**5. Conclusions**

backbone [61]. Mice receiving the vaccine exhibited increased virus-specific serum antibody response as compared to those receiving the DNA vaccine alone. These results suggest that RLR signaling can enhance antibody development after vaccination by activation of innate immunity and improved adaptive immune responses. However, in a previous study Koyama et al. found a defect in antigen-specific B and T cell activation in MyD88-defficient mice, unlike MAVS-deficient mice, suggesting that adaptive immune responses against influenza A virus are governed by the TLR pathway [62]. On the contrary, MAVS-deficient mice infected with WNV displayed uncontrolled inflammation including elevated systemic IFN, proinflamma‐ tory cytokine and chemokine levels, and enhanced humoral responses marked by complete loss of virus neutralization activity with a failure to protect against WNV infection [63]. This work defined an innate/adaptive immune interface mediated through MAVS-dependent RLR signaling that regulates the quantity, quality and balance of the immune response to WNV. Using MDA5-defficient mice, Anz et al. showed that the loss of T regulatory cell function on infection with encephalomyocarditis virus (EMCV) is strictly dependent on RLR signaling [64]. In a different study, a normal and effective adaptive immune response against respiratory syncytial virus (RSV) infection was reported in the absence of both MAVS and MyD88, RLRand TLR-adaptor molecules, respectively [65]. All together, the results suggest that, in the case of flavivirus and picornavirus infections, RLR signaling is important in the control of the quantity, quality and balance of the adaptive immune response, with specific and differential mechanisms of regulation operating depending on the specific virus infections. The under‐ standing of the molecular features underlying these processes could offer new strategies for immune and antiviral therapy targeting the RLR pathway for therapeutic control of the viral

The vaccine development field is evolving from traditional whole cell vaccines to more defined and safer subunit vaccines. In the idea of exploiting the innate responses in vaccine adjuvant design, a growing demand for the use of immunopotentiators in poorly immunogenic subunit vaccines is arising with the development of a new generation of vaccine adjuvants. New vaccine adjuvants are designed to improve the recruitment and activation of dendritic cells, then enabling transition from the innate to adaptive immune system for priming of B- and Tcell responses. Endogenous or therapeutically induced early type-I IFN responses may confer protection until adaptive immunity is activated to an extent that the pathogen can be elimi‐ nated. In that context, PRRs come into sight as targets of new vaccine adjuvants beside their

Evidence is now emerging that many empiric vaccines and adjuvants inherently stimulate PRRs, like the yellow fever vaccine 17D, one of the most effective vaccines available, shown to activate multiple DC subsets through stimulation of several TLRs (including TLR-7, -8 and -9) [66], highlighting the potential of vaccination strategies that use combinations of different

The current vaccine adjuvants licensed for use in human vaccines are limited [67], but other PRR agonists in clinical stages of development are emerging as potential vaccine adjuvant candidates [68], such as the TLR3 and MDA5 agonist poly I:C, a promising mucosal adjuvant for intranasal H5N1 influenza vaccination [69]. Clearly, TLR agonists are the most clinically

infection and enhancement of the induced immune response.

202 Current Issues in Molecular Virology - Viral Genetics and Biotechnological Applications

PRRs ligands to stimulate polyvalent immune responses.

role as sentinels in innate immunity.

Understanding how the innate immune system senses the infection of different viruses with a variety of genome structures and signals and the crosstalk between different PRRs will help to understand the complex regulation of immunity to infection. The increasing knowledge on the nature of PAMPs and sensor specificity will surely contribute to the development of safer and more effective vaccines for infectious diseases. PRR agonists arise as promising molecules due to their synergistic effects on cytokine production and contributing to effective immune responses. The success of rationally designed vaccine formulations in the near future will likely correlate with the advances on understanding cell signalling mechanisms as well as PRR adjuvanticity and response outcomes. Targeted immunomodulatory strategies will require knowledge of the virus-specific aspects of the pathway. Viral proteins with IFN antagonistic activity are potential drug targets for antiviral strategies. Moreover, small, synthetic and noninfectious RNAs mimicking viral PAMPs can act as potent IFN inducers and exert an antiviral effect in vivo, providing new insight into broad-spectrum antiviral development strategies.

**LGP2** Laboratory of genetics and physiology-2

Use of RNA Domains in the Viral Genome as Innate Immunity Inducers for Antiviral Strategies and Vaccine

Improvement

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http://dx.doi.org/10.5772/56099

**MDA5** Melanoma differentiation-associated gene 5

**NLR** Nucleotide oligomerization domain (NOD-like receptor)

**MAP Kinase** Mitogen-activated protein kinase **MAVS** Mitochondrial antiviral signaling

**MHC** Major histocompatibility complex

**PAMP** Pathogen-associated molecular pattern

**Poly I:C** Polyriboinosinic-polyribocytidylic acid **Poly ICLC** poly-L-lysine and carboxymethyl cellulose

**miRNA** Micro RNA

**NCR** Non-coding region **NK** Natural killer

**ODNs** Oligodeoxynucleotides **ORF** Open reading frame

**pDC** Plasmacytoid dendritic cell **PFU** Plaque forming units **PKR** Protein kinase R

**PRR** Pattern-recognition receptor **RIG-I** Retinoic acid-inducible gene-1

**RSV** Respiratory syncytial virus **siRNA** Small interfering RNA **SL** Stem-loop structure **ss** Single-stranded **svRNA** Small viral RNA

**TIR** Toll/Interleukin-1 receptor

**TRIM** Tripartite motif protein **VPg** Virus encoded protein

**TRIF** TIR-domain-containing adaptor inducing interferon

**Table 1.** List of the acronyms and abbreviations used in this chapter

**TLR** Toll-like receptor **TNF** Tumor necrosis factor

**WNV** West Nile virus

**RLR** RIG-I-like receptor
