Staphylococcus aureus-Induced Plasmacytoid Dendritic Cell Activation Is Based on an IgG-Mediated Memory Response 1 Marijo Parcina,* Constanze Wendt,* Friedrich Goetz, † Rainer Zawatzky, ‡ Ulrich Za ¨hringer, § Klaus Heeg,* and Isabelle Bekeredjian-Ding 2 * Type I IFNs represent a major antimicrobial defense mechanism due to their property of enhancing immune responses by priming both innate and adaptive immune cells. Plasmacytoid dendritic cells (pDC) are the major source of type I IFN in the human body and represent innate immune cells involved in first-line defense against invading pathogens. Although pDC activation has been extensively studied upon stimulation with synthetic TLR ligands, viruses, and intracellular bacteria, there is only scarce infor- mation on extracellular bacteria. In this study we show that the triggering of human pDC-derived IFN- secretion by Staphylo- coccus aureus is independent of TLR2 and specific for coagulase-positive staphylococci. Specificity of the pDC response to S. aureus is independent of the bacterial virulence factors protein A and -toxin but is mediated by Ag-specific IgG and CD32. S. aureus- induced pDC activation can be blocked by inhibitory DNA oligonucleotides and chloroquine, suggesting that engagement of TLR7/9 by bacterial nucleic acids after CD32-mediated uptake of these compounds may play a central role in this process. Altogether, we propose that in marked contrast to nonselective TLR2-dependent activation of most innate immune cells, pDC activation by S. aureus represents an Ag-specific memory response since it requires the presence of class-switched immunoglobulins. The Journal of Immunology, 2008, 181: 3823–3833. P lasmacytoid dendritic cells (pDC) 3 represent very potent producers of proinflammatory cytokines and are the major source of type I IFNs in the human body (1–3). They are thought to serve as a first-line defense in infection, since type I IFNs prime both innate and adaptive immune cells, preparing them for a quicker and more efficient antimicrobial response (4–6). Ad- ditionally, IFN-inducible genes are up-regulated, and antimicrobial effector molecules subsequently become available intracellularly or after secretion into the extracellular space. In both cases these mediators either target the invading intracellular or extracellular pathogen via direct neutralization or by indirectly stimulating ac- cessory immune cells that will in turn phagocytize and lyse the microbe. pDC activation occurs in response to microbial stimuli, for ex- ample, either by viral or bacterial infection of the cell or by re- ceptor-mediated recognition of molecules that contain motifs spe- cific for microbial pathogens. Among the pattern recognition receptors involved in this process, TLRs have most extensively been studied in pDC activation (7). The TLRs that are highly ex- pressed on human pDC recognize microbial nucleic acids by de- tecting unmethylated CpG motifs in DNA (TLR9) or binding mi- crobial RNA or synthetic guanosine analogs (TLR7) (8 –11). Additionally, TLR2 and its co-receptors TLR1, TLR6, CD36 and the putative co-receptor TLR10 are expressed, albeit at low levels, making the pDC potentially responsive to TLR2-active bacterial lipopeptides (LP) (2, 12–15). Although MyD88 (TLR)-indepen- dent pathways leading to pDC-derived type I IFN secretion have been described (11, 16, 17), in most cases viral infection has been demonstrated to activate pDC in a TLR7- or TLR9-dependent manner, being bound to the nature of the virus (18 –21). Thus, DNA viruses such as herpes simplex virus are thought to trigger a TLR9-dependent response, while ssRNA viruses such as influenza virus engage TLR7 (22–28). In marked contrast to viral infection, little is known about the role of pDC in bacterial infection (29, 30). There are a few studies available that state that intracellular bacteria such as chlamydia, salmonella, mycobacteria, and listeria can induce type I IFN pro- duction (31–36), while even fewer reports observe type I IFN se- cretion in response to extracellular bacteria including Staphylococ- cus aureus and Escherichia coli (37– 40). Although triggering innate immune cells other than pDC by primarily extracellularly located bacteria mainly occurs via cell wall-associated TLR2-ac- tive LP or endotoxin-mediated engagement of TLR4, there is little information on the molecular mechanisms involved in human pDC activation. In the present study we investigated the mechanisms involved in stimulation of human pDC by extracellularly located bacteria. We chose S. aureus since it has repeatedly been shown to induce type I IFN in human peripheral blood leukocytes (38 – 40). We provide evidence that, in contrast to other innate immune cells, the recog- nition of staphylococci by human pDC is independent of TLR2 *Department of Medical Microbiology and Hygiene, University Hospital Heidelberg, Heidelberg, Germany; † Microbial Genetics, University Tuebingen, Tuebingen, Ger- many; ‡ Deutsches Krebsforschungszentrum, Heidelberg, Germany; and § Division of Immunochemistry, Research Center Borstel, Borstel, Germany Received for publication January 2, 2008. Accepted for publication July 17, 2008. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 I.B.-D. is supported by a research fellowship from the European Society of Clinical Microbiology and Infectious Diseases (ESCMID) and the Olympia Morata program of the University of Heidelberg. This study is part of the doctoral thesis of M.P. The study was further supported by the Deutsche Forschungsgemeinschaft (DFG) Sonder- forschungsbereiche (SFB) 405 “Immune Tolerance and Its Disturbances” to K.H., DFG Priority Program “Innate Immunity” SPP 1110 to U.Z., and DFG SFB 766 “Bacterial Cell Envelope: Structure, Function and Infection Interface” to F.G. 2 Address correspondence and reprint requests to Dr. Isabelle Bekeredjian-Ding, De- partment of Medical Microbiology and Hygiene, University Hospital Heidelberg, Im Neuenheimer Feld 324, 1.OG, D-69120 Heidelberg, Germany. E-mail address: isabelle.bekeredjian-ding@med.uni-heidelberg.de 3 Abbreviations used in this paper: pDC, plasmacytoid dendritic cells; LP, lipopep- tides; NDV, Newcastle disease virus; ODN, oligodesoxynucleotide; SpA, staphylo- coccal protein A; WT, wild type. Copyright © 2008 by The American Association of Immunologists, Inc. 0022-1767/08/$2.00 The Journal of Immunology www.jimmunol.org