The interplay between the CD4-lineage transcription factor ThPok and the CD8-lineage transcription factor, runt-related transcription factor 3 (Runx3), in T-cell development has been extensively documented. However, little is known about the roles of these transcription factors in invariant natural killer T (iNKT) cell development. CD1d-restricted iNKT cells are committed to the CD4 1 CD8 2 and CD4 2 CD8 2 sublineages, which respond to antigen stimulation with rapid and potent release of T helper (Th) 1 and Th2 cytokines. However, previous reports have demonstrated a new population of CD8 1 NKT cells in ThPok-deficient mice. In the current study, we sought to determine whether Runx3 was involved in the re-expression of CD8 and function of iNKT cells in the absence of ThPok. We used mice lacking Runx3, ThPok or both and verified that Runx3 was partially responsible for the appearance of CD8 1 iNKT cells in ThPok knockout mice. Additionally, Runx3 participated in the immune response mediated by iNKT cells in a model of a-galactosylceramide-induced acute hepatitis. These results indicate that Runx3 is crucial for the phenotypic and functional changes observed in ThPok-deficient iNKT cells. and the canonical T-cell receptor a (TCRa) chain (Va14-Ja18 in mice and V24-J18 in humans) coupled to specific Vb chains (Vb8, Vb7 and Vb2 in mice and Vb11 in humans). These receptors are responsible for the interaction of NKT cells with CD1d molecules expressed on CD4 1 CD8 1 double-positive (DP) thymocytes. 1 NKT cells are classified into type I (defined as invariant (iNKT) or classical NKT cells) and type II based on the antigens recognized by each and their TCR repertoires. 2 Our study focused on type I NKT cells (hereafter referred to as iNKT cells). iNKT cells recognize the glycosphingolipid antigen a-galactosylceramide (a-GalCer) and a-GalCer loading with the tetrameric form of CD1d is often used to define the iNKT cell subset because of the high affinity of CD1d for the iNKT cell TCR. 3 Although iNKT cells originate from CD4 1 CD8 1 (DP) thymocytes 4 and are selected by MHC class