The untranslated regions (UTRs) of the foot-and-mouth disease virus (FMDV) genome contain multiple functional elements. In the 5′ UTR, the internal ribosome entry site (IRES) element governs cap-independent translation initiation, whereas the S region is presumably involved in RNA replication. The 3′ UTR, composed of two stem–loops and a poly(A) tract, is required for viral infectivity and stimulates IRES activity. Here, it was found that the 3′ end established two distinct strand-specific, long-range RNA–RNA interactions, one with the S region and another with the IRES element. These interactions were not observed with the 3′ UTR of a different picornavirus. Several results indicated that different 3′ UTR motifs participated in IRES or S region interactions. Firstly, a high-order structure adopted by both the entire IRES and the 3′ UTR was essential for RNA interaction. In contrast, the S region interacted with each of the stem–loops. Secondly, S–3′ UTR interaction but not IRES–3′ UTR interaction was dependent on a poly(A)-dependent conformation. However, no other complexes were observed in mixtures containing the three transcripts, suggesting that these regions did not interact simultaneously with the 3′ UTR probe. Cellular proteins have been found to bind the S region and one of these also binds to the 3′ UTR in a competitive manner. Our data suggest that 5′–3′-end bridging through both direct RNA–RNA contacts and RNA–protein interactions may play an essential role in the FMDV replication cycle.

Foot-and-mouth disease virus (FMDV) is the causative agent of an acute vesicular disease affecting pigs, cattle and other domestic and wild animals worldwide. The aim of the host interferon (IFN) response is to limit viral replication and spread. Detection of the viral genome and products by specialized cellular sensors initiates a signaling cascade that leads to a rapid antiviral response involving the secretion of type I- and type III-IFNs and other antiviral cytokines with antiproliferative and immunomodulatory functions. During co-evolution with their hosts, viruses have acquired strategies to actively counteract host antiviral responses and the balance between innate response and viral antagonism may determine the outcome of disease and pathogenesis. FMDV proteases Lpro and 3C have been found to antagonize the host IFN response by a repertoire of mechanisms. Moreover, the putative role of other viral proteins in IFN antagonism is being recently unveiled, uncovering sophisticated immune evasion strategies different to those reported to date for other members of the Picornaviridae family. Here, we review the interplay between antiviral responses induced by FMDV infection and viral countermeasures to block them. Research on strategies used by viruses to modulate immunity will provide insights into the function of host pathways involved in defense against pathogens and will also lead to development of new therapeutic strategies to fight virus infections.

We constructed foot-and-mouth disease virus (FMDV) mutants bearing independent deletions of the two stem-loop structures predicted in the 3′ noncoding region of viral RNA, SL1 and SL2, respectively. Deletion of SL2 was lethal for viral infectivity in cultured cells, while deletion of SL1 resulted in viruses with slower growth kinetics and downregulated replication associated with impaired negative-strand RNA synthesis. With the aim of exploring the potential of an RNA-based vaccine against foot-and-mouth disease using attenuated viral genomes, full-length chimeric O1K/C-S8 RNAs were first inoculated into pigs. Our results show that FMDV viral transcripts could generate infectious virus and induce disease in swine. In contrast, RNAs carrying the ΔSL1 mutation on an FMDV O1K genome were innocuous for pigs but elicited a specific immune response including both humoral and cellular responses. A single inoculation with 500 μg of RNA was able to induce a neutralizing antibody response. This response could be further boosted by a second RNA injection. The presence of the ΔSL1 mutation was confirmed in viruses isolated from serum samples of RNA-inoculated pigs or after transfection and five passages in cell culture. These findings suggest that deletion of SL1 might contribute to FMDV attenuation in swine and support the potential of RNA technology for the design of new FMDV vaccines.

The RNA helicase LGP2 (Laboratory of Genetics and Physiology 2) is a non-signaling member of the retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), whose pivotal role on innate immune responses against RNA viruses is being increasingly uncovered. LGP2 is known to work in synergy with melanoma differentiation-associated gene 5 (MDA5) to promote the antiviral response induced by picornavirus infection. Here, we describe the activity of the foot-and-mouth disease virus (FMDV) Leader protease (Lpro) targeting LGP2 for cleavage. When LGP2 and Lpro were co-expressed, cleavage products were observed in an Lpro dose-dependent manner while co-expression with a catalytically inactive Lpro mutant had no effect on LGP2 levels or pattern. We further show that Lpro localizes and immunoprecipitates with LGP2 in transfected cells supporting their interaction within the cytoplasm. Evidence of LGP2 proteolysis was also detected during FMDV infection. Moreover, the inhibitory effect of LGP2 overexpression on FMDV growth observed was reverted when Lpro was co-expressed, concomitant with lower levels of IFN-β mRNA and antiviral activity in those cells. The Lpro target site in LGP2 was identified as an RGRAR sequence in a conserved helicase motif whose replacement to EGEAE abrogated LGP2 cleavage by Lpro. Taken together, these data suggest that LGP2 cleavage by the Leader protease of aphthoviruses may represent a novel antagonistic mechanism for immune evasion. Foot-and-mouth disease virus (FMDV) is the causative agent of a devastating disease affecting livestock worldwide. FMDV is considered an extremely successful pathogen able to replicate and spread rapidly among its hosts. The induction of type-I interferon (IFN) response is a crucial event in mammalian cells against infections, and viruses have evolved a variety of mechanisms to avoid it. LGP2 (Laboratory of Genetics and Physiology 2) is a cellular protein involved in sensing viral RNA during infection and plays a relevant role on regulation of the signaling pathways leading to IFN induction. Here we show that LGP2 is specifically targeted for cleavage by the FMDV-encoded Leader protease (Lpro) and a correlation with a decrease in the IFN levels induced in infected cells. Our data unveil a new viral mechanism of immune evasion based on direct cleavage of LGP2 by the Leader proteases of aphthoviruses. To our knowledge this is the first report of the proteolytic cleavage of LGP2 by a virally-encoded protease. Our findings will contribute to a better understanding of the virus-host interplay involved in pathogenesis and to the development of more efficient strategies to combat infectious diseases.

Vaccines are considered one of the greatest global health achievements, improving the welfare of society by saving lives and substantially reducing the burden of infectious diseases. However, few vaccines are fully effective, for reasons ranging from intrinsic limitations to more contingent shortcomings related, e.g., to cold chain transport, handling and storage. In this context, subunit vaccines where the essential antigenic traits (but not the entire pathogen) are presented in rationally designed fashion have emerged as an attractive alternative to conventional ones. In particular, this includes the option of fully synthetic peptide vaccines able to mimic well-defined B- and T-cell epitopes from the infectious agent and to induce protection against it. Although, in general, linear peptides have been associated to low immunogenicity and partial protection, there are several strategies to address such issues. In this review, we report the progress towards the development of peptide-based vaccines against foot-and-mouth disease (FMD) a highly transmissible, economically devastating animal disease. Starting from preliminary experiments using single linear B-cell epitopes, recent research has led to more complex and successful second-generation vaccines featuring peptide dendrimers containing multiple copies of B- and T-cell epitopes against FMD virus or classical swine fever virus (CSFV). The usefulness of this strategy to prevent other animal and human diseases is discussed.