Invariant TCR-triggered protein kinase D activation mediates NKT cell development

Development of invariant natural killer T (iNKT) cells in the thymus requires cell–cell interaction through invariant TCR (iTCR) and CD1d, which induces expression of the transcription factor, promyelocytic leukemia zinc finger (PLZF). However, the signaling pathway linking iTCR and PLZF remains unclear. Here, we report that a serine/threonine kinase, protein kinase D (PKD), plays a pivotal role in iNKT cell development. In T cell–specific PKD-deficient (Prkd2/3^∆CD4) mice, PLZF induction and iNKT cell generation were severely impaired, which were rescued by introduction of a PLZF transgene. We identified the transcription factor Ikaros as a substrate of PKD upon iTCR stimulation. Knock-in mice carrying a phosphorylation-defective mutant Ikaros (Ikzf1^S267/275A) exhibited an impairment of iNKT cell development, whereas conventional T cells were normal. In iNKT cells, Ikaros binds to the upstream region of the PLZF gene to induce its transcription. Mutant mice lacking the Ikaros-binding site (Zbtb16^∆IBS) generated fewer iNKT cells than WT mice. These results suggest that PKD links iTCRs to PLZF induction through Ikaros, thereby mediating iNKT cell development.