Background: Dendritic cells (DCs) are a complex group of cells that play a critical role in vertebrate immunity. Lymph-node resident DCs (LN-DCs) are subdivided into conventional DC (cDC) subsets (CD11b and CD8α in mouse; BDCA1 and BDCA3 in human) and plasmacytoid DCs (pDCs). It is currently unclear if these various DC populations belong to a unique hematopoietic lineage and if the subsets identified in the mouse and human systems are evolutionary homologs. To gain novel insights into these questions, we sought conserved genetic signatures for LN-DCs and in vitro derived granulocyte-macrophage colony stimulating factor (GM-CSF) DCs through the analysis of a compendium of genome-wide expression profiles of mouse or human leukocytes.

Understanding the mechanisms that help promote protective immune responses to pathogens is a major challenge in biomedical research and an important goal for the design of innovative therapeutic or vaccination strategies. While natural killer (NK) cells can directly contribute to the control of viral replication, whether, and how, they may help orchestrate global antiviral defense is largely unknown. To address this question, we took advantage of the well-defined molecular interactions involved in the recognition of mouse cytomegalovirus (MCMV) by NK cells. By using congenic or mutant mice and wild-type versus genetically engineered viruses, we examined the consequences on antiviral CD8 T cell responses of specific defects in the ability of the NK cells to control MCMV. This system allowed us to demonstrate, to our knowledge for the first time, that NK cells accelerate CD8 T cell responses against a viral infection in vivo. Moreover, we identify the underlying mechanism as the ability of NK cells to limit IFN-α/β production to levels not immunosuppressive to the host. This is achieved through the early control of cytomegalovirus, which dramatically reduces the activation of plasmacytoid dendritic cells (pDCs) for cytokine production, preserves the conventional dendritic cell (cDC) compartment, and accelerates antiviral CD8 T cell responses. Conversely, exogenous IFN-α administration in resistant animals ablates cDCs and delays CD8 T cell activation in the face of NK cell control of viral replication. Collectively, our data demonstrate that the ability of NK cells to respond very early to cytomegalovirus infection critically contributes to balance the intensity of other innate immune responses, which dampens early immunopathology and promotes optimal initiation of antiviral CD8 T cell responses. Thus, the extent to which NK cell responses benefit the host goes beyond their direct antiviral effects and extends to the prevention of innate cytokine shock and to the promotion of adaptive immunity.

As initially demonstrated with murine cytomegalovirus (MCMV), plasmacytoid dendritic cells (pDCs) are the major source of IFN-α/β in response to a variety of viruses in vivo. However, contradictory results have been obtained pertaining to the mechanisms promoting IFN-α/β production by pDCs in response to MCMV. In this study we show that TLR7 and TLR9 exert redundant functions for IFN-α/β, IL-12p40, and TNF-α production by pDCs in vivo during MCMV infection. In contrast, we confirm that systemic production of IL-12p70 strictly depends on TLR9. The combined loss of TLR7 and TLR9 recapitulates critical features of the phenotype of MyD88-deficient mice, including a dramatic decrease in systemic IFN-α/β levels, an increase in viral load, and increased susceptibility to MCMV-induced mortality. This is the first demonstration of the implication of TLR7 in the recognition of a DNA virus.

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Type I interferons (IFNs) are central to antiviral defense, but how they orchestrate immune cell function is incompletely understood. We determined that IFNs produced during murine cytomegalovirus (MCMV) infection differentially affect dendritic cells (DCs) and natural killer (NK) cells. IFNs induce cell-intrinsic responses in DCs, activating antiproliferative, antiviral, and lymphocyte-activating gene networks, consistent with high activity of the transcription factor STAT1 in these cells. By comparison, NK cells exhibit lower STAT1 expression and reduced IFN responsiveness. Rather, IFNs indirectly affect NK cells by inducing IL-15, which activates the transcription factor E2F and stimulates genes promoting cell expansion. IFN cell-intrinsic responses are necessary in DCs, but not NK cells, for MCMV resistance. Thus, sensitivity to IFN-induced cytokines and differences in IFN receptor signaling program immune cells to mount distinct responses that promote viral control.

Plasmacytoid dendritic cells (pDCs) are an important source of IFN-alpha/beta in response to a variety of viruses in vivo, including murine cytomegalovirus (MCMV). However, the respective contributions of various infected organs, and within these of pDCs, conventional dendritic cells and other cells, to the systemic production of IFN-alpha/beta or other innate cytokines during viral infections in vivo is largely unknown. Whether a specialization of pDC subsets in the production of different patterns of innate cytokines exists in vivo in response to a viral infection has not been investigated. Here, by analyzing for the first time directly ex vivo, at the single-cell level, the simultaneous production of up to three cytokines in pDCs isolated from MCMV-infected mice, we show that (i) pDCs are the quasi-exclusive source of IFN-alpha/beta, IL-12 and tumor necrosis factor (TNF)-alpha, early during MCMV infection, in two immunocompetent mouse lines and with two viral strains, (ii) pDC activation for IFN-alpha/beta production is organ specific and (iii) a significant proportion of pDCs simultaneously produce IFN-alpha/beta, TNF-alpha and IL-12, although TNF-alpha and IFN-alpha/beta appear more often co-expressed with one another than each of them with IL-12. Altogether, these results show a broad and non-redundant role of pDCs in early innate detection of, and defense against, viral infection. The data also show differences in the responsiveness of pDCs from different tissues and suggest distinct molecular requirements for pDC production of various cytokines. These observations must be taken into account when designing new antiviral vaccination strategies aimed at harnessing pDC responses.

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Plasmacytoid dendritic cells (pDC) are the major source of type I interferons (IFN‐I) during viral infections, in response to triggering of endosomal Toll‐like receptors (TLRs) 7 or 9 by viral single‐stranded RNA or unmethylated CpG DNA, respectively. Synthetic ligands have been used to disentangle the underlying signaling pathways. The adaptor protein AP3 is necessary to transport molecular complexes of TLRs, synthetic CpG DNA, and MyD88 into endosomal compartments allowing interferon regulatory factor 7 (IRF7) recruitment whose phosphorylation then initiates IFN‐I production. High basal expression of IRF7 by pDC and its further enhancement by positive IFN‐I feedback signaling appear to be necessary for robust cytokine production. In contrast, we show here that in vivo during mouse cytomegalovirus (MCMV) infection pDC produce high amounts of IFN‐I downstream of the TLR9‐to‐MyD88‐to‐IRF7 signaling pathway without requiring IFN‐I positive feedback, high IRF7 expression, or AP3‐driven endosomal routing of TLRs. Hence, the current model of the molecular requirements for professional IFN‐I production by pDC, established by using synthetic TLR ligands, does not strictly apply to a physiological viral infection.