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.

Deficiency of the cytokine receptor common gamma chain (gamma c) results in abnormal lymphoid development and a severe immunodeficiency disease due to the combined loss of the receptors for interleukins (IL)-2, -4, -7, -9, and -15. We have observed the development of secondary hematopoiesis with circulating hematopoietic progenitor cells in adult mice harboring a null mutation in gamma c. These extramedullary changes were not secondary to bone marrow failure or to an inability to maintain circulating blood counts. These results suggested that gamma c-dependent cytokine signaling pathways modulate hematopoietic development. An intrinsic defect in gamma c- hematopoietic stem cell commitment appeared unlikely, as fetal liver hematopoiesis was unaltered in gamma c- embryos. Furthermore, the absence of natural killer cells in gamma c- mice was not responsible for the observed hematopoietic changes. Peripheral TCR alpha beta T cells from gamma c- mice were characterized by an activated phenotype (CD62Llo, CD44hi, CD69hi) and showed increased levels of transcripts for hematopoietic stimulating cytokines, including IL-3 and granulocyte/macrophage-colony-stimulating factor. A predominance of these cells was detected in the bone marrow, suggesting a role for residual T cells in the enhanced hematopoiesis. Strikingly, the elimination of residual T cells from gamma c- mice reduced splenic and circulating hematopoietic precursor frequencies to normal levels. These results clearly implicate a deregulated TCR alpha beta T cell population in the observed hematopoietic changes in gamma c- mice, and emphasize the importance of gamma c-dependent cytokine interactions in modulating mature T cell responses.

Mutations in the gene encoding the common cytokine receptor gamma chain (γc) are responsible for human X-linked severe combined immunodeficiency disease (SCIDX1). We have used a γc-deficient mouse model to test the feasibility and potential toxicity of γc gene transfer as a therapy for SCIDX1. A retrovirus harboring the murine γc chain was introduced into γc-deficient bone marrow cells, which were then transplanted into alymphoid RAG2/γcdouble-deficient recipient mice. Circulating lymphocytes appeared 4 weeks postgraft and achieved steady-state levels by 8 weeks. The mature lymphocytes present in the grafted mice had integrated the γc transgene, expressed γc transcripts, and were able to proliferate in response to γc-dependent cytokines. The γc-transduced animals demonstrated (1) normal levels of immunoglobulin subclasses, including immunoglobulin G1 (IgG1) and IgG2a (which are severely decreased in γc- mice); (2) the ability to mount an antigen-specific, T-dependent antibody response showing effective in vivo T-B cell cooperation, and (3) the presence of gut-associated cryptopatches and intraepithelial lymphocytes. Importantly, peripheral B and T cells were still present 47 weeks after a primary graft, and animals receiving a secondary graft of γc-transduced bone marrow cells demonstrated peripheral lymphoid reconstitution. That γc gene transfer to hematopoietic precursor cells can correct the immune system abnormalities in γc- mice supports the feasibility of in vivo retroviral gene transfer as a treatment for human SCIDX1.