Plasmacytoid dendritic cells efficiently cross-prime naive T cells in vivo after TLR activation

Abstract

Cross-presentation is a crucial mechanism in tumoral and microbial immunity because it allows internalized cell associated or exogenous antigens (Ags) to be delivered into the major histocompatibility complex I pathway. This pathway is important for the development of CD8 ؉ T-cell responses and for the induction of tolerance. In mice, cross-presentation is considered to be a unique property of CD8␣ ؉ conventional dendritic cells (DCs). Here we show that splenic plasmacytoid IntroductionCross-presentation is a mechanism that involves the uptake and processing of exogenous antigens (Ags) within the major histocompatibility complex (MHC) class I pathway. 1 This process differs from the classical MHC class I processing pathway in which MHC class I molecules present Ags that are synthesized within the cells. Cross-presentation occurs subsequent to transport of Ag from the endosome to cytosol and digestion by the proteasome and may concern different forms of exogenous Ags, such as soluble, 2 particulate, 3 or cell-associated Ags. 4 However, alternative mechanisms have been described, and the cellular and molecular pathways that lead to the presentation of endocytosed proteins on MHC class I molecules are not yet fully defined. 5 Cross-presentation is a crucial mechanism allowing induction of naive CD8 ϩ T-cell responses against exogenous Ag and has been implicated in the elicitation of protective cytotoxic T lymphocyte (CTL) responses against tumors as well as in the generation of immune responses against clinically relevant pathogens that do not infect tissues of hemopoietic origin. 6,7 In addition to cross-priming of naive CD8 ϩ T cells, cross-presentation appears to be central to the maintenance of peripheral tolerance to self-Ag, called "cross-tolerance." 8,9 With very few exceptions, 10,11 cross-presentation is restricted to dendritic cells (DCs) 2,12 and macrophages 13,14 but is not normally seen in other nucleated cells. DCs are considered the only Ag-presenting cell (APC) able to prime naive T cells and are thus involved in the induction of both adaptive immunity and tolerance. The outcome of T-cell responses depends on DC maturation state, as immature or semimature DCs have been described to induce tolerance. 15 Immature DCs are present in the periphery where they sample Ags. They are characterized by high expression of receptors involved in endocytosis and phagocytosis. 16 Maturation of DCs can be induced by pathogens or by inflammatory signals. Pathogens are recognized through pathogen-associated molecular patterns, which interact with pattern recognition receptors, including Toll-like receptors (TLRs), constitutively expressed by DCs. 17,18 Maturation involves the migration of DCs to lymphoid organs, the activation of proteolytic activity for Ag processing, and the up-regulation of MHC and costimulatory molecules. After maturation, DCs prime naive T cells, leading to their differentiation into effector T cells. 19 Several populations of DCs have been described, and it is unclear whether all these DC types...

Type I Interferons Mediate the Innate Cytokine Response to Recombinant Fowlpox Virus but Not the Induction of Plasmacytoid Dendritic Cell-Dependent Adaptive Immunity

“…This is profound, considering the important role type I IFNs play in regulating adaptive immunity to a number of other viral infections and, of particular note here, the dsDNA viruses VV and adenovirus (AdV5), which lack the ability to elicit an adaptive immune response in the absence of type I IFNs (61,62). We therefore propose that the main role of pDCs here is not to provide type I IFNs but to act as antigen-presenting cells (APCs) (13), a property recently shown to involve ligation of TLR7 and TLR9 (35), which are, coincidently, the two same TLRs that we have found to be responsible for innate recognition of FPV (E. L. Lousberg, submitted for publication). In antigen presentation experiments which involved the proliferation of OT-I T cells in vitro and in vivo, we demonstrated that pDCs had an important role in antigen presentation and were in fact responsible for presenting antigen to a significantly greater extent than cDCs when DCs were harvested from a previously immunized mouse and coincubated with OT-I T cells.…”

“…We have previously shown that pDCs are essential for the induction of a cytolytic response directed against an FPV-expressed antigen (11); however, the results from our current study suggest that this effect did not lie in the capacity of pDCs to secrete large amounts of type I IFNs, as originally postulated, as here we clearly show that they appear to make no contribution to the induction of adaptive immune responses to recombinant FPV. Recent reports have indicated that pDCs can also present antigens and therefore contribute directly to the development of adaptive immune responses (35,55). With this in mind, we sought to determine whether the crucial role of pDCs in the formation of anti-FPV adaptive immunity may lie in their antigen presentation functions.…”

CpG Promotes Cross-Presentation of Dead Cell-Associated Antigens by Pre-CD8α+ Dendritic Cells

“…DQ-OVA is a self-quenched conjugate of OVA that exhibits green fluorescence after proteolysis and red fluorescence when the fragments accumulate in organelles at high concentrations (31,32). After 20 min incubation, the percentage of DC having degraded DQ-OVA (green fluorescence) was lower when DC were pretreated with CpG compared with untreated DC, although both populations had captured DQ-OVA as revealed by pulse-chase experiments (Fig.…”

“…DQ-OVA (Invitrogen) is a self-quenched dye conjugate of OVA that exhibits green fluorescence upon degradation and red fluorescence when it accumulates in organelles at high concentration (31,32). Flt3l DC were incubated for the indicated times with DQ-OVA (10 mg/ml) at 37˚C or at 4˚C in the presence of 0.1% azide and were stained with anti-CD11c Abs and analyzed by flow cytometry (Canto flow cytometer; BD Biosciences).…”

Conventional but Not Plasmacytoid Dendritic Cells Foster the Systemic Virus–Induced Type I IFN Response Needed for Efficient CD8 T Cell Priming

“…However, these two DC functions are not so strictly compartmentalized. Indeed, although less efficient than cDCs, TLR-or virus-licensed pDCs are able to cross-present particulate Ags and to cross-prime naive CD8 T cells (11). Moreover, there is growing evidence that pDCs are dispensable for IFN-I responses to certain viruses, suggesting that other cells must be involved in the production of IFNs-I.…”