Plasmacytoid pre-dendritic cells (pDCs) are specialized in responding to nucleic acids, and link innate with adaptive immunity. Although the response of pDCs to viruses is well established, whether pDCs can respond to extracellular bacteria remains controversial. Here, we demonstrate that extracellular bacteria such as Neisseria meningitidis, Haemophilus influenzae, and Staphylococcus aureus activate pDCs to produce IFN-α, TNF-α, IL-6, and to upregulate CD86 expression. We observed that pDCs were present within tonsillar crypts and oro-nasopharyngeal epithelium, where they may contact extracellular bacteria, in situ. Tonsil epithelium-conditioned supernatants inhibited IFN-α, TNF-α, and IL-6 triggered by the direct contact of N. meningitidis or S. aureus with pDCs. However, pDC priming of naive T cells was not affected, suggesting that tonsil epithelium microenvironment limits local inflammation while preserving adaptive immunity in response to extracellular bacteria. Our results reveal an important and novel function of pDCs in the initiation of the mucosal innate and adaptive immunity to extracellular bacteria. Immunol. 2013Immunol. . 43: 1264Immunol. -1273 Immunity to infection 1265 local inflammation to life-threatening septicemia and meningitis. The major location of these bacteria is the human mucosa [2][3][4]. The presence of N. meningitidis at the oro-nasopharynx mucosa was associated with the induction of a naturally acquired immunity able to activate T and B cells [5,6] but the underlying mechanisms by which this response is initiated remain largely unknown. Plasmacytoid pre-dendritic cells (pDCs) play a major role in innate antimicrobial immunity [7,8]. They recognize principally nucleic acids from microbes through endosomal and cytosolic receptors [9,10]. Upon stimulation, pDCs produce large amounts of type-I interferons, IL-6, TNF-α, and upregulate surface costimulatory molecules [7,11]. Although pDCs as well as type-I interferons have an important function in antiviral immunity, their role in the response to extracellular bacterial infection remains highly controversial [9,12]. Here, in an effort to reconstruct the mucosal microenvironment present in the initial steps of antibacterial immunity, we used human primary cells and address the following questions: (i) Can pDCs respond to extracellular bacteria in a mucosal context? (ii) Which is the role of the epithelium in this response and the subsequent T-cell priming?
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Extracellular bacteria activate pDCs to produce IFN-α, TNF-α, and IL-6 and increase CD86 expressionWe evaluated the direct effect of whole live extracellular bacteria on pDC. Freshly purified pDCs from human blood were cultured for 24 h, and supernatants were collected to quantify cytokine secretion. As expected, pDCs stimulated with CpG secreted high levels of IFN-α, TNF-α, and IL-6 secretion (Fig. 1A). When pDCs were stimulated with the Gram-negative extracellular bacteria N. meningitidis we observed a significant induction of all three cytokines: mean of 10,000 p...
