A Distinct IL-18-Induced Pathway to Fully Activate NK T Lymphocytes Independently from TCR Engagement

Abstract

NK T lymphocytes are characterized by their ability to promptly generate IL-4 and IFN-γ upon TCR engagement. Here, we demonstrate that these cells can also be fully activated in the absence of TCR cross-linking in response to the proinflammatory cytokine IL-18 associated with IL-12. NK T cells stimulated with IL-18 plus IL-12 proliferated, killed Fas+ target cells, and produced high levels of IFN-γ without IL-4. In these conditions, IFN-γ production was at least 10-fold higher than that upon TCR cross-linking. Interestingly, a 2-h pretreatment with IL-12 plus IL-18 sufficed to maintain the high IFN-γ-producing potential during subsequent stimulation with anti-TCR mAbs or with the specific Ag α-galactosylceramide. Similar effects were observed in vivo, because splenic CD4+ NK T cells from MHC class II-deficient mice secreted IFN-γ without further stimulation when removed 2 h after a single injection of IL-12 plus IL-18. In conclusion, our evidence for activation of NK T lymphocytes in response to IL-18 plus IL-12 in the absence of TCR engagement together with the maintenance of preferential IFN-γ vs IL-4 production upon subsequent exposure to specific Ags is consistent with the active participation of this cell population in innate as well as acquired cellular immune responses.