IL-12 is a potent cytokine that impairs the growth of several tumors in vivo in natural as well as in therapeutic conditions. Although IL-12 can enhance a number of immunological antitumor mechanisms, including those mediated by NK cells and CTL, recent reports have suggested that the mouse CD1d-restricted Vα14-Jα18 NKT cell was the essential cell type recruited in most, if not all tumor rejection models, including the B16 melanoma. In this study, we have examined and compared the role of NKT cells, T cells, NK cells, and other non-T non-B cells in the rejection of B16 melanoma cells after exogenous administration of IL-12. Surprisingly, our results failed to confirm a necessary role for NKT cells in this model. Instead, we found that NK cells mediated the rejection of liver metastases, whereas other γc-dependent non-T non-B cells, possibly lymphoid dendritic cells, were required for rejection of skin tumors. These findings challenge the view that NKT cells are systematically required for IL-12-mediated rejection of tumors, and instead reveal that a variety of effector pathways can be recruited depending on the tumor microenvironment.

IL-12 is a potent cytokine that impairs the growth of several tumors. in vivo in natural as well as in therapeutic conditions. Although IL-12 can enhance a number of immunological antitumor mechanisms, including those mediated by NK cells and CTL, recent reports have suggested that the mouse CDld-restricted Valpha14-Jalpha18 NKT cell was the essential cell type recruited in most, if not all tumor rejection models, including the B16 melanoma. In this study, we have examined and compared the role of NKT cells, T cells, NK cells, and other non-T non-B cells in the rejection of B16 melanoma cells after exogenous administration of IL-12. Surprisingly, our results failed to confirm a necessary role for NKT cells in this model. Instead, we found that NK cells mediated the rejection of liver metastases, whereas other gammac-dependent non-T non-B cells, possibly lymphoid dendritic cells, were required for rejection of skin tumors. These findings challenge the view that NKT cells are systematically required for IL-12-mediated rejection of tumors, and instead reveal that a variety of effector pathways can be recruited depending on the tumor microenvironment.

CD1d-restricted autoreactive natural killer (NK1.1+) T cells function as regulatory cells in various disease conditions. Using improved tetramer tracking methodology, we identified a NK1.1- thymic precursor and followed its differentiation and emigration to tissues by direct cell transfer and in situ cell labeling studies. A major lineage expansion occurred within the thymus after positive selection and before NK receptor expression. Surprisingly, cytokine analysis of the developmental intermediates between NK and NK+ stages showed a T helper cell TH2 to TH1 conversion, suggesting that the regulatory functions of NK T cells may be developmentally controlled. These findings characterize novel thymic and postthymic developmental pathways that expand autoreactive cells and differentiate them into regulatory cells.

To define the phenotype and T cell receptor (TCR) repertoire of CD1d-dependent T cells, we compared the populations of T cells that persisted in major histocompatibility complex (MHC)-deficient mice, which lack mainstream T cells, with those from MHC/CD1d doubly deficient mice, which lack both mainstream and CD1d-dependent T cells. Surprisingly, up to 80% of the CD1d-dependent T cells were stained by tetramers of CD1d/α-galactosylceramide, which specifically identify the previously described CD1d autoreactive Vα14-Jα18/Vβ8 natural killer (NK) T cells. Furthermore, zooming in on the CD1d-dependent non-Vα14 T cells, we found that, like Vα14 NK T cells, they mainly expressed recurrent, CD1d autoreactive TCR families and had a natural memory phenotype. Thus, CD1d-restricted T cells differ profoundly from MHC-peptide–specific T cells by their predominant use of autoreactive and semiinvariant, rather than naive and diverse, TCRs. They more closely resemble other lineages of innate lymphocytes such as B-1 B cells, γδ T cells, and NK cells, which express invariant or semiinvariant autoreactive receptors. Finally, we demonstrate that the MHC-restricted TCR repertoire is essentially non–cross-reactive to CD1d. Altogether, these findings imply that lipid recognition by CD1d-restricted T cells may have largely evolved as an innate rather than an adaptive arm of the mouse immune system.