No abstract

Naive CD4 lymphocytes undergo a polarization process in the periphery to become induced Th17 (iTh17) cells. Using retinoic acid–related orphan receptor γt (RORγt)-gfp mice, we found that RORγt and the transcription factor promyelocytic leukemia zinc finger (PLZF) are valuable new markers to identify the recently described natural Th17 (nTh17) cell population. nTh17 cells are thymically committed to promptly produce large amounts of IL-17 and IL-22. In this study, we show that, in addition to responding to TCR cross-linking, nTh17 cells secrete IL-17 and IL-22 when stimulated with IL-23 plus IL-1β, either in recombinant form or in supernatants from TLR4-activated dendritic cells. This innate-like ability of RORγt+ nTh17 cells to respond to TLR4-induced cytokines was not shared by iTh17 cells. The other distinct properties of RORγt+ nTh17 cells are their high expression of PLZF and their absence from lamina propria; iTh17 cells are found therein. RORγt+ nTh17 cells are present in the thymus of germ-free RORγt-gfp and IL-6−/− RORγt-gfp mice, indicating that these cells do not require symbiotic microbiota or IL-6 for their generation. Finally, we found that PLZF+RORγt+ nTh17 cells represent one of the primary IL-17–producing innate-like T cell populations in a TLR7 imiquimod model of psoriasis-like disorder, indicating their involvement in this kind of lesion. Collectively, our results reveal RORγt and PLZF as characteristic markers for identifying nTh17 cells and demonstrate one of their novel properties: the ability to respond promptly to TLR-dependent proinflammatory stimuli without TCR engagement, placing them as members of the innate-like T cell family.

Invariant NKT (iNKT) cells differentiate in the thymus into three distinct lineages defined by their cytokine and transcription factor expression. Signaling lymphocyte activation molecule (SLAM)-associated protein (SAP) is essential for early stages of iNKT cell development, but its role during terminal differentiation of iNKT1, iNKT2, or iNKT17 cells remains unclear. Taking advantage of SAP-deficient mice expressing a Vα14-Jα18 TCRα transgene, we found that SAP is critical not only for IL-4 production but also for the terminal differentiation of IL-4-producing iNKT2 cells. Furthermore, without SAP, the IL-17 producing subset is expanded, while IFN-γ-producing iNKT1 differentiation is only moderately compromised. Lack of SAP reduced the expression of the transcription factors GATA-3 and promyelocytic leukemia zinc finger, but enhanced the levels of retinoic acid receptor-related orphan receptor γt. In the absence of SAP, lineage commitment was actually shifted toward the emergence of iNKT17 over iNKT2 cells. Collectively, our data unveil a new critical regulatory function for SAP in thymic iNKT cell fate decisions.