Threonyl-tRNA synthetase (TRS) is an aminoacyl-tRNA synthetase that catalyzes the aminoacylation of tRNA by transferring threonine. In addition to an essential role in translation, TRS was extracellularly detected in autoimmune diseases and also exhibited pro-angiogenetic activity. TRS is reported to be secreted into the extracellular space when vascular endothelial cells encounter tumor necrosis factor-a. As T helper (Th) type 1 response and IFN-g levels are associated with autoimmunity and angiogenesis, in this study, we investigated the effects of TRS on dendritic cell (DC) activation and CD4 T cell polarization. TRS-treated DCs exhibited up-regulated expression of activation-related cell-surface molecules, including CD40, CD80, CD86, and MHC class II. Treatment of DCs with TRS resulted in a significant increase of IL-12 production. TRS triggered nuclear translocation of the NF-kB p65 subunit along with the degradation of IkB proteins and the phosphorylation of MAPKs in DCs. Additionally, MAPK inhibitors markedly recovered the degradation of IkB proteins and the increased IL-12 production in TRS-treated DCs, suggesting the involvement of MAPKs as the upstream regulators of NF-kB in TRSinduced DC maturation and activation. Importantly, TRS-stimulated DCs significantly increased the populations of IFN-g + CD4 T cells, and the levels of IFN-g when co-cultured with CD4 + T cells. The addition of a neutralizing anti-IL-12 mAb to the cell cultures of TRStreated DCs and CD4 + T cells resulted in decreased IFN-g production, indicating that TRSstimulated DCs may enhance the Th1 response through DC-derived IL-12. Injection of OT-II mice with OVA-pulsed, TRS-treated DCs also enhanced Ag-specific Th1 responses in vivo. Importantly, injection with TRS-treated DC exhibited increased populations of IFNg +-CD4 + and-CD8 + T cells as well as secretion level of IFN-g, resulting in viral clearance

In addition to essential roles in protein synthesis, lysyl-tRNA synthetase (KRS) is secreted to trigger a proinflammatory function that induces macrophage activation and TNF-a secretion. KRS has been associated with autoimmune diseases such as polymyositis and dermatomyositis. In this study, we investigated the immunomodulatory effects of KRS on bone marrow-derived dendritic cells (DCs) of C57BL/6 mice and subsequent polarization of Th cells and analyzed the underlying mechanisms. KRS-treated DCs increased the expression of cell surface molecules and proinflammatory cytokines associated with DC maturation and activation. Especially, KRS treatment significantly increased production of IL-12, a Th1-polarizing cytokine, in DCs. KRS triggered the nuclear translocation of the NF-kB p65 subunit along with the degradation of IkB proteins and the phosphorylation of MAPKs in DCs. Additionally, JNK, p38, and ERK inhibitors markedly recovered the degradation of IkB proteins, suggesting the involvement of MAPKs as the upstream regulators of NF-kB in the KRS-induced DC maturation and activation. Importantly, KRS-treated DCs strongly increased the differentiation of Th1 cells when cocultured with CD4 + T cells. The addition of anti-IL-12-neutralizing Ab abolished the secretion of IFN-g in the coculture, indicating that KRS induces Th1 cell response via DC-derived IL-12. Moreover, KRS enhanced the OVA-specific Th1 cell polarization in vivo following the adoptive transfer of OVA-pulsed DCs. Taken together, these results indicated that KRS effectively induced the maturation and activation of DCs through MAPKs/NF-kB-signaling pathways and favored DC-mediated Th1 cell response.

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Mst1 is a multifunctional serine/threonine kinase that is highly expressed in several immune organs. The role of Mst1 in the activation of dendritic cells (DCs), a key player of adaptive immunity, is poorly understood. In this study, we investigated the role of Mst1 in GM-CSF-induced bone marrow-derived DCs and the underlying mechanisms. Mst1−/− DCs in response to GM-CSF expressed higher levels of activation/maturation-related cell surface molecules, such as B7 and MHC class II than Mst1+/+ DCs. Furthermore, the expression of proinflammatory cytokines, such as IL-23, TNF-α, and IL-12p40, was increased in Mst1−/− DCs, indicating that Mst1-deficiency may induce the hyperactivation of DCs. Additionally, Mst1−/− DCs exhibited a stronger capacity to activate allogeneic T cells than Mst1+/+ DCs. Silencing of Mst1 in DCs promoted their hyperactivation, similar to the phenotypes of Mst1−/− DCs. Mst1−/− DCs exhibited an increase in Akt1 phosphorylation and c-myc protein levels. In addition, treatment with an Akt1 inhibitor downregulated the protein level of c-myc increased in Mst1-deficient DCs, indicating that Akt1 acts as an upstream inducer of the de novo synthesis of c-myc. Finally, Akt1 and c-myc inhibitors downregulated the increased expression of IL-23p19 observed in Mst1-knockdown DCs. Taken together, these data demonstrate that Mst1 negatively regulates the hyperactivation of DCs through downregulation of the Akt1/c-myc axis in response to GM-CSF, and suggest that Mst1 is one of the endogenous factors that determine the activation status of GM-CSF-stimulated inflammatory DCs.

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