Hiyaa S. Ghosh

Plasmacytoid Dendritic Cells: Recent Progress and Open Questions

Plasmacytoid dendritic cells (pDCs) are specialized in rapid and massive secretion of type I interferon (IFN-α/β) in response to foreign nucleic acids. Combined with their antigen presentation capacity, this powerful functionality enables pDCs to orchestrate innate and adaptive immune responses. pDCs combine features of both lymphocytes and classical dendritic cells and display unique molecular adaptations to nucleic acid sensing and IFN production. In the decade since the identification of the pDC as a distinct immune cell type, our understanding of its molecular underpinnings and role in immunity has progressed rapidly. Here we review select aspects of pDC biology including cell fate establishment and plasticity, specific molecular mechanisms of pDC function, and the role of pDCs in T cell responses, antiviral immunity, and autoimmune diseases. Important unresolved questions remain in these areas, promising exciting times in pDC research for years to come.

SIRT1 Negatively Regulates the Mammalian Target of Rapamycin

The IGF/mTOR pathway, which is modulated by nutrients, growth factors, energy status and cellular stress regulates aging in various organisms. SIRT1 is a NAD+ dependent deacetylase that is known to regulate caloric restriction mediated longevity in model organisms, and has also been linked to the insulin/IGF signaling pathway. Here we investigated the potential regulation of mTOR signaling by SIRT1 in response to nutrients and cellular stress. We demonstrate that SIRT1 deficiency results in elevated mTOR signaling, which is not abolished by stress conditions. The SIRT1 activator resveratrol reduces, whereas SIRT1 inhibitor nicotinamide enhances mTOR activity in a SIRT1 dependent manner. Furthermore, we demonstrate that SIRT1 interacts with TSC2, a component of the mTOR inhibitory-complex upstream to mTORC1, and regulates mTOR signaling in a TSC2 dependent manner. These results demonstrate that SIRT1 negatively regulates mTOR signaling potentially through the TSC1/2 complex.

Sirt1 interacts with transducin-like enhancer of split-1 to inhibit nuclear factor κB-mediated transcription

Sirt1 is an NAD+-dependent deacetylase that plays a role in cellular processes such as transcriptional regulation, stress response, longevity and apoptosis. Sirt1 deacetylates histone proteins and certain transcription factors such as p53, CTIP2 (chicken ovalbumin upstream promoter-transcription factor-interacting protein 2), FOXO (forkhead box O) and NF-kappaB (nuclear factor kappaB). To identify potential Sirt1-interacting factors, we performed a yeast two-hybrid screen. The screen identified TLE1 (transducin-like enhancer of split-1) as a possible Sirt1-interacting factor, which was then confirmed by co-immunoprecipitation. TLE1 is a non-DNA binding co-repressor for several transcriptional factors including NF-kappaB. We have demonstrated using co-transfection assays that Sirt1 and TLE1 repress NF-kappaB activity. The catalytic mutant of Sirt1, Sirt1-H363Y, and the N-terminal Sirt1 fragment (amino acids 1-270) also show similar repression activity, suggesting that the deacetylase activity of Sirt1 may not be critical for its effect on NF-kappaB activity. Furthermore, analysis in Sirt1-null MEFs (murine embryonic fibroblasts) and HeLa cells stably expressing siRNA (small interfering RNA) specific to Sirt1 or TLE1 demonstrate that both Sirt1 and TLE1 are required for negative regulation of NF-kappaB activity. Taken together, these results suggest that the interaction between Sirt1 and TLE1 is important for mediating repression of NF-kappaB activity.

SIRT1 associates with eIF2-alpha and regulates the cellular stress response

SIRT1 is a NAD+ dependent protein deacetylase known to increase longevity in model organisms. SIRT1 regulates cellular response to oxidative and/or genotoxic stress by regulating proteins such as p53 and FOXO. The eukaryotic initiation factor-2, eIF2, plays a critical role in the integrated stress response pathway. Under cellular stress, phosphorylation of the alpha subunit of eIF2 is essential for immediate shut-off of translation and activation of stress response genes. Here we demonstrate that SIRT1 interacts with eIF2α. Loss of SIRT1 results in increased phosphorylation of eIF2α. However, the downstream stress induced signaling pathway is compromised in SIRT1-deficient cells, indicated by delayed expression of the downstream target genes CHOP and GADD34 and a slower post-stress translation recovery. Finally, SIRT1 co-immunoprecipitates with mediators of eIF2α dephosphorylation, GADD34 and CreP, suggesting a role for SIRT1 in the negative feedback regulation of eIF2α phosphorylation.

Transcription factor Runx2 controls the development and migration of plasmacytoid dendritic cells

Exit of mature pDCs from the bone marrow requires the transcription factor Runx2, in part via Runx2-driven expression of CCR5.

ETO family protein Mtg16 regulates the balance of dendritic cell subsets by repressing Id2

Transcriptional cofactor of the ETO family Mtg16 promotes pDCs and restricts cDC differentiation in part by repressing Id2.

Protein Tyrosine Phosphatase PTPRS Is an Inhibitory Receptor on Human and Murine Plasmacytoid Dendritic Cells

Plasmacytoid dendritic cells (pDCs) are primary producers of type I interferon (IFN) in response to viruses. The IFN-producing capacity of pDCs is regulated by specific inhibitory receptors, yet none of the known receptors are conserved in evolution. We report that within the human immune system, receptor protein tyrosine phosphatase sigma (PTPRS) is expressed specifically on pDCs. Surface PTPRS was rapidly downregulated after pDC activation, and only PTPRS(-) pDCs produced IFN-α. Antibody-mediated PTPRS crosslinking inhibited pDC activation, whereas PTPRS knockdown enhanced IFN response in a pDC cell line. Similarly, murine Ptprs and the homologous receptor phosphatase Ptprf were specifically co-expressed in murine pDCs. Haplodeficiency or DC-specific deletion of Ptprs on Ptprf-deficient background were associated with enhanced IFN response of pDCs, leukocyte infiltration in the intestine and mild colitis. Thus, PTPRS represents an evolutionarily conserved pDC-specific inhibitory receptor, and is required to prevent spontaneous IFN production and immune-mediated intestinal inflammation.