Pigeon RIG-I Function in Innate Immunity against H9N2 IAV and IBDV

Transcriptome-wide analyses of early immune responses in lumpfish leukocytes upon stimulation with poly(I:C)

Background

Both bacterial and viral diseases are a major threat to farmed fish. As the antiviral immune mechanisms in lumpfish (Cyclopterus lumpus L.) are poorly understood, lumpfish leukocytes were stimulated with poly(I:C), a synthetic analog of double stranded RNA, which mimic viral infections, and RNA sequencing was performed.

Methods

To address this gap, we stimulated lumpfish leukocytes with poly(I:C) for 6 and 24 hours and did RNA sequencing with three parallels per timepoint. Genome guided mapping was performed to define differentially expressed genes (DEGs).

Results

Immune genes were identified, and transcriptome-wide analyses of early immune responses showed that 376 and 2372 transcripts were significantly differentially expressed 6 and 24 hours post exposure (hpe) to poly(I:C), respectively. The most enriched GO terms when time had been accounted for, were immune system processes (GO:0002376) and immune response (GO:0006955). Analysis of DEGs showed that among the most highly upregulated genes were TLRs and genes belonging to the RIG-I signaling pathway, including LGP2, STING and MX, as well as IRF3 and IL12A. RIG-I was not identified, but in silico analyses showed that genes encoding proteins involved in pathogen recognition, cell signaling, and cytokines of the TLR and RIG-I signaling pathway are mostly conserved in lumpfish when compared to mammals and other teleost species.

Conclusions

Our analyses unravel the innate immune pathways playing a major role in antiviral defense in lumpfish. The information gathered can be used in comparative studies and lay the groundwork for future functional analyses of immune and pathogenicity mechanisms. Such knowledge is also necessary for the development of immunoprophylactic measures for lumpfish, which is extensively cultivated for use as cleaner fish in the aquaculture for removal of sea lice from Atlantic salmon (Salmo salar L.).

Role of CIV NS1 Protein in Innate Immunity and Viral Replication

The innate immune pathway serves as the first line of defense against viral infections and plays a crucial role in the host's immune response in clearing viruses. Prior research has indicated that the influenza A virus has developed various strategies to avoid host immune responses. Nevertheless, the role of the NS1 protein of the canine influenza virus (CIV) in the innate immune pathway remains unclear. In this study, eukaryotic plasmids of NS1, NP, PA, PB1, and PB2 were constructed, and it was found that these proteins interact with melanoma differentiation-associated gene 5 (MDA5) and antagonize the activation of IFN-β promoters by MDA5. We selected the NS1 protein for further study and found that NS1 does not affect the interaction between the viral ribonucleoprotein (RNP) subunit and MDA5, but that it downregulates the expression of the laboratory of genetics and physiology 2 (LGP2) and retinoic acid-inducible gene-I (RIG-I) receptors in the RIG-I pathway. Additionally, NS1 was found to inhibit the expression of several antiviral proteins and cytokines, including MX dynamin like GTPase 1 (MX1), 2'-5'oligoadenylate synthetase (OAS), Signal Transducers and Activators of Transcription (STAT1), tripartite motif 25 (TRIM25), interleukin-2 (IL-2), IFN, IL-8, and IL-1β. To further investigate the role of NS1, a recombinant H3N2 virus strain (rH3N2) and an NS1-null virus (rH3N2ΔNS1) were rescued using reverse-genetic technology. The rH3N2ΔNS1 virus exhibited lower viral titers compared to rH3N2, but had a stronger activation effect on the receptors LGP2 and RIG-I. Furthermore, when compared to rH3N2, rH3N2ΔNS1 exhibited a more pronounced activation of antiviral proteins such as MX1, OAS, STAT1, and TRIM25, as well as antiviral cytokines such as IL-6, IFN-β, and IL-1β. These findings suggest a new mechanism by which NS1, a nonstructural protein of CIV, facilitates innate immune signaling and provides new avenues for the development of antiviral strategies.

A Role for the Chicken Interferon-Stimulated Gene CMPK2 in the Host Response Against Virus Infection

Virus infection can lead to the production of interferon, which activates the JAK/STAT pathway and induces the expression of multiple downstream interferon-stimulated genes (ISGs) to achieve their antiviral function. Cytidine/uridine monophosphate kinase 2 (CMPK2) gene has been identified as an ISG in human and fish, and is also known as a rate-limiting enzyme in mitochondria to maintain intracellular UTP/CTP levels, which is necessary for de novo mitochondrial DNA synthesis. By mining previous microarray data, it was found that both Avian Influenza Virus (AIV) and Newcastle Disease Virus (NDV) infection can lead to the significant upregulation of chicken CMPK2 gene. However, little is known about the function of CMPK2 gene in chickens. In the present study, the open reading frame (ORF) of chicken CMPK2 (chCMPK2) was cloned from DF-1, a chicken embryo fibroblasts cell line, and subjected to further analysis. Sequence analysis showed that chCMPK2 shared high similarity in amino acid with CMPK2 sequences from all the other species, especially reptiles. A thymidylate kinase (TMK) domain was identified in the C-terminus of chCMPK2, which is highly conserved among all species. In vitro, AIV infection induced significant increases in chCMPK2 expression in DF-1, HD11, and the chicken embryonic fibroblasts (CEF), while obvious increase only detected in DF-1 cells and CEF cells after NDV infection. In vivo, the expression levels of chCMPK2 were up-regulated in several tissues from AIV infected chickens, especially the brain, spleen, bursa, kidney, intestine, heart and thymus, and notable increase of chCMPK2 was detected in the bursa, kidney, duodenum, lung, heart, and thymus during NDV infection. Here, using MDA5 and IFN-β knockdown cells, we demonstrated that as a novel ISG, chCMPK2 could be regulated by the MDA5/IFN-β pathway. The high expression level of exogenous chCMPK2 displayed inhibitory effects on AIV and NDV as well as reduced viral RNA in infected cells. We further demonstrated that Asp135, a key site on the TMK catalytic domain, was identified as critical for the antiviral activities of chCMPK2. Taken together, these data demonstrated that chCMPK2 is involved in the chicken immune system and may play important roles in host anti-viral responses.

Evolution of RNA sensing receptors in birds

Birds are important hosts for many RNA viruses, including influenza A virus, Newcastle disease virus, West Nile virus and coronaviruses. Innate defense against RNA viruses in birds involves detection of viral RNA by pattern recognition receptors. Several receptors of different classes are involved, such as endosomal toll-like receptors and cytoplasmic retinoic acid-inducible gene I-like receptors, and their downstream adaptor proteins. The function of these receptors and their antagonism by viruses is well established in mammals; however, this has received less attention in birds. These receptors have been characterized in a few bird species, and the completion of avian genomes will permit study of their evolution. For each receptor, functional work has established ligand specificity and activation by viral infection. Engagement of adaptors, regulation by modulators and the supramolecular organization of proteins required for activation are incompletely understood in both mammals and birds. These receptors bind conserved nucleic acid agonists such as single- or double-stranded RNA and generally show purifying selection, particularly the ligand binding regions. However, in birds, these receptors and adaptors differ between species, and between individuals, suggesting that they are under selection for diversification over time. Avian receptors and signalling pathways, like their mammalian counterparts, are targets for antagonism by a variety of viruses, intent on escape from innate immune responses.

The expression of IFN-β is suppressed by the viral 3D polymerase via its impact on PGAM5 expression during enterovirus D68 infection

Enterovirus D68 (EV-D68) belongs to the Picornaviridae family and can lead to severe clinical manifestations in the respiratory system. The 3D-polymerase (3D^poly) is an important nonstructural protein during EV-D68 replication, but few studies have addressed its interaction with the host antiviral response during EV-D68 infection. Here, we used human bronchial epithelial cells to investigate the impact of the 3D^poly on the mitochondrial dynamics and innate immune response. The results showed that the number and morphology of the mitochondria in 16HBE cells was affected during the early stage of infection, and these effects included the cellular apoptosis. Moreover, we found that the 3D^poly of EV-D68 can interact with PGAM5 and promote mitofusin 2 protein upregulation, and subsequently, 3D^poly impairs IFN-β expression by impacting the activation of the RIG-I receptor signaling pathway. Our findings suggest that during EV-D68 replication, the 3D^poly, via its interaction with PGAM5, can affect the mitochondrial dynamics and suppress the expression of IFN-β by impacting the RIG-I-like receptor signal pathway.

Antiviral Activity of Canine RIG-I against Canine Influenza Virus and Interactions between Canine RIG-I and CIV

RIG-I functions as a virus sensor that induces a cellular antiviral response. Although it has been investigated in other species, there have been no further studies to date on canine RIG-I against canine influenza virus (CIV). In the present study, we cloned the RIG-I gene of beagle dogs and characterized its expression, subcellular localization, antiviral response, and interactions with CIV proteins. RIG-I was highly expressed and mainly localized in the cytoplasm, with low levels detected in the nucleus. The results revealed that overexpression of the CARD domain of RIG-I and knockdown of RIG-I showed its ability to activate the RLR pathway and induced the expression of downstream interferon-stimulated genes. Moreover, overexpression of canine RIG-I suppressed the replication of CIV. The association between RIG-I and CIV was evaluated with the luciferase assay and by indirect immunofluorescence and bimolecular fluorescence complementation analyses. The results showed that CIV nonstructural protein 1 (NS1) can strongly suppress the RIG-I–mediated innate immune response, and the novel interactions between CIV matrix proteins (M1 and M2) and canine RIG-I were disclosed. These findings provide a basis for investigating the antiviral mechanism of canine RIG-I against CIV, which can lead to effective strategies for preventing CIV infection in dogs.

Host immune responses of pigeons infected with Newcastle disease viruses isolated from pigeons

Newcastle disease (ND), affecting over 250 bird species, is caused by the Newcastle disease virus (NDV). ND is one of the leading causes of morbidity and mortality in pigeons. Most studies investigating NDV in pigeons have focused on the epidemiology and pathogenicity of the virus. However, the host immune responses in pigeons infected with NDVs remains largely unclear. In this study, we investigated the host immune responses in pigeons infected with two NDV stains, a pigeon paramyxovirus type 1(PPMV-1) strain, GZH14, and a genotype II virus, KP08. Although no mortality was observed upon infection with either virus, obvious neurological effects were observed in the GZH14-infected pigeons but not in the KP08-infected pigeons. Both viruses could replicate in the examined tissues, namely brain, lung, spleen, trachea, kidney, and bursa of Fabricius. The expression level of RIG-I, IL-6, IL-1β, CCL5, and IL-8 were up-regulated by both viruses in the brain, lung and spleen at 3 and 7 days post-infection. Notably, these proinflammatory cytokines and chemokines showed more intense expression in the brain, when induced by the GZH14 strain than with the KP08 strain. These results indicate that the intense inflammatory responses induced by PPMV-1 in the brain may be a critical determinant of neurological symptoms in pigeons infected with PPMV-1. Our study provides new insight into the pathogenicity of PPMV-1 in pigeons attributable to the host immune responses.

Creating Disease Resistant Chickens: A Viable Solution to Avian Influenza?

Influenza A virus (IAV) represents an ongoing threat to human and animal health worldwide. The generation of IAV-resistant chickens through genetic modification and/or selective breeding may help prevent viral spread. The feasibility of creating genetically modified birds has already been demonstrated with the insertion of transgenes that target IAV into the genomes of chickens. This approach has been met with some success in minimising the spread of IAV but has limitations in terms of its ability to prevent the emergence of disease. An alternate approach is the use of genetic engineering to improve host resistance by targeting the antiviral immune responses of poultry to IAV. Harnessing such resistance mechanisms in a “genetic restoration” approach may hold the greatest promise yet for generating disease resistant chickens. Continuing to identify genes associated with natural resistance in poultry provides the opportunity to identify new targets for genetic modification and/or selective breeding. However, as with any new technology, economic, societal, and legislative barriers will need to be overcome before we are likely to see commercialisation of genetically modified birds.

Applications of Gene Editing in Chickens: A New Era Is on the Horizon

The chicken represents a valuable model for research in the area of immunology, infectious diseases as well as developmental biology. Although it was the first livestock species to have its genome sequenced, there was no reverse genetic technology available to help understanding specific gene functions. Recently, homologous recombination was used to knockout the chicken immunoglobulin genes. Subsequent studies using immunoglobulin knockout birds helped to understand different aspects related to B cell development and antibody production. Furthermore, the latest advances in the field of genome editing including the CRISPR/Cas9 system allowed the introduction of site specific gene modifications in various animal species. Thus, it may provide a powerful tool for the generation of genetically modified chickens carrying resistance for certain pathogens. This was previously demonstrated by targeting the Trp38 region which was shown to be effective in the control of avian leukosis virus in chicken DF-1 cells. Herein we review the current and future prospects of gene editing and how it possibly contributes to the development of resistant chickens against infectious diseases.

Functional Evolution of Avian RIG-I-Like Receptors

RIG-I-like receptors (retinoic acid-inducible gene-I-like receptors, or RLRs) are family of pattern-recognition receptors for RNA viruses, consisting of three members: retinoic acid-inducible gene I (RIG-I), melanoma differentiation-associated gene 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2). To understand the role of RLRs in bird evolution, we performed molecular evolutionary analyses on the coding genes of avian RLRs using filtered predicted coding sequences from 62 bird species. Among the three RLRs, conservation score and dN/dS (ratio of nonsynonymous substitution rate over synonymous substitution rate) analyses indicate that avian MDA5 has the highest conservation level in the helicase domain but a lower level in the caspase recruitment domains (CARDs) region, which differs from mammals; LGP2, as a whole gene, has a lower conservation level than RIG-I or MDA5. We found evidence of positive selection across all bird lineages in RIG-I and MDA5 but only on the stem lineage of Galliformes in LGP2, which could be related to the loss of RIG-I in Galliformes. Analyses also suggest that selection relaxation may have occurred in LGP2 during the middle of bird evolution and the CARDs region of MDA5 contains many positively selected sites, which might explain its conservation level. Spearman’s correlation test indicates that species-to-ancestor dN/dS of RIG-I shows a negative correlation with endogenous retroviral abundance in bird genomes, suggesting the possibility of interaction between immunity and endogenous retroviruses during bird evolution.

Emerging avian influenza infections: Current understanding of innate immune response and molecular pathogenesis

The highly pathogenic avian influenza viruses (HPAIVs) cause severe disease in gallinaceous poultry species, domestic ducks, various aquatic and terrestrial wild bird species as well as humans. The outcome of the disease is determined by complex interactions of multiple components of the host, the virus, and the environment. While the host-innate immune response plays an important role for clearance of infection, excessive inflammatory immune response (cytokine storm) may contribute to morbidity and mortality of the host. Therefore, innate immunity response in avian influenza infection has two distinct roles. However, the viral pathogenic mechanism varies widely in different avian species, which are not completely understood. In this review, we summarized the current understanding and gaps in host-pathogen interaction of avian influenza infection in birds. In first part of this article, we summarized influenza viral pathogenesis of gallinaceous and non-gallinaceous avian species. Then we discussed innate immune response against influenza infection, cytokine storm, differential host immune responses against different pathotypes, and response in different avian species. Finally, we reviewed the systems biology approach to study host-pathogen interaction in avian species for better characterization of molecular pathogenesis of the disease. Wild aquatic birds act as natural reservoir of AIVs. Better understanding of host-pathogen interaction in natural reservoir is fundamental to understand the properties of AIV infection and development of improved vaccine and therapeutic strategies against influenza.