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-alpha beta T cells and NK cells. For this purpose, we studied common gamma chain (gamma c)-deficient mice, which demonstrate a selective defect in CD3(-) NK cell development relative to conventional TCR-alpha beta T cells. NK thymocytes differentiate in gamma c(-) mice as shown by the normal percentage of TCR V beta 8(+) CD4(-)CD8(-) cells and the normal quantity of thymic V alpha 14-J alpha 281 mRNA that characterize the NK T repertoire. However, gamma c-deficient NK thymocytes fail to coexpress the NK-associated markers NKR-P1 or Ly49, yet retain characteristic expression of the cytokine receptors interleukin (IL)-7R alpha and IL-2R beta. Despite these phenotypic abnormalities, gamma c(-) 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-alpha beta 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 gamma c-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 se lection via their invariant TCR to confer the IL-4-producing phenotype, followed by acquisition of NK-associated markers and maturation/export to the periphery.