TLR3 is essential for the induction of protective immunity against Punta Toro Virus infection by the double-stranded RNA (dsRNA), poly(I:C12U), but not Poly(I:C): differential recognition of synthetic dsRNA molecules

Negative regulation of the type I interferon signaling pathway by synthetic Toll-like receptor 7 ligands

Ten Toll-like receptor (TLR) family members have been reported in humans. Here, the endoplasmatic receptors TLR9, TLR8, TLR7, and TLR3 respond to nucleic acids and derivatives or to small molecules (TLR7 and 8). Another cytoplasmic RNA receptor, retinoic acid inducible gene I (RIG-I), is stimulated by 5' triphosphate double-stranded RNA. We discovered that TLR7 small-molecule agonists inhibit nucleic acid-mediated TLR3, TLR7, TLR9, or RIG-I-dependent interferon-α (IFN-α) immune response. Other cytokines and chemokines stimulated by nucleic acid agonists remained unaffected. The observed blockage of TLR3, TLR7, TLR9, and RIG-I-mediated IFN-α response appears to be driven by a competitive mechanism at the type I IFN pathway. Besides type I IFN, IFN response genes such as IFIT-1, Mx1, OAS1, or IRF7 were affected, which indicates that the key element driving the inhibition is located in the type I IFN pathway. Indeed, the heterotrimeric complex formation of phosphor-signal transducer and activator of transcription factor 1 (STAT1), phosphor-STAT2, and IRF9 (called ISGF3, IFN-stimulated gene factor 3) is inhibited through the TLR7 small-molecule agonists by phosphor-STAT2 blockage. These findings provide novel insights into the use of synthetic TLR7 or TLR7/8 small molecules as ligands for immune activation and suppression.

A double-blind, placebo-controlled, randomized, clinical trial of the TLR-3 agonist rintatolimod in severe cases of chronic fatigue syndrome

Background

Chronic fatigue syndrome/myalgic encephalomyelitis (CFS/ME) is a severely debilitating disease of unknown pathogenesis consisting of a variety of symptoms including severe fatigue. The objective of the study was to examine the efficacy and safety of a TLR-3 agonist, rintatolimod (Poly I: C₁₂U), in patients with debilitating CFS/ME.

Methods and Findings

A Phase III prospective, double-blind, randomized, placebo-controlled trial comparing twice weekly IV rintatolimod versus placebo was conducted in 234 subjects with long-standing, debilitating CFS/ME at 12 sites. The primary endpoint was the intra-patient change from baseline at Week 40 in exercise tolerance (ET). Secondary endpoints included concomitant drug usage, the Karnofsky Performance Score (KPS), Activities of Daily Living (ADL), and Vitality Score (SF 36). Subjects receiving rintatolimod for 40 weeks improved intra-patient placebo-adjusted ET 21.3% (p = 0.047) from baseline in an intention-to-treat analysis. Correction for subjects with reduced dosing compliance increased placebo-adjusted ET improvement to 28% (p = 0.022). The improvement observed represents approximately twice the minimum considered medically significant by regulatory agencies. The rintatolimod cohort vs. placebo also reduced dependence on drugs commonly used by patients in an attempt to alleviate the symptoms of CFS/ME (p = 0.048). Placebo subjects crossed-over to receive rintatolimod demonstrated an intra-patient improvement in ET performance at 24 weeks of 39% (p = 0.04). Rintatolimod at 400 mg twice weekly was generally well-tolerated.

Conclusions/Significance

Rintatolimod produced objective improvement in ET and a reduction in CFS/ME related concomitant medication usage as well as other secondary outcomes.

Trial Registration

ClinicalTrials.gov NCT00215800

Targeting pattern recognition receptors in cancer immunotherapy

Pattern recognition receptors (PRRs) are known for many years for their role in the recognition of microbial products and the subsequent activation of the immune system. The 2011 Nobel Prize for medicine indeed rewarded J. Hoffmann/B. Beutler and R. Steinman for their revolutionary findings concerning the activation of the immune system, thus stressing the significance of understanding the mechanisms of activation of the innate immunity. Such immunostimulatory activities are of major interest in the context of cancer to induce long-term antitumoral responses. Ligands for the toll-like receptors (TLRs), a well-known family of PRR, have been shown to have antitumoral activities in several cancers. Those ligands are now undergoing extensive clinical investigations both as immunostimulant molecules and as adjuvant along with vaccines. However, when considering the use of these ligands in tumor therapy, one shall consider the potential effect on the tumor cells themselves as well as on the entire organism. Recent data indeed demonstrate that TLR activation in tumor cells could trigger both pro- or antitumoral effect depending on the context. This review discusses this balance between the intrinsic activation of PRR in tumor cells and the extrinsic microenvironment activation in term of overall effect of PRR ligands on tumor development. We review recent advances in the field and underline appealing prospects for clinical development of PRR agonists in the light of our current knowledge on their expression and activation.

Regulatory B cell production of IL-10 inhibits lymphoma depletion during CD20 immunotherapy in mice

Current therapies for non-Hodgkin lymphoma commonly include CD20 mAb to deplete tumor cells. However, the response is not durable in a substantial proportion of patients. Herein, we report our studies in mice testing the hypothesis that heterogeneity in endogenous tissue CD20+ B cell depletion influences in vivo lymphoma therapy. Using highly effective CD20 mAbs that efficiently deplete endogenous mature B cells and homologous CD20+ primary lymphoma cells through monocyte- and antibody-dependent mechanisms, we found that lymphoma depletion and survival were reduced when endogenous host B cells were not depleted, particularly a rare IL-10-producing B cell subset (B10 cells) known to regulate inflammation and autoimmunity. Even small numbers of adoptively transferred B10 cells dramatically suppressed CD20 mAb-mediated lymphoma depletion by inhibiting mAb-mediated monocyte activation and effector function through IL-10-dependent mechanisms. However, the activation of innate effector cells using a TLR3 agonist that did not activate B10 cells overcame the negative regulatory effects of endogenous B10 cells and enhanced lymphoma depletion during CD20 immunotherapy in vivo. Thus, we conclude that endogenous B10 cells are potent negative regulators of innate immunity, with even small numbers of residual B10 cells able to inhibit lymphoma depletion by CD20 mAbs. Consequently, B10 cell removal could provide a way to optimize CD20 mAb-mediated clearance of malignant B cells in patients with non-Hodgkin lymphoma.

Short-interfering RNAs induce retinal degeneration via TLR3 and IRF3

The discovery of sequence-specific gene silencing by endogenous double-stranded RNAs (dsRNA) has propelled synthetic short-interfering RNAs (siRNAs) to the forefront of targeted pharmaceutical engineering. The first clinical trials utilized 21-nucleotide (nt) siRNAs for the treatment of neovascular age-related macular degeneration (AMD). Surprisingly, these compounds were not formulated for cell permeation, which is required for bona fide RNA interference (RNAi). We showed that these "naked" siRNAs suppress neovascularization in mice not via RNAi but via sequence-independent activation of cell surface Toll-like receptor-3 (TLR3). Here, we demonstrate that noninternalized siRNAs induce retinal degeneration in mice by activating surface TLR3 on retinal pigmented epithelial cells. Cholesterol conjugated siRNAs capable of cell permeation and triggering RNAi also induce the same phenotype. Retinal degeneration was not observed after treatment with siRNAs shorter than 21-nts. Other cytosolic dsRNA sensors are not critical to this response. TLR3 activation triggers caspase-3-mediated apoptotic death of the retinal pigment epithelium (RPE) via nuclear translocation of interferon regulatory factor-3. While this unexpected adverse effect of siRNAs has implications for future clinical trials, these findings also introduce a new preclinical model of geographic atrophy (GA), a late stage of dry AMD that causes blindness in millions worldwide.

Identifying innate immune pathways of the chicken may lead to new antiviral therapies

Zoonotic viruses, such as highly pathogenic avian influenza (HPAI), present a significant threat to both the poultry industry and public health. The present method of controlling avian influenza (AI) relies on good farming practice with limited use of vaccination in some countries. However, new ways to control disease outbreaks might be possible with additional knowledge of the natural host response to virus. Moreover, manipulation of the innate immune system in mammals improves the outcomes following viral infection. A similar approach might be applied to the chicken, nevertheless, a greater knowledge of the chicken innate immune system is required. This review outlines important mammalian antiviral mechanisms that have been modulated to strengthen viral immunity and highlights the potential application of these strategies in the chicken, especially in regards, to AI.

Toll-like receptor, RIG-I-like receptors and the NLRP3 inflammasome: key modulators of innate immune responses to double-stranded RNA viruses

Double-stranded RNA (dsRNA), the genetic material for many RNA viruses, induces robust host immune responses via pattern recognition receptors, which include Toll-like receptor 3 (TLR3), retinoic acid-inducible gene-I-like receptors (RLRs) and the multi-protein NLRP3 inflammasome complex. The engagement of dsRNA receptors or inflammasome activation by viral dsRNA initiates complex intracellular signaling cascades that play essential roles in inflammation and innate immune responses, as well as the resultant development of adaptive immunity. This review focuses on signaling pathways mediated by TLR3, RLRs and the NLRP3 inflammasome, as well as the potential use of agonists and antagonists that target these pathways to treat disease.

The pharmacokinetics of Toll-like receptor agonists and the impact on the immune system

Toll-like receptor (TLR) ligation activates both the innate and adaptive immune systems, and plays an important role in antiviral and anti-tumor immunity. Therefore, a significant amount of effort has been devoted to exploit the therapeutic potential of TLR agonists. Depending on the therapeutic purpose, either as adjuvants to vaccine, chemotherapy or standalone therapy, TLR agonists have been administered via different routes. Both preclinical and clinical studies have suggested that the route of administration has significant effects on pharmacokinetics, and that understanding these effects is critical to the success of TLR agonist drug development. This article will summarize the pharmacokinetics of TLR agonists with different administration routes, with an emphasis on clinical studies of TLR ligands in oncologic applications.

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).

Recent insights into the role of Toll-like receptors in viral infection

Toll-like receptors (TLRs) have a central role in innate immunity as they detect conserved pathogen-associated molecular patterns (PAMPs) on a range of microbes, including viruses, leading to innate immune activation and orchestration of the adaptive immune response. To date, a large number of viruses have been shown to trigger innate immunity via TLRs, suggesting that these receptors are likely to be important in the outcome to viral infection. This suggestion is supported by the observation that many viruses have evolved mechanisms not only to evade the innate immune system, but also to subvert it for the benefit of the virus. In this review we will discuss earlier evidence, mainly from knock-out mice studies, implicating TLRs in the innate immune response to viruses, in light of more recent clinical data demonstrating that TLRs are important for anti-viral immunity in humans.

Vaccine adjuvants: putting innate immunity to work

Adjuvants enhance immunity to vaccines and experimental antigens by a variety of mechanisms. In the past decade, many receptors and signaling pathways in the innate immune system have been defined and these innate responses strongly influence the adaptive immune response. The focus of this review is to delineate the innate mechanisms by which adjuvants mediate their effects. We highlight how adjuvants can be used to influence the magnitude and alter the quality of the adaptive response in order to provide maximum protection against specific pathogens. Despite the impressive success of currently approved adjuvants for generating immunity to viral and bacterial infections, there remains a need for improved adjuvants that enhance protective antibody responses, especially in populations that respond poorly to current vaccines. However, the larger challenge is to develop vaccines that generate strong T cell immunity with purified or recombinant vaccine antigens.

Immunomodulatory effects of dsRNA and its potential as vaccine adjuvant

dsRNA can be detected by pattern recognition receptors, for example, TLR3, MDA-5, NLRP3 to induce proinflammatory cytokines responsible for innate/adaptive immunity. Recognized by endosomal TLR3 in myeloid DCs (mDCs), dsRNA can activate mDCs into mature antigen presenting cells (mAPCs) which in turn present antigen epitopes with MHC-I molecules to naïve T cells. Coadministration of protein and synthetic dsRNA analogues can elicit an antigen-specific Th1-polarized immune response which stimulates the CD8+ CTL response and possibly dampen Th17 response. Synthetic dsRNA analogues have been tested as vaccine adjuvant against viral infections in animal models. However, a dsRNA receptor, TLR3 can be expressed in tumor cells while other members of TLR family, for example, TLR4 and TLR2 have been shown to promote tumor progression, metastasis, and chemoresistance. Thus, the promising potential of dsRNA analogues as a tumor therapeutic vaccine adjuvant should be evaluated cautiously.

dsRNA and the innate antiviral immune response

Innate immunity is the first line of defense against viral infections. It is based on a mechanism of sensing pathogen-associated molecular patterns through host germline-encoded pattern recognition receptors. dsRNA is arguably the most important viral pathogen-associated molecular pattern due to its expression by almost all viruses at some point during their replicative cycle. Viral dsRNA has been studied for over 55 years, first as a toxin, then as a type I interferon inducer, a viral mimetic and an immunomodulator for therapeutic purposes. This article will focus on dsRNA, its structure, generation (both endogenous and viral), host sensing mechanisms and induction of type I interferons. The possible therapeutic applications of these findings will also be discussed. The goal of this article is to give an overview of these mechanisms, highlighting novel findings, while providing a historical perspective.

A new mouse-adapted strain of SARS-CoV as a lethal model for evaluating antiviral agents in vitro and in vivo

Severe acute respiratory syndrome (SARS) is a highly lethal emerging disease caused by coronavirus SARS-CoV. New lethal animal models for SARS were needed to facilitate antiviral research. We adapted and characterized a new strain of SARS-CoV (strain v2163) that was highly lethal in 5- to 6-week-old BALB/c mice. It had nine mutations affecting 10 amino acid residues. Strain v2163 increased IL-1alpha, IL-6, MIP-1alpha, MCP-1, and RANTES in mice, and high IL-6 expression correlated with mortality. The infection largely mimicked human disease, but lung pathology lacked hyaline membrane formation. In vitro efficacy against v2163 was shown with known inhibitors of SARS-CoV replication. In v2163-infected mice, Ampligen was fully protective, stinging nettle lectin (UDA) was partially protective, ribavirin was disputable and possibly exacerbated disease, and EP128533 was inactive. Ribavirin, UDA, and Ampligen decreased IL-6 expression. Strain v2163 provided a valuable model for anti-SARS research.

PolyI:polyC12U adjuvant-combined intranasal vaccine protects mice against highly pathogenic H5N1 influenza virus variants

The highly pathogenic avian H5N1 influenza virus has the potential to incite a global pandemic. Therefore, there is an urgent need to develop effective vaccines against these viruses. Because it is difficult to predict which strain of influenza will cause a pandemic, it is advantageous to develop vaccines that will confer cross-protective immunity against variants of the influenza virus. Recently, we reported that the Toll-like receptor 3 agonist, polyI:polyC(12)U (Ampligen), has been proven to be safe in a Phase III human trial, and is an effective mucosal adjuvant for intranasal H5N1 influenza vaccination. Intranasal administration of an Ampligen adjuvanted pre-pandemic H5N1 vaccine (NIBRG14), which was derived from the A/Vietnam/1194/2004 strain, resulted in the secretion of vaccine-specific IgA and IgG in nasal mucosa and serum, respectively, and protected mice against homologous A/Vietnam/1194/2004 and heterologous A/Hong Kong/483/97 and A/Indonesia/6/2005 viral challenge.

Efficacy of favipiravir (T-705) and T-1106 pyrazine derivatives in phlebovirus disease models

Several studies have reported favipiravir (T-705) to be effective in treating a number of viral diseases modeled in rodent systems. Notably, the related pyrazine derivative, T-1106, was found to be more effective than T-705 in treating yellow fever virus infection in hamsters. Based on these findings, we hypothesized that T-1106 may be more effective in treating hepatotropic Punta Toro virus (PTV, Phlebovirus) infection in rodents. In cell culture, the inhibitory concentrations of the compounds against various phleboviruses ranged from 3 to 55microM for T-705 and from 76 to 743microM for T-1106. In PTV-challenged hamsters, a model that generally presents with high liver viral loads, T-1106 was more effective at reducing mortality. However, in mice infected with PTV, a model wherein systemic infection is more prominent, the greater efficacy exhibited by T-1106 in the hamster system was not apparent. In contrast, T-705 was superior in preventing mortality in hamsters challenged with Pichinde virus (PICV, Arenavirus), an infection characterized as diffuse and pantropic. Remarkably, T-1106 has proven more active in vivo than would have been expected from our cell culture results, and our in vivo findings suggest that it is more effective in infections characterized predominantly by high levels of hepatic viral burden.

Punta Toro virus (Bunyaviridae, Phlebovirus) infection in mice: strain differences in pathogenesis and host interferon response

The Adames strain of Punta Toro virus (PTV-A, Bunyaviridae, Phlebovirus) causes an acute lethal disease in hamsters and mice. The Balliet strain of the virus (PTV-B) is generally considered to be avirulent. The difference in hamster susceptibility is likely due to the ability of PTV-A to suppress interferon (IFN)-beta similarly to that described for Rift Valley fever virus. Here we investigated strain differences in PTV pathogenesis and the IFN response in mice. Although PTV-B infection in mice did not induce systemic IFN-beta release, primary macrophages produced dramatically higher levels when exposed to the virus in culture. The importance of IFN in resistance to PTV infection was borne out in studies employing STAT-1 knock-out mice. Also, a number of genes specific to IFN response pathways were upregulated in PTV-B-infected macrophages. Our findings provide new insights into the type I IFN response during PTV infection in the mouse model of phleboviral disease.

Hepatitis C virus single-stranded RNA induces innate immunity via Toll-like receptor 7

BACKGROUND/AIMS

Innate immune responses to HCV infection are triggered through host recognition of pathogen-associated molecular patterns. Interferons are critical for the protection against HCV infection. However, the pathways linking virus recognition to IFN induction remain poorly understood.

METHODS

Immune cells and Huh-7 cells were infected with HCV cell culture (HCVcc) or transfected with HCV-derived immunostimulatory RNA oligonucleotides (ORNs), and immune activation was assessed.

RESULTS

We found that HCVcc suppressed immune responses because the HCVcc protein impaired the PBMC and pDC responses. However, HCVcc genomic RNA had an immunostimulatory effect. HCV encodes G/U-rich ssRNA TLR7 ligands that significantly activate innate immunity, and induced IFN-alpha production. Moreover, HCV-derived ORNs also activated IRF7 and NF-kappaB in Huh-7 cells. In particular, the HCV 3'-UTR strongly induced cytokine production. Different lengths of polyuridine tract in the 3'-UTR of different HCV strains induced IFN-alpha production. These data demonstrate that the HCV-specific G/U fragment is a motif sequence, and is recognized by TLR7 as a PAMP. The requirement for TLR7 to recognize HCV RNA was confirmed using specific inhibitors, RNAi and by TLR7overexpression.

CONCLUSION

These results provide an insight into the development of immune adjuvant for vaccines and for the production of new antiviral drugs.

Not all polyriboinosinic-polyribocytidylic acids (Poly I:C) are equivalent for inducing maturation of dendritic cells: implication for alpha-type-1 polarized DCs

This study compares the behavior of 2 commercially available polyriboinosinic-polyribocytidylic acids (poly I:C1 and poly I:C2) and the structural analog poly I:C12U in regard to dendritic cell (DC) maturation. When the Toll-like receptor 3 (TLR3) agonists are tested in combination with interferon-alpha, tumor necrosis factor-alpha, interleukin (IL)-1beta, and interferon-gamma (the so-called alpha-type-1 DC), the 3 different cocktails generate phenotypically mature DCs, but with different functional properties. Higher migratory capacity is observed with poly I:C1, the only poly I:C allowing spontaneous release of IL-12p70 by DCs. However, upon CD40 triggering, cocktails containing poly I:C2 or poly I:C12U allow a far higher production of IL-12p70 compared with those containing poly I:C1. Using a TLR signaling pathway reverse transcription profiler polymerase chain reaction to analyze changes in gene expression after treatment of DCs with the agonists alone, we show that 39% of the 84 tested genes are differentially regulated between the 3 conditions. Poly I:C12U induces far fewer regulated genes than the 2 other poly I:Cs. These different behaviors could be due to alternative ways of sensing double-stranded RNA, which do not rely solely on TLR3 but also on other types of receptors, depending on the size of poly I:Cs. As the 2 poly I:Cs tested here have very different molecular weights, this could partly explain the observed differences. In conclusion, neither the poly I:Cs nor their structural analog poly I:C12U have an equivalent behavior. This should be taken into an account not only when they are used in cocktails for DC maturation but also when analyzing signaling pathways with synthetic double-stranded RNA analogs.

Activation of interferon regulatory factor-3 via toll-like receptor 3 and immunomodulatory functions detected in A549 lung epithelial cells exposed to misplaced U1-snRNA

U1-snRNA is an integral part of the U1 ribonucleoprotein pivotal for pre-mRNA splicing. Toll-like receptor (TLR) signaling has recently been associated with immunoregulatory capacities of U1-snRNA. Using lung A549 epithelial/carcinoma cells, we report for the first time on interferon regulatory factor (IRF)-3 activation initiated by endosomally delivered U1-snRNA. This was associated with expression of the IRF3-inducible genes interferon-β (IFN-β), CXCL10/IP-10 and indoleamine 2,3-dioxygenase. Mutational analysis of the U1-snRNA-activated IFN-β promoter confirmed the crucial role of the PRDIII element, previously proven pivotal for promoter activation by IRF3. Notably, expression of these parameters was suppressed by bafilomycin A₁, an inhibitor of endosomal acidification, implicating endosomal TLR activation. Since resiquimod, an agonist of TLR7/8, failed to stimulate A549 cells, data suggest TLR3 to be of prime relevance for cellular activation. To assess the overall regulatory potential of U1-snRNA-activated epithelial cells on cytokine production, co-cultivation with peripheral blood mononuclear cells (PBMC) was performed. Interestingly, A549 cells activated by U1-snRNA reinforced phytohemagglutinin-induced interleukin-10 release by PBMC but suppressed that of tumor necrosis factor-α, indicating an anti-inflammatory potential of U1-snRNA. Since U1-snRNA is enriched in apoptotic bodies and epithelial cells are capable of performing efferocytosis, the present data in particular connect to immunobiological aspects of apoptosis at host/environment interfaces.

PolyI:C-induced reduction in uptake of soluble antigen is independent of dendritic cell activation

Dendritic cells (DC) are key players in the initiation and modulation of adaptive immune responses due to their ability to acquire and present antigen and stimulate T cells. For the induction of effector T cell functions, antigen must be presented by activated DC. In this study, we have compared uptake of antigen by mouse DC in the presence of different Toll-like receptor (TLR) agonists, which are potent inducers of DC activation. Here we show that the reduction in uptake of soluble antigen in the presence of the viral double-stranded RNA (dsRNA) analogues polyinosinic-polycytidylic acid and Ampligen is independent of TLR-mediated DC activation. A reduction in antigen uptake by bone marrow-derived and splenic DC was also observed in response to other RNA homopolymers such as polyinosinic and polyguanylic acids, which are known inhibitors of scavenger receptor-mediated endocytosis. Pinocytosis and mannose receptor-mediated uptake of soluble antigen were not affected by any of the tested nucleic acids. The reduction in antigen uptake by dsRNA did not negatively influence the T cell stimulating properties of the DC. In summary, we conclude that the decrease in antigen endocytosis observed in the presence of a variety of TLR agonists is independent of TLR signalling and is caused by competition for specific surface receptors that are involved in the uptake of these TLR agonists and the antigen.

A biotechnological product and its potential as a new immunomodulator for treatment of animal phlebovirus infection: Punta Toro virus

Intracellular pathogens with widespread drug-resistance contribute substantially to the increasing rates in morbidity and mortality due to emerging and reemerging diseases. Thus, the development of new drugs, including those that can enhance the immune response, is urgently needed. The immunomodulator, P-MAPA, a proteinaceous aggregate of ammonium and magnesium phospholinoleate-palmitoleate anhydride derived from Aspergillus oryzae, have been shown to induce antitumor activities. The ability of this compound to elicit protective immunity against viral infections has not been fully explored. Here, we report findings on the use of P-MAPA as an antiviral agent in a mouse model of acute phleboviral (Punta Toro virus) disease. A dose administered i.p. 24h post-infectious challenge (100mg/kg dose of P-MAPA) was remarkably effective at preventing death due to Punta Toro virus infection. This dose also reduced systemic viral burden and liver discoloration assayed on day 3 of infection. Taken together, our findings indicate that non-specific immunotherapy with P-MAPA appears to be an effective treatment for blocking Punta Toro virus-induced disease and suggest that further exploration with other viral disease models is warranted.

Synthetic double-stranded RNAs are adjuvants for the induction of T helper 1 and humoral immune responses to human papillomavirus in rhesus macaques

Toll-like receptor (TLR) ligands are being considered as adjuvants for the induction of antigen-specific immune responses, as in the design of vaccines. Polyriboinosinic-polyribocytoidylic acid (poly I:C), a synthetic double-stranded RNA (dsRNA), is recognized by TLR3 and other intracellular receptors. Poly ICLC is a poly I:C analogue, which has been stabilized against the serum nucleases that are present in the plasma of primates. Poly I:C₁₂U, another analogue, is less toxic but also less stable in vivo than poly I:C, and TLR3 is essential for its recognition. To study the effects of these compounds on the induction of protein-specific immune responses in an animal model relevant to humans, rhesus macaques were immunized subcutaneously (s.c.) with keyhole limpet hemocyanin (KLH) or human papillomavirus (HPV)16 capsomeres with or without dsRNA or a control adjuvant, the TLR9 ligand CpG-C. All dsRNA compounds served as adjuvants for KLH-specific cellular immune responses, with the highest proliferative responses being observed with 2 mg/animal poly ICLC (p = 0.002) or 6 mg/animal poly I:C₁₂U (p = 0.001) when compared with immunization with KLH alone. Notably, poly ICLC—but not CpG-C given at the same dose—also helped to induce HPV16-specific Th1 immune responses while both adjuvants supported the induction of strong anti-HPV16 L1 antibody responses as determined by ELISA and neutralization assay. In contrast, control animals injected with HPV16 capsomeres alone did not develop substantial HPV16-specific immune responses. Injection of dsRNA led to increased numbers of cells producing the T cell–activating chemokines CXCL9 and CXCL10 as detected by in situ hybridization in draining lymph nodes 18 hours after injections, and to increased serum levels of CXCL10 (p = 0.01). This was paralleled by the reduced production of the homeostatic T cell–attracting chemokine CCL21. Thus, synthetic dsRNAs induce an innate chemokine response and act as adjuvants for virus-specific Th1 and humoral immune responses in nonhuman primates.

Novel adjuvants that facilitate the induction of strong cellular immunity could be of help in the design of vaccine strategies to combat infections such as HIV or tuberculosis. Our immune cells possess archaic receptors recognizing structures of infectious pathogens, and the interaction of these receptors with their ligands results in an activation of the immune system. Here we exploited synthetic forms of one of these ligands, i.e., dsRNA, to define an adjuvant for the induction of cellular immune responses in primates. We injected model and viral proteins together with three different forms of dsRNA subcutaneously (s.c.) in rhesus macaques, and all compounds served as adjuvants for the induction of cellular immunity without the incidence of major side effects. These adjuvant effects depended on the adjuvant dose and coincided with profound alterations in the chemokine production in the draining lymph nodes. dsRNA also helped to induce cellular and humoral immune responses against capsomeres of low immunogenicity derived from the human papillomavirus 16, the causative agent in about 50% of all cases of cervical cancer worldwide. Therefore, formulations involving synthetic dsRNA are promising candidates for development of novel vaccines.

Prophylaxis with cationic liposome-DNA complexes protects hamsters from phleboviral disease: importance of liposomal delivery and CpG motifs

Cationic liposome-DNA complexes (CLDC) are cationic/neutral lipid carriers complexed with plasmid DNA that when administered systemically results in a robust T(H)1 cytokine response. CLDC have been shown to be effective in prophylaxis and therapeutic treatment of animal models of viral disease. To determine the contribution of liposomal delivery and CpG content of the plasmid DNA to the efficacy of CLDC; plasmid, CpG-free plasmid DNA, or CpG-containing oligodeoxynucleotides (ODN) with and without liposomes, as well as poly(I:C(12)U), were evaluated for their ability to elicit protection against lethal Punta Toro virus (PTV, Bunyaviridae, phlebovirus) challenge in hamsters. CLDC-containing plasmid significantly improved survival, decreased systemic and liver viral loads, and reduced liver damage due to progression of viral infection. Mouse-reactive ODNs complexed with liposomes failed to protect hamsters, whereas ODNs known to cross-react with human and mouse (CpG 2006) or non-liposomal poly(I:C(12)U) showed survival benefit but did not limit liver injury. Liposomes complexed with a non-CpG motif-containing plasmid reduced liver viral load and tissue damage, but did not protect hamsters from death. To evaluate the mechanisms of the enhanced activity of CLDC, microarray experiments examined differences in the gene expression profile. The results suggest a broad T(H)1 response elicited by liposomal delivery of a diverse sequence containing CpG and non-CpG elements may be a more effective antiviral treatment than other nucleic acid based immunotherapeutics.

Targeting poly(I:C) to the TLR3-independent pathway boosts effector CD8 T cell differentiation through IFN-alpha/beta

Poly(I:C) is an adjuvant used for antitumor treatment and vaccines because of its prominent effects on CD8 T cells and NK cells. Poly(I:C) binds TLR3 and this interaction is thought to be central for driving cell-mediated immune responses. We investigated the importance of TLR3 in poly(I:C)-mediated endogenous CD8 T cell responses using the pathogenic T cell stimulant Staphylococcus aureus enterotoxin A. While the responsive CD8 T cells expanded comparably in both wild-type and TLR3(-/-) mice, differentiation of effector CD8 T cells was enhanced by poly(I:C) in the TLR3(-/-) mice. A higher percentage of Ag-specific CD8 T cells became IFN-gamma and TNF-alpha producers in the absence of TLR3 signaling. Consistent with this boosted response was the observation that TLR3-deficient cells synthesized less IL-10 compared with TLR3-sufficient cells in response to poly(I:C). Ultimately, however, the fundamental mechanism of CD8 effector T cell differentiation through the TLR3-independent pathway was shown to be completely IFN-alpha/beta-dependent. Administration of IFN-alpha/beta-neutralizing Abs abolished the poly(I:C) effects in TLR3(-/-) mice. These findings reveal specific roles of how dsRNA receptors shape CD8 T cell responses, which should be considered as poly(I:C) is authenticated as a therapeutic adjuvant used in vaccines.

Innate sensors of influenza virus: clues to developing better intranasal vaccines

Mucosal immunity acquired by natural infection with influenza viruses at the respiratory tract is more effective and cross-protective against subsequent variant virus infection than systemic immunity induced by parenteral immunization with inactivated vaccines. To develop an effective influenza vaccine, it is beneficial to mimic the process of natural infection that bridges innate and adaptive immune systems. The innate immune system that recognizes influenza virus infection consists of several classes of pattern-recognition receptors, including the Toll-like receptors, the retinoic acid-inducible gene-I-like receptors and the NOD-like receptors. Here, we review our current understanding of the mechanism of innate recognition of influenza and how the signals emanating from the innate sensors control adaptive immunity. Further, we discuss the potential roles of these receptors in developing intranasal influenza vaccines.

A clinical grade poly I:C-analogue (Ampligen) promotes optimal DC maturation and Th1-type T cell responses of healthy donors and cancer patients in vitro

Maturation of dendritic cells (DC) can be triggered in vitro by inflammatory cytokines or Toll-like receptor (TLR) ligands such as CpG or polyI:C. Corresponding, well-characterized agents which can be applied in clinical settings are sparse. We have evaluated a clinical grade, non-toxic analogue of polyI:C, poly(I:C12U) (Ampligen), as a potential adjuvant for cancer immunotherapy, for its ability to drive maturation of human myeloid DC. Our results provide evidence that poly(I:C12U) is effective in inducing optimal phenotypic (elevated levels of MHC-Class I/Class II, CD83, CCR7, CD86 and CD40 molecules) and functional maturation of human DC in vitro, capable of promoting the production of the inflammatory (Th1-type) cytokine IL-12, with significantly lower levels of IL-10 production, compared to that induced by the parent compound polyI:C. Importantly, poly(I:C12U) has a comparable effect on the maturation and function of DC derived either from healthy donors or cancer patients indicating that it is able to overcome any immune suppressive factors associated with the tumour bearing state. These characteristics make poly(I:C12U) a suitable agent for use as an adjuvant in cancer directed immunotherapeutic regimes.

TLR3 is an endogenous sensor of tissue necrosis during acute inflammatory events

Ligands from dying cells are a source of Toll-like receptor (TLR) activating agents. Although TLR3 is known to respond to RNA from necrotic cells, the relative importance of this response in vivo during acute inflammatory processes has not been fully explored. We observed the involvement of TLR3 activation during experimental polymicrobial septic peritonitis and ischemic gut injury in the absence of an exogenous viral stimulus. In TLR3-deficient mice, increased chemokine/cytokine levels and neutrophil recruitment characterized the initial inflammatory responses in both injury models. However, the levels of inflammatory chemokines and tumor necrosis factor α quickly returned to baseline in tlr3^−/− mice, and these mice were protected from the lethal effects of sustained inflammation. Macrophages from tlr3^−/− mice responded normally to other TLR ligands but did not respond to RNA from necrotic neutrophils. Importantly, an immunoneutralizing antibody directed against TLR3 attenuated the generation of inflammatory chemokines evoked by byproducts from necrotic neutrophils cultured with wild-type macrophages. In vivo, anti-TLR3 antibody attenuated the tissue injury associated with gut ischemia and significantly decreased sepsis-induced mortality. Collectively, these data show that TLR3 is a regulator of the amplification of immune response and serves an endogenous sensor of necrosis, independent of viral activation.

Accessing the therapeutic potential of immunostimulatory nucleic acids

Short synthetic CpG oligodesoxynucleotides (CpG ODN) that activate plasmacytoid dendritic cells (PDC) and B-cells via the endosomal Toll-like receptor (TLR) 9 are the prototypical immunostimulatory nucleic acids (NAs), and their therapeutic potential is currently evaluated in numerous clinical trials. In recent years, NA recognition has emerged as a general means of virus detection by the innate immune system that involves at least seven receptors located in either the endosome or the cytosol. Endosomal TLRs are expressed selectively, and predominantly by immune cell types. By contrast, the expression of cytosolic NA receptors is ubiquitous, significantly widening the range of cell types that can be stimulated therapeutically by NAs to include even tumor cells. Here we discuss unique properties of each of these receptors, and argue that an understanding of the molecular basis of receptor-ligand interactions, and the development of chemically defined, selective ligands is required in order to fully realize the promise that NA immunetherapeutics hold.

Function of TRADD in tumor necrosis factor receptor 1 signaling and in TRIF-dependent inflammatory responses

Tumor necrosis factor receptor 1 (TNFR1) and Toll-like receptors (TLRs) regulate immune and inflammatory responses. Here we show that the TNFR1-associated death domain protein (TRADD) is critical in TNFR1, TLR3 and TLR4 signaling. TRADD deficiency abrogated TNF-induced apoptosis, prevented recruitment of the ubiquitin ligase TRAF2 and ubiquitination of the adaptor RIP1 in the TNFR1 signaling complex, and considerably inhibited but did not completely abolish activation of the transcription factor NF-kappaB and mitogen-activated protein kinases 'downstream' of TNFR1. TRIF-dependent cytokine production induced by the synthetic double-stranded RNA poly(I:C) and lipopolysaccharide was lower in TRADD-deficient mice than in wild-type mice. Moreover, TRADD deficiency inhibited poly(I:C)-mediated RIP1 ubiquitination and activation of NF-kappaB and mitogen-activated protein kinase signaling in fibroblasts but not in bone marrow macrophages. Thus, TRADD is an essential component of TNFR1 signaling and has a critical but apparently cell type-specific function in TRIF-dependent TLR responses.

Sequence- and target-independent angiogenesis suppression by siRNA via TLR3

Clinical trials of small interfering RNA (siRNA) targeting vascular endothelial growth factor-A (VEGFA) or its receptor VEGFR1 (also called FLT1), in patients with blinding choroidal neovascularization (CNV) from age-related macular degeneration, are premised on gene silencing by means of intracellular RNA interference (RNAi). We show instead that CNV inhibition is a siRNA-class effect: 21-nucleotide or longer siRNAs targeting non-mammalian genes, non-expressed genes, non-genomic sequences, pro- and anti-angiogenic genes, and RNAi-incompetent siRNAs all suppressed CNV in mice comparably to siRNAs targeting Vegfa or Vegfr1 without off-target RNAi or interferon-alpha/beta activation. Non-targeted (against non-mammalian genes) and targeted (against Vegfa or Vegfr1) siRNA suppressed CNV via cell-surface toll-like receptor 3 (TLR3), its adaptor TRIF, and induction of interferon-gamma and interleukin-12. Non-targeted siRNA suppressed dermal neovascularization in mice as effectively as Vegfa siRNA. siRNA-induced inhibition of neovascularization required a minimum length of 21 nucleotides, a bridging necessity in a modelled 2:1 TLR3-RNA complex. Choroidal endothelial cells from people expressing the TLR3 coding variant 412FF were refractory to extracellular siRNA-induced cytotoxicity, facilitating individualized pharmacogenetic therapy. Multiple human endothelial cell types expressed surface TLR3, indicating that generic siRNAs might treat angiogenic disorders that affect 8% of the world's population, and that siRNAs might induce unanticipated vascular or immune effects.

Sensing of viral infection and activation of innate immunity by toll-like receptor 3

Toll-like receptors (TLRs) form a major group of transmembrane receptors that are involved in the detection of invading pathogens. Double-stranded RNA is a marker for viral infection that is recognized by TLR3. TLR3 triggering activates specific signaling pathways that culminate in the activation of NF-kappaB and IRF3 transcription factors, as well as apoptosis, enabling the host to mount an effective innate immune response through the induction of cytokines, chemokines, and other proinflammatory mediators. In this review, we describe the paradoxical role of TLR3 in innate immunity against different viruses and in viral pathogenesis but also the evidence for TLR3 as a "danger" receptor in nonviral diseases. We also discuss the structure and cellular localization of TLR3, as well as the complex signaling and regulatory events that contribute to TLR3-mediated immune responses.

The microbial mimic poly IC induces durable and protective CD4+ T cell immunity together with a dendritic cell targeted vaccine

CD4(+) Th1 type immunity is implicated in resistance to global infectious diseases. To improve the efficacy of T cell immunity induced by human immunodeficiency virus (HIV) vaccines, we are developing a protein-based approach that directly harnesses the function of dendritic cells (DCs) in intact lymphoid tissues. Vaccine proteins are selectively delivered to DCs by antibodies to DEC-205/CD205, a receptor for antigen presentation. We find that polyriboinosinic:polyribocytidylic acid (poly IC) independently serves as an adjuvant to allow a DC-targeted protein to induce protective CD4(+) T cell responses at a mucosal surface, the airway. After two doses of DEC-targeted, HIV gag p24 along with poly IC, responder CD4(+) T cells have qualitative features that have been correlated with protective function. The T cells simultaneously make IFN-gamma, tumor necrosis factor (TNF)-alpha, and IL-2, and in high amounts for prolonged periods. The T cells also proliferate and continue to secrete IFN-gamma in response to HIV gag p24. The adjuvant role of poly IC requires Toll-like receptor (TLR) 3 and melanoma differentiation-associated gene-5 (MDA5) receptors, but its analog poly IC(12)U requires only TLR3. We suggest that poly IC be tested as an adjuvant with DC-targeted vaccines to induce numerous multifunctional CD4(+) Th1 cells with proliferative capacity.

Animal models of highly pathogenic RNA viral infections: hemorrhagic fever viruses

A diverse group of highly pathogenic RNA viruses cause a severe multisystemic illness in humans commonly referred to as viral hemorrhagic fever (VHF). Although they can vary widely in clinical presentation, all VHFs share certain features that include intense fever, malaise, bleeding and shock. Effective antiviral therapies for most of the VHFs are lacking. Complicating development of intervention strategies is the relative infrequency and unpredictability of VHF outbreaks making human clinical trials extremely challenging or unfeasible. Therefore, animal models that can recapitulate human disease are essential to the development of effective antivirals and vaccines. In general, a good animal model of VHF will demonstrate systemic dispersion of the virus through infection of mononuclear phagocytes and dendritic cells, which induces the release of inflammatory mediators that increase vascular permeability and facilitate coagulation. The culmination of this process leads to significant loss of plasma volume and terminal hypovolemic shock. Although it is clear that nonhuman primate models are the most faithful to human disease, the more accessible and less costly rodent models, including those based on infection with related surrogate viruses, can reproduce certain components of VHF and can serve as suitable preclinical models for initial development of effective countermeasures. Such models are sufficient for testing of drugs that directly block viral replication, but may be inadequate for evaluating therapies that depend for their success on the activation or inhibition of host responses.