Mouse CD8alpha+ DCs and human BDCA3+ DCs are major producers of IFN-lambda in response to poly IC

Novel antiviral therapeutics to control foot-and-mouth disease

Foot-and-mouth disease virus (FMDV) causes a highly contagious disease of cloven-hoofed animals. Vaccines require ∼7 days to induce protection; thus, before this time, vaccinated animals are still susceptible to the disease. Our group has previously shown that swine inoculated with 1×10(11) focus forming units (FFU) of a replication-defective human adenovirus containing the gene for porcine interferon alpha (Adt-pIFN-α) are sterilely protected from FMDV serotypes A24, O1 Manisa, or Asia 1 when the animals are challenged 1 day postadministration, and protection can last for 3-5 days. Polyriboinosinic-polyribocytidylic acid stabilized with poly-l-lysine and carboxymethyl cellulose (poly ICLC) is a synthetic double-stranded RNA that is a viral mimic and activates multiple innate immune pathways through interaction with toll-like receptor 3 and MDA-5. It is a potent inducer of IFNs. In this study, we initially examined the effect of poly IC and IFN-α on FMDV replication and gene induction in cell culture. Poly ICLC alone or combined with Adt-pIFN-α was then evaluated for its therapeutic efficacy in swine against intradermal challenge with FMDV A24, 1 day post-treatment. Groups of swine were subcutaneously inoculated either with poly ICLC alone (4 or 8 mg) or in combination with different doses of Adt-pIFN-α (2.5×10(9), 1×10(9), or 2.5×10(8) FFU). While different degrees of protection were achieved in all the treated animals, a dose of 8 mg of poly ICLC alone or combined with 1×10(9) FFU of Adt-pIFN-α was sufficient to sterilely protect swine when challenged 24 h later with FMDV A24. IFN-stimulated gene (ISG) expression in peripheral blood mononuclear cells at 1 day post-treatment was broader and higher in protected animals than in nonprotected animals. These data indicate that poly ICLC is a potent stimulator of IFN and ISGs in swine and at an adequate dose is sufficient to induce complete protection against FMD.

IFN-λ and IgE-mediated allergic disease: a potential future role?

Reduced early microbial exposure has become a leading candidate to explain the rise in allergic disease, and research has focused on studying the interaction between the developing immune system and the microbial environment. However, despite intense interest, the pathways that lead to dysregulation of the immune system in allergic disease are still poorly understood. The newly described type III IFN-λ molecules were initially shown to exhibit antiviral activity, but these molecules are also likely to have an important role to play in the immune-epithelial interface, given their immunomodulatory functions and restricted receptor expression to immune and epithelial cells. Previous studies on the role of IFN-λ in allergic disease have been limited to allergic asthma. More recently, a genetic variation flanking IL28B encoding IFN-λ3 has been associated with allergic disease. Here, we examine this family and suggest how IFN-λ may be an important player in allergic disease.

TLR13 recognizes bacterial 23S rRNA devoid of erythromycin resistance-forming modification

Host protection from infection relies on the recognition of pathogens by innate pattern-recognition receptors such as Toll-like receptors (TLRs). Here, we show that the orphan receptor TLR13 in mice recognizes a conserved 23S ribosomal RNA (rRNA) sequence that is the binding site of macrolide, lincosamide, and streptogramin group (MLS) antibiotics (including erythromycin) in bacteria. Notably, 23S rRNA from clinical isolates of erythromycin-resistant Staphylococcus aureus and synthetic oligoribonucleotides carrying methylated adenosine or a guanosine mimicking a MLS resistance-causing modification failed to stimulate TLR13. Thus, our results reveal both a natural TLR13 ligand and specific mechanisms of antibiotic resistance as potent bacterial immune evasion strategy, avoiding recognition via TLR13.

Deciphering the transcriptional network of the dendritic cell lineage

Although much progress has been made in the understanding of the ontogeny and function of dendritic cells (DCs), the transcriptional regulation of the lineage commitment and functional specialization of DCs in vivo remains poorly understood. We made a comprehensive comparative analysis of CD8(+), CD103(+), CD11b(+) and plasmacytoid DC subsets, as well as macrophage DC precursors and common DC precursors, across the entire immune system. Here we characterized candidate transcriptional activators involved in the commitment of myeloid progenitor cells to the DC lineage and predicted regulators of DC functional diversity in tissues. We identified a molecular signature that distinguished tissue DCs from macrophages. We also identified a transcriptional program expressed specifically during the steady-state migration of tissue DCs to the draining lymph nodes that may control tolerance to self tissue antigens.

IFN-λ inhibits HIV-1 integration and post-transcriptional events in vitro, but there is only limited in vivo repression of viral production

The lambda interferons (IL-28a, 28b, and IL-29) inhibit the replication of many viruses, but their role in the inhibition of HIV-1 infection remains unclear. During this study, we monitored IL-29 production in HIV-1 infected individuals and analyzed the in vitro and in vivo inhibition of HIV-1 production. Prior treatment with IL-28a or IL-29 induced an antiviral state in cultured primary T-cells, which suppressed HIV-1 integration and post-transcriptional events. The antiviral factors MxA, OAS, and PKR were up-regulated. In HIV-1 infected patients, IL-29 level was increased along with the depletion of CD4⁺ T-cells in peripheral blood, while the elevated IL-29 did not show a significantly negative correlation with viral load. Further analysis of HIV-1 infected individuals showed that IL-29 was positively correlated with IFN-β and anti-inflammatory cytokine IL-10, and was negatively correlated with IFN-γ, which might suggest that IFN-λ participates in modulating antiviral immune responses during HIV-1 infection in vivo. Together, although IFN-λ impeded HIV-1 infection of T-cells in vitro, IFN-λ showed only limited in vivo repression of viral production. The modulation of IFN-λ on inflammatory factors might be worthy for further concentrating on for better understanding the host immune response during HIV-1 infection.

Dendritic cell and macrophage heterogeneity in vivo

Macrophage and dendritic cell (DC) are hematopoietic cells found in all tissues in the steady state that share the ability to sample the environment but have distinct function in tissue immunity. Controversies remain on the best way to distinguish macrophages from DCs in vivo. In this Perspective, we discuss how recent discoveries in the origin of the DC and macrophage lineage help establish key functional differences between tissue DC and macrophage subsets. We also emphasize the need to further understand the functional heterogeneity of the tissue DC and macrophage lineages to better comprehend the complex role of these cells in tissue homeostasis and immunity.

CpG and poly(I:C) stimulation of dendritic cells and fibroblasts limits herpes simplex virus type 1 infection in an IFNβ-dependent and -independent way

Viral activation of toll-like receptors (TLRs) on dendritic cells (DCs) leads to production of various cytokines, including antiviral type I interferons (IFNs). Synthetic ligands specific for TLRs are also able to induce the production of type I IFNs (IFNα/β) by DCs, suggesting that these ligands have potential as antiviral drugs. In this in vitro study we extensively investigated the antiviral activity of various TLR ligands. Mouse bone marrow (BM) cells were differentiated into plasmacytoid and conventional DCs (pDCs and cDCs), stimulated with various TLR ligands and tested the antiviral abilities of collected supernatants in an in vitro herpes simplex virus type 1 (HSV-1) infection model. We observed a significant IFNβ-, (but not IFNα-) dependent reduction in HSV-1 infection when a mixed pDC/cDC population was stimulated with the TLR9 ligand CpG. In the absence of pDCs, TLR stimulation resulted in less pronounced antiviral effects. The most pronounced antiviral effect was observed when both DC subsets were stimulated with poly(I:C). A similar noticeable antiviral effect was observed when fibroblasts (L929 cells) were stimulated directly with poly(I:C). These poly(I:C)-mediated antiviral effects were only partially IFNβ-mediated and probably TLR independent. These data demonstrate that TLR ligands are not only able to produce type I IFN but can indeed act as antiviral drugs. In particular poly(I:C), which exerts its antiviral effects even in the absence of DCs, may become a promising drug e.g. to prevent respiratory infections by topical intranasal application.

IFN-λs

For decades, type I IFNs have been considered indispensable and unique antiviral mediators for the activation of rapid innate antiviral protection. However, the recent discovery of type III IFNs is challenging this paradigm. Since their identification in 2002/2003 by two independent groups, type III IFNs or IFN-λs, also known as IL-28/29, have been the subject of increased study with consequent recognition of their importance in virology and immunology. Initial reports suggested that IFN-λs functionally resemble type I IFNs. Although IFN-λs and classical type I IFNs (IFN-α/β) utilize distinct receptor complexes for signaling, both types of IFNs activate similar intracellular signaling pathways and biological activities, including the ability to induce antiviral state in cells, and both type I and type III IFNs are induced by viral infection. However, different antiviral potency, pattern of their induction and differential tissue expression of their corresponding receptor subunits suggest that the type I and type III IFN antiviral systems do not merely duplicate each other. Recent studies have started to reveal unique biological activities of IFN-λs in and beyond innate antiviral immunity.

Interferon-lambda and therapy for chronic hepatitis C virus infection

Interferon (IFN)-α, a type-I IFN, is widely used to treat chronic hepatitis C virus infection, but the broad expression of IFN-α receptors often leads to adverse reactions in many organs. Here, we examine IFN-λ, a type-III IFN, as a therapeutic alternative to IFN-α. Like IFN-α, IFN-λ also induces antiviral activity in hepatocytes, but might induce fewer adverse reactions because its receptor is largely restricted to cells of epithelial origin. We also discuss the recent discovery of single nucleotide polymorphisms (SNPs) near the human IFN-λ3 gene, IL28B, that correlate strongly with the ability to achieve a sustained virological response to therapy with pegylated IFN-α plus ribavirin in patients with chronic hepatitis C.

Chemokines: a new dendritic cell signal for T cell activation

Dendritic cells (DCs) are the main inducers and regulators of cytotoxic T lymphocyte (CTL) responses against viruses and tumors. One checkpoint to avoid misguided CTL activation, which might damage healthy cells of the body, is the necessity for multiple activation signals, involving both antigenic as well as additional signals that reflect the presence of pathogens. DCs provide both signals when activated by ligands of pattern recognition receptors and “licensed” by helper lymphocytes. Recently, it has been established that such T cell licensing can be facilitated by CD4⁺ T helper cells (“classical licensing”) or by natural killer T cells (“alternative licensing”). Licensing regulates the DC/CTL cross-talk at multiple layers. Direct recruitment of CTLs through chemokines released by licensed DCs has recently emerged as a common theme and has a crucial impact on the efficiency of CTL responses. Here, we discuss recent advances in our understanding of DC licensing for cross-priming and implications for the temporal and spatial regulation underlying this process. Future vaccination strategies will benefit from a deeper insight into the mechanisms that govern CTL activation.

Complexity of dendritic cell subsets and their function in the host immune system

Dendritic cells (DCs) are professional antigen-presenting cells that are critical for induction of adaptive immunity and tolerance. Traditionally DCs have been divided into two discrete subtypes, which comprise conventional and non-conventional DCs. They are distributed across various organs in the body and comprise a heterogeneous population, which has been shown to display differences in terms of surface marker expression, function and origins. Recent studies have shed new light on the process of DC differentiation and distribution of DC subtypes in various organs. Although monocytes, macrophages and DCs share a common macrophage-DC progenitor, a common DC progenitor population has been identified that exclusively gives rise to DCs and not monocytes or macrophages. In this review, we discuss the recent advances in our understanding of DC differentiation and subtypes and provide a comprehensive overview of various DC subtypes with emphasis on their function and origins. Furthermore, in light of recent developments in the field of DC biology, we classify DCs based on the precursor populations from which the various DC subsets originate. We classify DCs derived from common DC progenitor and pre-DC populations as conventional DCs, which includes both migratory and lymphoid-resident DC subsets and classify monocyte-derived DCs and plasmacytoid DCs as non-conventional DCs.

IFN-λs and BDCA3+/CD8α+ dendritic cells: towards the design of novel vaccine adjuvants?

IFN-λs are related to type I interferons but their receptor has a more cell-restricted pattern of expression. Consequently, one can expect that IFN-λs have antiviral and anti-tumoral activities as well as immunomodulatory properties with less adverse side-effects than type I interferons. However, their roles in physiopathology and immunoregulation remain to be fully elucidated. The study under evaluation identifies that murine CD8α(+) dendritic cells and their recently described human equivalent, BDCA3(+) dendritic cells, are the major producers of IFN-λs in response to dsRNA poly I:C. This study illustrates a new function for a DC subset conserved between species. These findings may impact the development of vaccine or therapeutic strategies based on DC targeting.

IL-28A (IFN-λ2) modulates lung DC function to promote Th1 immune skewing and suppress allergic airway disease

IL-28 (IFN-λ) cytokines exhibit potent antiviral and antitumor function but their full spectrum of activities remains largely unknown. Recently, IL-28 cytokine family members were found to be profoundly down-regulated in allergic asthma. We now reveal a novel role of IL-28 cytokines in inducing type 1 immunity and protection from allergic airway disease. Treatment of wild-type mice with recombinant or adenovirally expressed IL-28A ameliorated allergic airway disease, suppressed Th2 and Th17 responses and induced IFN-γ. Moreover, abrogation of endogenous IL-28 cytokine function in IL-28Rα^−/− mice exacerbated allergic airway inflammation by augmenting Th2 and Th17 responses, and IgE levels. Central to IL-28A immunoregulatory activity was its capacity to modulate lung CD11c⁺ dendritic cell (DC) function to down-regulate OX40L, up-regulate IL-12p70 and promote Th1 differentiation. Consistently, IL-28A-mediated protection was absent in IFN-γ^−/− mice or after IL-12 neutralization and could be adoptively transferred by IL-28A-treated CD11c⁺ cells. These data demonstrate a critical role of IL-28 cytokines in controlling T cell responses in vivo through the modulation of lung CD11c⁺ DC function in experimental allergic asthma.

Antiviral responses induced by the TLR3 pathway

Antiviral responses are successively induced in virus-infected animals, and include primary innate immune responses such as type I interferon (IFN) and cytokine production, secondary natural killer (NK) cell responses, and final cytotoxic T lymphocyte (CTL) responses and antibody production. The endosomal Toll-like receptors (TLRs) and cytoplasmic RIG-I-like receptors (RLRs), which recognize viral nucleic acids, are responsible for virus-induced type I IFN production. RLRs are expressed in most tissues and cells and are primarily implicated in innate immune responses against various viruses through type I IFN production, whereas nucleic acid-sensing TLRs, TLRs 3, 7, 8 and 9, are expressed on the endosomal membrane of dendritic cells (DCs) and play distinct roles in antiviral immunity. TLR3 recognizes viral double-stranded RNA taken up into the endosome and serves to protect the host against viral infection by the induction of a range of responses including type I IFN production and DC-mediated activation of NK cells and CTLs, although the deteriorative role of TLR3 has also been reported in some virus infections. Here, we review the current knowledge on the role of TLR3 during viral infection, and the current understanding of the TLR3-signalling cascade that operates via the adaptor protein TICAM-1 (also called TRIF).