Surface molecules on stimulated plasmacytoid dendritic cells are sufficient to cross-activate resting myeloid dendritic cells
Introduction
Dendritic cells (DCs) play a key role in the induction of the immune response because of their double function as antigen-presenting cells (APCs) and pathogen sensors [1], [2], [3], [4]. As APCs, DCs capture antigens and present them to T cells, inducing T-cell activation and the subsequent adaptive immune response [1], [2], [3], [4]. As pathogen sensors, DCs recognize foreign microorganisms, undergo activation, and, as a consequence, produce soluble factors that drive the innate immune response against a specific class of pathogens, such as intracellular viruses or extracellular bacteria [1], [2], [3], [4]. DCs recognize pathogens through specialized receptors termed pathogen recognition receptors that bind molecular patterns specific to the foreign microorganisms but absent in the host [1], [2], [3], [4]. Among pathogen recognition receptors, Toll-like receptors (TLRs) comprise a well-studied category expressed on the cell surface or in the endosome compartment [5], [6], [7]. The endosomal TLRs, such as TLR3, TLR7, TLR8, or TLR9, recognize viral or bacterial nucleic acids, whereas cell surface TLRs, such as TLR2, TLR4, and TLR5, recognize bacterial surface–exposed structures like protein of the flagella (TLR5), lipopolysaccharide (LPS; TLR4), or lipopeptides (TLR2) [5], [6], [7].
In human blood, 2 populations of DCs, namely plasmacytoid (pDCs) and myeloid (mDCs) DCs [1], [3], [8], are observed. Both DC populations, when appropriately stimulated, function as APCs through the processing and presentation of engulfed antigens and the upregulation of costimulatory molecules such as CD40, CD80, or CD86 for T-cell activation [1], [3], [8]. Nevertheless, these 2 DC populations have complementary phenotypes and function in terms of their response to triggers. In fact, pDCs express TLR9, but not TLR4, respond to the TLR9 agonist CpG but not to the TLR4 agonist LPS, and produce massive amounts of type 1 interferons, tumor necrosis factor (TNF)-α, low amounts of interleukin (IL)-6 and IL-8, and no IL-1β [8]. By contrast, mDCs express TLR4 but not TLR9 and respond to LPS but not to CpG by the production of IL-1β, TNF-α, and high amounts of IL-6 and IL-8 [8]. In addition, whereas mDCs (like almost all cell types) produce type 1 interferons slowly in the presence of a viral productive infection, using an autocrine or paracrine amplification loop, pDCs produce these cytokines quickly and massively, also in the absence of a productive infection [9]. This observation is the basis of the idea that pDCs serve as viral infection sensors [9]. Our recent publication reinforced this idea by demonstrating that pDCs, in contrast to mDCs, are unable to respond to bacteria and lack phagocytic activity [8]. We also demonstrated that, similar to that in mice [10], human mDCs and pDCs cooperate in inducing immune responses. In particular, we observed a reciprocal cross-maturation when both DC types are cultured together and selectively stimulated using CpG (pDCs) or LPS (mDCs). Cross-activated DCs upregulate costimulatory molecules, but do not display cytokine secretion ability, a phenomenon that we termed “sterile antigen presentation” [8]. Therefore, this mDC–pDC crosstalk is critical for the maturation of the nondirectly stimulated DCs and thus potentially enhances induction of the T-cell response, but has no impact on the range of cytokines secreted, which would influence the quality of the immune response induced. The human mDC–pDC crosstalk is contact dependent, even if cytokines amplify the phenomenon. In particular, by neutralizing the activity of TNF-α or interferon (IFN)-α, we demonstrated that TNF-α has a stronger effect on pDC cross-maturation induced by mDCs than IFN-α has on the mDC cross-maturation by pDCs [8].
Section snippets
Donors
Buffy coats from healthy human immunodeficiency virus (HIV)–, hepatitis B virus–, and hepatitis C virus–negative donors were obtained from the Blood Transfusion Section, Alta Val D'Elsa Hospital (Poggibonsi, Italy). Informed consent was obtained before all blood donations. The study protocol was approved by the Novartis Research Center ethical committee and conforms to the ethical guidelines of the 1975 Declaration of Helsinki.
Cell purification
Peripheral blood mononuclear cells (PBMCs) were obtained from buffy
Results and discussion
Although IFN-α is described to modulate the immune response and to induce DC maturation [8], [12], we observed, in our in vitro system, that the cross-maturation of mDCs is mainly determined by contact with pDCs [8]. In Fig. 1A, we illustrate that mDCs cultured overnight with pDCs in the presence of CpG upregulate CD40, whereas the same cells cultured alone are not responsive to CpG, as expected. The neutralization of IFN-α partially reduced mDC cross-maturation, whereas the physical separation
Acknowledgments
This work was supported in part by a grant from the European Commission Sixth Framework Program (Contract LSHB-CT-2004−512074 DC-THERA Network of Exellence).
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