in CD3⁻ NK cell development relative to conventional TCR-αβ T cells. NK thymocytes differentiate in γc⁻ mice as shown by the normal percentage of TCR Vβ8⁺ CD4⁻CD8⁻ cells and the normal quantity of thymic Vα14–Jα281 mRNA that characterize the NK T repertoire. However, γc-deficient NK thymocytes fail to coexpress the NK-associated markers NKR-P1 or Ly49, yet retain characteristic expression of the cytokine receptors interleukin (IL)-7Rα and IL-2Rβ. Despite these phenotypic abnormalities, γc⁻ NK thymocytes could produce normal amounts of IL-4. These results define a maturational progression of NK thymocyte differentiation where intrathymic selection and IL-4–producing capacity can be clearly dissociated from the acquisition of the NK phenotype. Moreover, these data suggest a closer ontogenic relationship of NK T cells to TCR-αβ T cells than to NK cells with respect to cytokine dependency. We also failed to detect peripheral NK T cells in these mice, demonstrating that γc-dependent interactions are required for export and/or survival of NK T cells from the thymus. These results suggest a stepwise pattern of differentiation for thymically derived NK T cells: primary selection via their invariant TCR to confer the IL-4–producing phenotype, followed by acquisition of NK-associated markers and maturation/export to the periphery.