Receptors and ligands involved in viral induction of type I interferon production by plasmacytoid dendritic cells

Alphaherpesvirus-induced activation of plasmacytoid dendritic cells depends on the viral glycoprotein gD and is inhibited by non-infectious light particles

Plasmacytoid dendritic cells (pDC) are important innate immune cells during the onset of viral infections as they are specialized in the production of massive amounts of antiviral type I interferon (IFN). Alphaherpesviruses such as herpes simplex virus (HSV) or pseudorabies virus (PRV) are double stranded DNA viruses and potent stimulators of pDC. Detailed information on how PRV activates porcine pDC is lacking. Using PRV and porcine primary pDC, we report here that PRV virions, so-called heavy (H-)particles, trigger IFNα production by pDC, whereas light (L-) particles that lack viral DNA and capsid do not. Activation of pDC requires endosomal acidification and, importantly, depends on the PRV gD envelope glycoprotein and O-glycosylations. Intriguingly, both for PRV and HSV-1, we found that L-particles suppress H-particle-mediated activation of pDC, a process which again depends on viral gD. This is the first report describing that gD plays a critical role in alphaherpesvirus-induced pDC activation and that L-particles directly interfere with alphaherpesvirus-induced IFNα production by pDC.

Development of new therapeutical/adjuvant molecules by pepscan mapping of autophagy and IFN inducing determinants of rhabdoviral G proteins

Surface glycoproteins of enveloped virus are potent elicitors of both innate and adaptive host immune responses. Therefore, the identification of viral glycoprotein determinants directly implicated in the induction of these responses might be of special interest for designing new therapeutical/adjuvant molecules. In this work we review the contribution of the "pepscan" approach to the screening of viral functions in the sequence of glycoprotein G (gpG) of the fish rhabdovirus of viral hemorrhagic septicemia (VHSV). Among others, by scanning gpG peptides, it has been possible to identify and validate minimal determinants for gpG directly implicated in initiating the fish type I Interferon-associated immune responses as well as in the antiviral autophagy program. Further fine-tunning of the identified peptides in the gpG of VHSV has allowed designing novel adjuvants that decrease DNA vaccine requirements and identify possible innovative antiviral molecules. In addition, these results have also contributed to improve our knowledge on how to stimulate the fish immune system.

Biology of plasmacytoid dendritic cells and natural killer cells in HIV-1 infection

PURPOSE

This review summarizes recent literature on the biology of dendritic cells and natural killer cells in HIV-1 infection and the importance of crosstalk between them in the development of strong antiviral immunity.

RECENT FINDINGS

Type I interferons produced by dendritic cells in response to HIV-1 have been suggested to act as a double-edged sword, stemming HIV-1 replication on the one hand and causing T-cell loss on the other. Recent epidemiologic evidence demonstrates a strong association between the natural killer cell receptor KIR3DS1 (along with its presumed ligand HLA-B Bw4-80I) in the control of HIV-1 replication.

SUMMARY

Dendritic and natural killer cells play a central role in the innate immune response to viral infections through both the direct elimination of infected cells and modulation of each other's function.

Identification of a lineage negative cell population in bovine peripheral blood with the ability to mount a strong type I interferon response

Lineage negative dendritic cells, or natural interferon-producing cells (NIPC), also referred to as plasmacytoid dendritic cells (pDC) constitute a small population of leukocytes secreting high levels of type I interferon (IFNα/β) in response to certain danger signals. Here, we provide initial data towards the identification of so far uncharacterised circulating bovine pDC like cells. A lineage negative cell population (LIN(-) cells) was isolated from PBMC which showed characteristics similar to that of pDC in other species. Isolated LIN(-) cells presented lymphoid morphology with a semi-crescent nucleus, extensive ER and Golgi network; indicative of pDC. In addition phenotypic analysis of LIN(-) cells described them as distinct from other bovine DC subsets; expressing both lymphoid and myeloid surface markers. LIN(-) cells did not express lineage specific markers, but were MHC class II(+), CD45RO(+), CD80/86(+), CD6(+), WC1(+), CD26(+) and expressed the myeloid markers CD205, CD172a and CD11a. In keeping with pDC, LIN(-) cells express TLR7 mRNA transcripts; however, in a resting state do not express TLR8 or TLR9. Functionally, LIN(-) cells, but not PBMC, monocytes and monocyte derived DC produce large amounts of IFNα/β in response to different CpG oligonucleotides. Taken together, we present data suggesting that an enriched circulating population of bovine LIN(-) cells are uniquely capable of producing IFNα/β in response to CpG oligonucleotides and thus this population likely contain the functional equivalent of bovine pDC.

Hepatitis B virus impairs TLR9 expression and function in plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) play a key role in detecting pathogens by producing large amounts of type I interferon (IFN) by sensing the presence of viral infections through the Toll-Like Receptor (TLR) pathway. TLR9 is a sensor of viral and bacterial DNA motifs and activates the IRF7 transcription factor which leads to type I IFN secretion by pDCs. However, during chronic hepatitis B virus (HBV) infection, pDCs display an impaired ability to secrete IFN-α following ex vivo stimulation with TLR9 ligands. Here we highlight several strategies used by HBV to block IFN-α production through a specific impairment of the TLR9 signaling. Our results show that HBV particle internalisation could inhibit TLR9- but not TLR7-mediated secretion of IFN-α by pDCs. We observed that HBV down-regulated TLR9 transcriptional activity in pDCs and B cells in which TLR9 mRNA and protein levels were reduced. HBV can interfere with TLR9 activity by blocking the MyD88-IRAK4 axis and Sendai virus targeting IRF7 to block IFN-α production. Neutralising CpG motif sequences were identified within HBV DNA genome of genotypes A to H which displayed a suppressive effect on TLR9-immune activation. Moreover, TLR9 mRNA and protein were downregulated in PBMCs from patients with HBV-associated chronic hepatitis and hepatocellular carcinoma. Thus HBV has developed several escape mechanisms to avoid TLR9 activation in both pDCs and B lymphocytes, which may in turn contribute to the establishment and/or persistence of chronic infection.

Age-associated impaired plasmacytoid dendritic cell functions lead to decreased CD4 and CD8 T cell immunity

Increased susceptibility to infections, particularly respiratory viral infections, is a hallmark of advancing age. The underlying mechanisms are not well understood, and there is a scarcity of information regarding the contribution of the innate immune system, which is the first line of defense against infections. In the present study, we have investigated the effect of advancing age on plasmacytoid dendritic cell (PDC) function because they are critical in generating a robust antiviral response via the secretion of interferons (IFN). Our results indicate that PDCs from the aged are impaired in their capacity to secrete IFN-I in response to influenza virus and CPG stimulation. Additionally, we observed a severe reduction in the production of IFN-III, which plays an important role in defense against viral infections at respiratory mucosal surfaces. This reduction in IFN-I and IFN-III were a result of age-associated impaired phosphorylation of transcription factor, IRF-7. Furthermore, aged PDCs were observed to be impaired in their capacity to induce perforin and granzyme in CD8 T cells. Comparison of the antigen-presenting capacity of aged PDC with young PDC revealed that PDCs from aged subjects display reduced capacity to induce proliferation and IFN-gamma secretion in CD4 and CD8 T cells as compared with PDCs from young subjects. In summary, our study demonstrates that advancing age has a profound effect on PDC function at multiple levels and may therefore, be responsible for the increased susceptibility to infections in the elderly.

The role of myeloid receptors on murine plasmacytoid dendritic cells in induction of type I interferon

This study tested the hypothesis that a set of predominantly myeloid restricted receptors (F4/80, CD36, Dectin-1, CD200 receptor and mannan binding lectins) and the broadly expressed CD200 played a role in a key function of plasmacytoid DC (pDC), virally induced type I interferon (IFN) production. The Dectin-1 ligands zymosan, glucan phosphate and the anti-Dectin-1 monoclonal antibody (mAb) 2A11 had no effect on influenza virus induced IFNα/β production by murine splenic pDC. However, mannan, a broad blocking reagent against mannose specific receptors, inhibited IFNα/β production by pDC in response to inactivated influenza virus. Moreover, viral glycoproteins (influenza virus haemagglutinin and HIV-1 gp120) stimulated IFNα/β production by splenocytes in a mannan-inhibitable manner, implicating the function of a lectin in glycoprotein induced IFN production. Lastly, the effect of CD200 on IFN induction was investigated. CD200 knock-out macrophages produced more IFNα than wild-type macrophages in response to polyI:C, a MyD88-independent stimulus, consistent with CD200's known inhibitory effect on myeloid cells. In contrast, blocking CD200 with an anti-CD200 mAb resulted in reduced IFNα production by pDC-containing splenocytes in response to CpG and influenza virus (MyD88-dependent stimuli). This suggests there could be a differential effect of CD200 on MyD88 dependent and independent IFN induction pathways in pDC and macrophages. This study supports the hypothesis that a mannan-inhibitable lectin and CD200 are involved in virally induced type I IFN induction.

HIV-1-derived lentiviral vectors directly activate plasmacytoid dendritic cells, which in turn induce the maturation of myeloid dendritic cells

Lentiviral vectors (LV) can induce type I interferon (IFN I) production from murine plasmacytoid dendritic cells (pDC), but not myeloid (my)DC. Here, we investigated whether this mechanism is conserved in human DC. MyDC and pDC were isolated from peripheral blood and transduced with increasing vector concentrations. Compared with in vitro differentiated monocyte-derived DC, the transduction efficiency of peripheral blood DC was low (ranging from <1% to 45%), with pDC showing the lowest susceptibility to LV transduction. Phenotype and function of myDC were not directly modified by LV transduction; by contrast, pDC produced significant levels of IFN-α and tumor necrosis factor-α. pDC activation was dependent on functional vector particles and was mediated by Toll-like receptor 7/9 triggering. Coculture of myDC with pDC in the presence of LV resulted in myDC activation, with CD86 up-regulation and interleukin-6 secretion. These findings demonstrate that the induction of transgene-specific immunity is triggered by an innate immune response with pDC activation and consequent myDC maturation, a response that closely resembles the one induced by functional viruses. This information is important to design strategies aimed at using LV in humans for gene therapy, where adverse immune responses must be avoided, or for cancer immunotherapy, where inducing immunity is the goal.

North American porcine reproductive and respiratory syndrome viruses inhibit type I interferon production by plasmacytoid dendritic cells

Although enveloped viruses typically trigger the prodigious secretion of alpha interferon (IFN-α) by plasmacytoid dendritic cells (pDC), porcine pDC remain quiescent when exposed to porcine reproductive and respiratory syndrome virus (PRRSV). This inactivity is likely due to virus-mediated interference since the typical IFN-α response by either purified or nonsorted porcine pDC to transmissible gastroenteritis virus (TGEV) or the Toll-like receptor 9 agonist, oligodeoxynucleotide (ODN) D19, was markedly reduced in the presence of PRRSV. Suppression occurred independently of virus viability and acidification of pDC early endosomes but correlated with diminished levels of IFN-α mRNA. This change was attributed to an abrogation of transcription resulting from a decrease in the otherwise enhanced amounts of the requisite interferon regulatory factor 7 (IRF-7), whose gene expression in turn was limited as a consequence of a lessened availability of nuclear-localized signal transducer and activator of transcription 1 (STAT1). While PRRSV also inhibited tumor necrosis factor alpha (TNF-α) synthesis by pDC responding to either agent, only the interleukin-2 (IL-2) and IL-6 production instigated by ODN D19 exposure was blocked. Likewise, PRRSV did not impact a specific TGEV-associated enhancement of IL-8 expression. Moreover, an augmented phosphorylation of NF-κB seen in activated pDC was not only unaffected by PRRSV but actually occurred in its presence. Thus, as supported by a demonstrated resilience of pDC to PRRSV infection, this pathogen may interact with a cell surface protein(s) to selectively impede the completion of cascades involved in cytokine production by stimulated pDC.

Pepscan mapping of viral hemorrhagic septicemia virus glycoprotein G major lineal determinants implicated in triggering host cell antiviral responses mediated by type I interferon

Surface glycoproteins of enveloped virus are potent elicitors of type I interferon (IFN)-mediated antiviral responses in a way that may be independent of the well-studied genome-mediated route. However, the viral glycoprotein determinants responsible for initiating the IFN response remain unidentified. In this study, we have used a collection of 60 synthetic 20-mer overlapping peptides (pepscan) spanning the full length of glycoprotein G (gpG) of viral hemorrhagic septicemia virus (VHSV) to investigate what regions of this protein are implicated in triggering the type I IFN-associated immune responses. Briefly, two regions with ability to increase severalfold the basal expression level of the IFN-stimulated mx gene and to restrict the spread of virus among responder cells were mapped to amino acid residues 280 to 310 and 340 to 370 of the gpG protein of VHSV. In addition, the results obtained suggest that an interaction between VHSV gpG and integrins might trigger the host IFN-mediated antiviral response after VHSV infection. Since it is known that type I IFN plays an important role in determining/modulating the protective-antigen-specific immune responses, the identification of viral glycoprotein determinants directly implicated in the type I IFN induction might be of special interest for designing new adjuvants and/or more-efficient and cost-effective viral vaccines as well as for improving our knowledge on how to stimulate the innate immune system.

Dendritic cells--at the front-line of pathogen attack

Efficient immune defence function is dependent on the role played by dendritic cells (DCs), particularly the interaction between conventional DC (cDC) and plasmacytoid DC (pDC), together with other monocytic cells. This functionality of immune defences is open to manipulation by viral pathogens infecting DC, a situation further complicated by the diversity of mechanisms employed by different viruses and the subset of DC involved. The present review uses two virus examples--classical swine fever virus (CSFV) and porcine circovirus type 2 (PCV2)--to demonstrate the complexity of this host-pathogen scenario. CSFV is a monocytotropic RNA virus infecting and replicating in both cDC and pDC. This virus employs its non-structural Npro protein for antagonizing the Type I interferon (IFN) induction pathway. The Npro protein promotes proteasomal degradation of interferon regulatory factor (IRF)3, particularly notable in cDC. In contrast, CSFV infection induces IFNalpha production by pDC, probably due to a lack of interference by the Npro protein with the IRF7 more prominent in pDC. Such ability of the virus to inhibit cDC while augmenting IFNalpha production by pDC might lead to an exaggerated pDC response, relating to the immunopathological characteristics of the disease. PCV2 is an ssDNA containing virus, which in contrast to CSFV is inefficient in its capacity to replicate in DC. Recent evidence suggests that virus replication occurs in endothelial cells, with the DC being more involved through their particularly elevated endocytosis of the virus. PCV2 can accumulate to high levels both in vitro and in vivo, a phenomenon dependent on the virus capsid protein, inferring that the viral capsid or genome impedes DC endocytic degradation of the virus. Nevertheless, the presence of PCV2 in cDC does not interfere with processing of other antigens. The immunoregulatory characteristics of PCV2 are manifest as impairment of "danger" recognition by cells of the innate defences. This varies dependent on the "danger" signal and the cells responding, especially when one compares cDC and pDC. Overall, the PCV2-induced immunomodulation contrasts with that of CSFV in being a property dependent on the viral genome, particularly the dsDNA replicative form, and with immunoregulatory capacity for both cDC and pDC. Moreover, PCV2 compromises immune defence development against other pathogens rather than itself. In conclusion, the DC family represents a critical immune defence element open to modulation by virus infection, with serious consequences for host resistance to disease. The characteristics of the immune modulation depend on the virus and the DC subsets involved. Overall, the roles played by the pDC can be decisive in shaping the outcome of the infection and the characteristics of the virus-induced immunocompromisation.