Requirement of HMGB1 and RAGE for the maturation of human plasmacytoid dendritic cells

Abstract

Dendritic cells (DC) are key components of innate and adaptive immune responses. Plasmacytoid DC (PDC) are a specialized DC subset that produce high amounts of type I interferons in response to microbes. High mobility group box 1 protein (HMGB1) is an abundant nuclear protein, which acts as a potent pro-inflammatory factor when released extracellularly. We show that HMGB1 leaves the nucleus of maturing PDC following TLR9 activation, and that PDC express on the plasma membrane the bestcharacterized receptor for HMGB1, RAGE. Maturation and type I IFN secretion of PDC is hindered when the HMGB1/RAGE pathway is disrupted. These results reveal HMGB1 and RAGE as the first known autocrine loop modulating the maturation of PDC, and suggest that antagonists of HMGB1/RAGE might have therapeutic potential for the treatment of systemic human diseases. IntroductionDendritic cells (DC) are potent antigen-presenting cells bridging the innate and adaptive immune responses. To date, two subsets of DC have been identified in humans: myeloid DC and plasmacytoid DC (PDC), the latter also referred to as type I interferon (IFN-a)-producing cells [1][2][3][4][5][6]. PDC recognize viral or bacterial DNA patterns, consisting of unmethylated CpG motifs in the context of species-dependent surrounding sequences (CpG) [7]. As a consequence they produce, within a few hours, large amounts of . Based on their ability to induce IFN-a secretion by PDC, two types of CpG have been described: CpG 2216 (CpG-A), which induces high amounts of IFN-a, and CpG 2006 (CpG-B), which promotes survival, maturation and migration of PDC to the lymph nodes, but induces lower amounts of . PDC sustain the priming of naive T cells and their differentiation into Th1/Th2 effectors [15][16][17] or into regulatory T cells [16,[18][19][20][21][22].Recent studies shed light on the events involved in the recognition of microbial DNA by human PDC, the only subset of human DC that expresses the Toll-like receptor 9 (TLR9) [12]. Both CpG-A and CpG-B have been shown to activate PDC via TLR9 [23]. Upon phagocytosis PDC lyse microorganisms in phagolysosomes, where microbial DNA interacts with and activates TLR9 derived from the endoplasmic reticulum [24]. TLR9 activation leads to recruitment of the MyD88 adaptor protein and phosphorylation of the IL-1R-associated kinase (IRAK). Phosphorylated IRAK associates with the adaptor molecule TNF-associated factor 6 (TRAF6) [25]. Two separate signaling pathways, JNK and NF-jB, are then activated, promoting PDC maturation and survival. IFN-a induction, an event exclusively occurring in PDC, depends on the formation of a complex consisting of MyD88, TRAF6 and the IRF7 transcription factor, as well as on TRAF6-dependent ubiquitination [26].The events that determine the outcome of the interaction of PDC with T cells, and in particular the induction of effector or regulatory immune responses, are poorly characterized. Environmental signals that Here we report that HMGB1 is exported from the nucleus of PDC upon selective engagement of TLR9 an...

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