Polymerase III transcription is necessary for T cell priming by dendritic cells

Marisa Reverendo (1) , Rafael J Argüello , Christine Polte , Jan Valečka , Voahirana Camosseto , Nathalie Auphan-Anezin , Zoya Ignatova , Evelina Gatti , Philippe Pierre

1 CIML - Centre d'Immunologie de Marseille - Luminy

Exposure to microbe-associated molecular patterns (MAMPs) causes dendritic cells (DCs) to undergo a remarkable activation process characterized by changes in key biochemical mechanisms. These enhance antigen processing and presentation, as well as strengthen DC capacity to stimulate naïve T cell proliferation. Here, we show that in response to the MAMPS lipopolysaccharide and polyriboinosinic:polyribocytidylic acid (Poly I:C), RNA polymer-ase III (Pol lII)-dependent transcription and consequently tRNA gene expression are strongly induced in DCs. This is in part caused by the phosphorylation and nuclear export of MAF1 homolog negative regulator of Poll III (MAF1), via a synergistic casein kinase 2 (CK2)-and mammalian target of rapamycin-dependent signaling cascade downstream of Toll-like receptors (TLRs). De novo tRNA expression is necessary to augment protein synthesis and compensate for tRNA degradation driven by TLR-dependent DC exposure to type-I IFN. Although protein synthesis is not strongly inhibited in absence of RNA Pol III activity, it compromises the translation of key DC mRNAs, like those coding for costimulatory molecules and proinflammatory cytokines, which instead can be stored in stress granules, as shown for CD86 mRNA. TLR-dependent CK2 stimulation and subsequent RNA Pol III activation are therefore key for the acquisition by DCs of their unique T cell immune-stimulatory functions. casein kinase 2 | interferon | protein synthesis | CD86 | immunity D endritic cells (DCs) are key regulators of both protective immune responses and tolerance to self-antigens (1). DCs are professional antigen presenting cells (APCs), equipped with pattern recognition receptors, like Toll-like receptors (TLRs), capable of recognizing and responding to microbe-associated molecular patterns (MAMPs) (2). For example, lipopolysaccha-ride (LPS) detection by TLR4 promotes DC activation by triggering a series of signaling cascades resulting in massive changes in gene expression, membrane traffic, and metabolism. This matu-ration process ultimately culminates in the priming of naïve T cell recognizing antigenic peptides presented by major histocompati-bility complexes (MHCs) and costimulatory molecules at the surface of activated DCs (3, 4). LPS-stimulated DCs undergo a phase of rapid up-regulation of protein synthesis mediated in part through the mammalian target of rapamycin (mTORC1) signal transduction pathway. This up-regulation is necessary for cytokine production and rapid increase in surface MHC class II and costimulatory molecules, like B7.1/CD86 (5). Importantly the secretion of type-I IFN (IFN) contributes majorly to the speed and intensity of DC maturation in an autocrine manner (6). The role of RNA polymerase III (Pol III) activity and tRNA gene expression during DC activation has remained unexplored. Pol III is responsible for the transcription of some 300 different genes (class III genes), that are mostly tRNAs (7). In-depth analysis of Pol III activity has revealed a cascade of coordinated interactions of transcription factors to recruit Pol III and allow the transcription of tRNA genes. TFIIIC binding to intragenic conserved promoters is followed by assembly of initiation factor TFIIIB subunits (Brf1, Bdp1, and TBP) and binding to upstream sequences, that lead to the subsequent recruitment of the Pol III subunits (8-10). Pol III is normally controlled by the general negative regulator MAF1 (11), which binds to the polymerase and impairs its recruitment to the promoter DNA/TFIIB complex, and thus prevents transcription initiation. We show here that TLR agonists drive up global tRNA transcription during the first hours of DC activation. Following LPS stimulation, enhanced Pol III transcription is achieved through the concerted actions of casein kinase 2 (CK2) and mTORC1 on the MAF1 repressor. TLR-dependent nuclear translocation of both CK2 and mTORC1 causes MAF1 phos-phorylation and cytosolic accumulation over time, consequently allowing for Pol III transcriptional activation. This cascade of signaling events enhancing tRNAs expression is necessary to increase protein synthesis and harness DCs with the full immune-stimulatory potential required for priming naïve T cells. We also found that DC exposure to type-I IFN accelerates tRNA turnover. Enhanced Pol III transcription upon TLR triggering is therefore necessary to compensate for IFN-dependent tRNA decay, which would, in absence of this mechanism, drive the formation of stress granules (SGs) and prevent the translation of key mRNAs, such as CD86, in activated DCs. These SGs differ from the SGs formed by environmental stress, since they are independent of full protein synthesis inhibition and increased eukaryotic translation initiation factor 2 alpha subunit (eIF2α) phosphorylation, 2 hallmarks of SGs formed in response to oxidative stress (12). The CK2/MAF1/ Pol III axis represents therefore a previously unidentified signaling Significance RNA polymerase III-dependent transcription and increased tRNA expression are necessary for MAMP-stimulated DCs to stimulate naïve T cells. Augmented Pol III-dependent transcription is as essential as the switch to glycolysis and other energetic metabolism variations that are now considered as hallmarks of immune cell activation and are all necessary to increase protein synthesis in these cells.

Domaines: Sciences du Vivant [q-bio]; Immunologie

Marisa Reverendo, Rafael J Argüello, Christine Polte, Jan Valečka, Voahirana Camosseto, et al.. Polymerase III transcription is necessary for T cell priming by dendritic cells. Proceedings of the National Academy of Sciences of the United States of America, 2019, ⟨10.1073/pnas.1904396116⟩. ⟨hal-02363598⟩