A Single Amino Acid at Residue 188 of the Hexon Protein Is Responsible for the Pathogenicity of the Emerging Novel Virus Fowl Adenovirus 4

Genome characterization of a novel fowl adenovirus serotype 8b isolate and construction of the reverse genetic system for rapid genome manipulation

Inclusion body hepatitis (IBH) induced by fowl adenovirus serotype 8b (FAdV-8b) infection is an important avian infectious disease circulating around the globe, posing significant losses to the poultry industry. In this study, a FAdV-8b strain, CH/SDQD/2021, was isolated from IBH-affected chickens in Shandong province, China and the genetic properties of CH/SDQD/2021 were characterized. The full genome length of CH/SDQD/2021 is 44,000 bp, with a G+C content of 58 % and 32 open reading frames (ORF). Sequencing alignment and phylogenetic analysis indicated that the genome identity of CH/SDQD/2021 compared to 30 other FAdV-E strains retrieved from GenBank ranges from 89.72 % to 96.71 %. Animal regression test indicated that CH/SDQD/2021 infection induced IBH in one-week-old SPF chickens. Subsequently, a reverse genetic system was developed to facilitate rapid genome manipulation of FAdV-8b for gene function study and vaccine development. To explore potential foreign gene insertion sites in FAdV-8b, ORF0-1-2, ORF11 and ORF19 of CH/SDQD/2021 were substituted by the green fluorescent gene ZsGreen, respectively, and the corresponding recombinant viruses were successfully rescued. The results showed that comparing with the parental FAdV-8b, the replication efficiency of the ORF0-1-2-substituted recombinant was reduced, while the replication efficiency of the ORF11-substituted recombinant was promoted. The findings of this study enrich the epidemiological data for the prevalent FAdV strains in China. Furthermore, the establishment of the FAdV-8b reverse genetic system will provide an efficient technique platform for FAdV-8b gene function research at the whole virus level and developing related multivalent vaccine candidates.

Recombinant Marek's disease virus type 1 provides full protection against H9N2 influenza A virus in chickens

The H9N2 subtype of the avian influenza virus (AIV) poses a significant threat to the poultry industry and human health. Recombinant vaccines are the preferred method of controlling H9N2 AIV, and Marek's disease virus (MDV) is the ideal vector for recombinant vaccines. During this study, we constructed two recombinant MDV type 1 strains that carry the hemagglutinin (HA) gene of AIV to provide dual protection against both AIV and MDV. To assess the effects of different MDV insertion sites on the protective efficacy of H9N2 AIV, the HA gene of H9N2 AIV was inserted in UL41 and US2 of the MDV type 1 vector backbone to obtain recombinant viruses rMDV-UL41/HA and rMDV-US2/HA, respectively. An indirect immunofluorescence assay showed sustained expression of HA protein in both recombinant viruses. Additionally, the insertion of the HA gene in UL41 and US2 did not affect MDV replication in cell cultures. After immunization of specific pathogen-free chickens, although both the rMDV-UL41/HA and rMDV-US2/HA groups exhibited similar levels of hemagglutination inhibition antibody titers, only the rMDV-UL41/HA group provided complete protection against the H9N2 AIV challenge, and also offered complete protection against challenge with MDV. These results demonstrated that rMDV-UL41/HA could be used as a promising bivalent vaccine strain against both H9N2 avian influenza and Marek's disease in chickens.

A novel subunit vaccine based on Fiber1/2 knob domain provides full protection against fowl adenovirus serotype 4 and induces stronger immune responses than a Fiber2 subunit vaccine

Outbreaks of hepatitis-hydropericardium syndrome (HHS) caused by fowl adenovirus serotype 4 (FAdV-4) have resulted in huge economic losses to the poultry industry in China since 2015. However, commercially available vaccines against the FAdV-4 infection remain scarce. In our study, subunit vaccine candidates derived from the bacterially expressed recombinant Fiber1 knob domain and Fiber2 knob domain fusion protein (termed as Fiber1/2 knob subunit vaccine) and Fiber2 protein (termed as Fiber2 subunit vaccine) of the FAdV-4 SDSX strain were developed. Immunogenicity evaluation showed that the Fiber1/2 knob subunit vaccine induced the production of antibodies at 7 d postvaccination (dpv), earlier than the Fiber2 subunit vaccine. Moreover, the neutralizing antibody level of the Fiber1/2 subunit vaccine group was higher than the Fiber2 subunit vaccine group, showing significant differences at 14, 21, and 28 dpv. Immune protection test results revealed that both Fiber1/2 knob subunit and Fiber2 subunit vaccines could protect chickens from death against FAdV-4 challenge, although the weight of chickens in the Fiber1/2 knob subunit vaccine group decreased less. Furthermore, analysis of plasma Glutamic oxaloacetic transaminase (AST) and blood glutamic pyruvic transaminase (ALT) levels suggested that the Fiber1/2 subunit vaccine can significantly inhibit liver damage caused by FAdV-4 infection and is more effective in blocking the pathogenicity of FAdV-4 in target organs. In addition, the Fiber1/2 knob subunit vaccine further reduced the viral load in different tissues and virus shedding in chickens than the Fiber2 subunit vaccine. Overall, the Fiber1/2 knob subunit vaccine was more effective than the Fiber2 subunit vaccine. These findings lay the foundation for the development of more effective FAdV-4 subunit vaccines.

Hypervirulent fowl adenovirus serotype 4 elicits early innate immune response and promotes virus-induced cellular autophagy in the spleen

The recent emergence of hepatitis-hydropericardium syndrome caused by highly pathogenic fowl adenovirus serotype 4 (FAdV-4) has resulted in significant economic losses to the poultry industry. However, the early innate immune response of immune organs within 24 hpi and the induction of autophagy in vivo after FAdV-4 infection have not been fully elucidated. In this study, 35-day-old specific pathogen-free (SPF) chickens were artificially infected with hypervirulent FAdV-4, which resulted in a mortality rate of up to 90%. The results showed that FAdV-4 infection rapidly triggered the innate immune response in vivo of chickens, with the spleen eliciting a stronger innate immune response than the thymus and bursa. During the early stage of viral infection within 24 hpi, the main receptors TLR3/7/21, MDA5, and cGAS were activated via the NF-κB and TBK1/IRF7-dependent signaling pathways, which up-regulated production of inflammatory cytokines and type I interferons. Additionally, the expression levels of the autophagy-related molecules LC3B, Beclin1, and ATG5 were significantly up-regulated at 24 hpi, while degradation of SQSTM1/p62 was observed, suggesting that FAdV-4 infection elicits a complete autophagy response in the spleen. Besides, the colocalization of Fiber2 and LC3B suggested that FAdV-4 infection induced autophagy which benefits FAdV-4 replication in vivo. This study provides new insights into the immunoregulation signal pathways of the early innate immunity in response to hypervirulent FAdV-4 infection in vivo within 24 hpi and the close relationship between viral replication and autophagy.

Differences in the pathogenicity and molecular characteristics of fowl adenovirus serotype 4 epidemic strains in Guangxi Province, southern China

Starting in 2015, the widespread prevalence of hydropericardium-hepatitis syndrome (HHS) has led to considerable financial losses within China's poultry farming industry. In this study, pathogenicity assessments, whole-genome sequencing, and analyses were conducted on 10 new isolates of the novel genotype FAdV-4 during a HHS outbreak in Guangxi Province, China, from 2019 to 2020. The results indicated that strains GX2019-010 to GX2019-013 and GX2019-015 to GX2019-018 were highly virulent, while strain GX2020-019 exhibited moderate virulence. Strain GX2019-014 was characterized as a wild-type strain with low virulence, displaying no pathogenic effects when 0.5 mL containing 106 TCID50 virus was inoculated into the muscle of specific pathogen-free (SPF) chickens at 4 weeks of age, while 107 TCID50 and 108 TCID50 resulted in mortality rates of 80 and 100%, respectively. The whole genomes of strains GX2019-010 to GX2019-013, GX2019-015 to GX2019-018, and GX2020-019 showed high homology with other Chinese newly emerging highly pathogenic FAdV-4 strains, whereas GX2019-014 was closer to nonmutant strains and shared the same residues with known nonpathogenic strains (B1-7, KR5, and ON1) at positions 219AA and 380AA of the Fiber-2 protein. Our work enriches the research on prevalent strains of FAdV-4 in China, expands the knowledge on the virulence diversity of the novel genotype FAdV-4, and provides valuable reference material for further investigations into the key virulence-associated genetic loci of FAdV-4.

Biological features of fowl adenovirus serotype-4

Fowl adenovirus serotype 4 (FAdV-4) is highly pathogenic to broilers aged 3 to 5 weeks and has caused considerable economic loss in the poultry industry worldwide. FAdV-4 is the causative agent of hydropericardium-hepatitis syndrome (HHS) or hydropericardium syndrome (HPS). The virus targets mainly the liver, and HPS symptoms are observed in infected chickens. This disease was first reported in Pakistan but has now spread worldwide, and over time, various deletions in the FAdV genome and mutations in its major structural proteins have been detected. This review provides detailed information about FAdV-4 genome organization, physiological features, epidemiology, coinfection with other viruses, and host immune suppression. Moreover, we investigated the role and functions of important structural proteins in FAdV-4 pathogenesis. Finally, the potential regulatory effects of FAdV-4 infection on ncRNAs are also discussed.

Protective immune response induced by Leghorn male hepatoma cell-adapted fowl adenovirus-4

Fowl adenovirus-4 (FAdV-4) is a highly contagious virus that causes acute and lethal hepatitis. It leads to substantial economic losses in the poultry industry. Among the structural proteins of FAdV-4, hexon and fiber2 are associated with immunopathogenesis. A frameshift mutation was generated in the fiber2 protein by seral passages in the Leghorn male hepatoma (LMH) cell line. Immunization using the attenuated virus (80 times passaged) before the virulent FAdV-4 challenge protected hosts from the infection and cleared the invading virus. In immunized animals, activated CD4+ and CD8+ T cell populations were larger during the FAdV-4 challenge. The change in the B cell population was similar. Myeloid cells were highly increased during FAdV-4 infection after the immunization, but the immunization inhibited the expansion in both liver and spleen. The functional gene expression for immune modulation was strongly associated with immune cell changes in the liver, however, this association was not strong in the spleen. The present findings imply that genetic modification by cellular adaptation regulates immune cell phenotype and function in the target organ. In addition, we suggest the attenuated virus as a protective strategy against the novel FAdV-4 strains.

Identification of ORF1B as a unique nonstructural protein for fowl adenovirus serotype 4

Fowl adenovirus serotype 4 (FAdV-4), the causative agent of hepatitis-hydropericardium syndrome (HHS), is a double-stranded DNA virus. Although many structural proteins have been deeply studied, the coding potential of some other open reading frames (ORFs) and the biological functions of their products during virus infection have not been fully elucidated. Here, a unique nonstructural protein ORF1B of FAdV-4 was identified and its expression kinetics along infection was analyzed. Except that of FAdV-10, a member of the same genus as FAdV-4, FAdV-4 ORF1B shared as low homologous identity as 29.2% in amino acid sequence with the other ten counterparts. Structurally, ORF1B was mapped on the N-terminal region of the genome between 1485 nt to 1808 nt and predicted to only contain two α-helix. Confocal immunofluorescence assay with homemade rabbit polyclonal antibody demonstrated that ORF1B could be simultaneously observed with structural protein Fiber 1 in FAdV-4-infected cells. Western blot further showed that ORF1B could only be detected in the infected cells but not mature virions, suggesting ORF1B was a nonstructural protein. Subsequently, the expression level of ORF1B detected by qRT-PCR and IFA was gradually decreased along with FAdV-4 infection, suggesting ORF1B was an early gene transcript. These results will lay a solid foundation to further study the biological effect of ORF1B on the replication and pathogenicity of FAdV-4.

Prevalence and Molecular Characteristics of FAdV-4 from Indigenous Chicken Breeds in Yunnan Province, Southwestern China

Fowl adenovirus-induced hepatitis-pericardial effusion syndrome outbreaks have been increasingly reported in China since 2015, resulting in substantial economic losses to the poultry industry. The genetic diversity of indigenous chicken results in different immune traits, affecting the evolution of these viruses. Although the molecular epidemiology of fowl adenovirus serotype 4 (FAdV-4) has been well studied in commercial broiler and layer chickens, the prevalence and genetic characteristics of FAdV-4 in indigenous chickens remain largely unknown. In this study, samples were collected from six indigenous chicken breeds in Yunnan province, China. FAdV-positive samples were identified in five of the six indigenous chicken populations via PCR and 10 isolates were obtained. All FAdVs belonged to serotype FAdV-4 and species FAdV-C. The hexon, fiber, and penton gene sequence comparison analysis demonstrated that the prevalence of FAdV-4 isolates in these chickens might have originated from other provinces that exported chicks and poultry products to Yunnan province. Moreover, several distinct amino acid mutations were firstly identified in the major structural proteins. Our findings highlighted the need to decrease inter-regional movements of live poultry to protect indigenous chicken genetic resources and that the immune traits of these indigenous chickens might result in new mutations of FAdV-4 strains.

Critical Role of Viral Protein Hexon in Hypervirulent Fowl Adenovirus Serotype-4-Induced Autophagy by Interaction with BAG3 and Promotion of Viral Replication in LMH Cells

Hepatitis-pericardial syndrome (HHS) is an acute highly infectious avian disease caused by fowl adenovirus serotype 4 (FAdV-4), characterized by fulminant hepatitis and hydropericardium in broilers. Since 2015, a widespread epidemic has occurred in China due to the emergence of hypervirulent FAdV-4 (HPFAdV-4), causing huge losses to the stakeholders. However, the pathogenesis of HPFAdV-4 and the host responses to its infection remain elusive. Here, we show that infection of leghorn male hepatocellular (LMH) cells by HPFAdV-4 induced complete autophagy in cells and that the autophagy induced by recombinant HPFAdV-4-ON1 (rHPFAdV-4-ON1), a viral strain generated by replacing the hexon gene of wild-type HPFAdV-4 (HPFAdV-4-WT) with the one of nonpathogenic strain FAdV-4-ON1, was remarkably mitigated compared to that of the rHPFAdV-4-WT control, suggesting that HPFAdV-4 hexon is responsible for virus-induced autophagy. Importantly, we found that hexon interacted with a cellular protein, BAG3, a host protein that initiates autophagy, and that BAG3 expression increased in cells infected with HPFAdV-4. Furthermore, knockdown of BAG3 by RNA interference (RNAi) significantly inhibited HPFAdV-4- or hexon-induced autophagy and suppressed viral replication. On the contrary, expression of hexon markedly upregulated the expression of BAG3 via activating the P38 signaling pathway, triggering autophagy. Thus, these findings reveal that HPFAdV-4 hexon interacts with the host protein BAG3 and promotes BAG3 expression by activating P38 signaling pathway, thereby inducing autophagy and enhancing viral proliferation, which immensely furthers our understanding of the pathogenesis of HPFAdV-4 infection. IMPORTANCE HHS, mainly caused by HPFAdV-4, has caused large economic losses to the stakeholders in recent years. Infection of leghorn male hepatocellular (LMH) cells by HPFAdV-4 induced complete autophagy that is essential for HPFAdV-4 replication. By a screening strategy, the viral protein hexon was found responsible for virus-induced autophagy in cells. Importantly, hexon was identified as a factor promoting viral replication by interaction with BAG3, an initiator of host cell autophagy. These findings will help us to better understand the host response to HPFAdV-4 infection, providing a novel insight into the pathogenesis of HPFAdV-4 infection.

Pathogenicity and molecular characteristics of fowl adenovirus serotype 4 with moderate virulence in Guangxi Province, China

The GX2020-019 strain of fowl adenovirus serotype 4 (FAdV-4) was isolated from the liver of chickens with hydropericardium hepatitis syndrome in Guangxi Province, China, and was purified by plaque assay three times. Pathogenicity studies showed that GX2020-019 can cause typical FAdV-4 pathology, such as hydropericardium syndrome and liver yellowing and swelling. Four-week-old specific pathogen-free (SPF) chickens inoculated with the virus at doses of 10³ median tissue culture infectious dose (TCID₅₀), 10⁴ TCID₅₀, 10⁵ TCID₅₀, 10⁶ TCID₅₀, and 10⁷ TCID₅₀ had mortality rates of 0, 20, 60, 100, and 100%, respectively, which were lower than those of chickens inoculated with other highly pathogenic Chinese isolates, indicating that GX2020-019 is a moderately virulent strain. Persistent shedding occurred through the oral and cloacal routes for up to 35 days postinfection. The viral infection caused severe pathological damage to the liver, kidney, lung, bursa of Fabricius, thymus, and spleen. The damage to the liver and immune organs could not be fully restored 21 days after infection, which continued to affect the immune function of chickens. Whole genome analysis indicated that the strain belonged to the FAdV-C group, serotype 4, and had 99.7-100% homology with recent FAdV-4 strains isolated from China. However, the amino acid sequences encoded by ORF30 and ORF49 are identical to the sequences found in nonpathogenic strains, and none of the 32 amino acid mutation sites that appeared in other Chinese isolates were found. Our research expands understanding of the pathogenicity of FAdV-4 and provides a reference for further studies.

An efficient double-fluorescence approach for generating fiber-2-edited recombinant serotype 4 fowl adenovirus expressing foreign gene

Recently, the infection of serotype 4 fowl adenovirus (FAdV-4) in chicken flocks has become endemic in China, which greatly threatens the sustainable development of poultry industry. The development of recombinant FAdV-4 expressing foreign genes is an efficient strategy for controlling both FAdV-4 and other important poultry pathogens. Previous reverse genetic technique for generating the recombinant fowl adenovirus is generally inefficient. In this study, a recombinant FAdV-4 expressing enhanced green fluorescence protein (EGFP), FA4-EGFP, was used as a template virus and directly edited fiber-2 gene to develop an efficient double-fluorescence approach to generate recombinant FAdV-4 through CRISPR/Cas9 and Cre-Loxp system. Moreover, using this strategy, a recombinant virus FAdV4-HA(H9) stably expressing the HA gene of H9N2 influenza virus was generated. Chicken infection study revealed that the recombinant virus FAdV4-HA(H9) was attenuated, and could induce haemagglutination inhibition (HI) titer against H9N2 influenza virus at early time points and inhibit the viral replication in oropharynx. All these demonstrate that the novel strategy for constructing recombinant FAdV-4 expressing foreign genes developed here paves the way for rapidly developing attenuated FAdV-4-based recombinant vaccines for fighting the diseases caused by both FAdV-4 and other pathogens.

Molecular phylodynamics of fowl adenovirus serotype 11 and 8b from inclusion body hepatitis outbreaks

Fowl adenovirus (FAdV) serotypes are involved in a variety of clinical manifestations in poultry and has resulted in substantial economic loss to the poultry farmers. Despite the endemicity of Inclusion body hepatitis (IBH) in South Asian countries, including India, its etiology is not well studied. In western India, the rural poultry flocks obtained from the vaccinated parents were experiencing disease outbreaks with substantial economic losses due to heavy outbreaks and mortality. Therefore, the study was conducted to decipher the molecular epidemiology of the FAdV from field outbreaks in western India. A total of 37 commercial broiler poultry flocks and 29 village poultry flocks of western India were visited during 2019 to 2021. Out of these, 19.14% flocks showed incidence of IBH during the age of 15 to 35 days. The mortality ranged from 3.3 percent to 55.28 percent. The samples were subjected for amplification of partial hexon gene covering loop 1 and loop 2. The results revealed 48.28% positivity by PCR. The sequence analysis identified 14 isolates as species D serotype 11 with 0.97 to 0.99% divergence and two as species E serotype 8b with 0.99% divergence. The FAdV-11 isolates showed amino acid substitutions D195N, T399A, N417S, and N496H. The amino acids I188 and N195 were conserved in FAdV-11. The molecular clock in Bayesian methods was used to determine most common ancestor. The isolates MH379249 and MH379248 were determined the most recent common ancestor for FAdV-11 and FAdV-8b isolates. The analysis suggested evolution of 10 FAdV-11 strains in 2012, and four FAdV-11 strains and two FAdV-8b strains in 2018.

Phylogenetic and pathogenic characterization of current fowl adenoviruses in China

In recent years, fowl adenoviruses (FAdVs) continue to outbreak and cause huge economic losses to the poultry industry in China. The homologous recombination accounts for the diversity serotypes of adenovirus. However, the prevalence, recombination and pathogenicity of current FAdVs remain unclear. Herein, the prevalence, phylogenetic feature and pathogenicity of FAdVs in China in 2019 were characterized. Our findings showed that multiple species and serotypes of FAdVs currently circulate in China, including A, C, D and E species, and 1, 2, 4, 8a and 8b serotypes. Notably, the recombination occurred between FAdV-8a and FAdV-8b, and the recombination regions included Hexon, Fiber, ORF19 and ORF20. All five FAdVs replicated effectively in various chicken tissues, and viral shedding peaked at 4-8 dpi. Except CH/GDSZ/1905(FAdV-1/A), the remaining FAdVs caused obvious inclusion body hepatitis (IBH) in 3-week-old specific-pathogen-free (SPF) chickens, of which CH/JSXZ/1905(FAdV-4/C) caused hydropericardium-hepatitis syndrome (HHS) with a mortality rate of 62.5%. Taken together, our findings illustrate the prevalence, recombination and pathogenicity of current FAdVs in China and strengthen surveillance and further pathogenicity studies of FAdVs are extremely urgent.

Complete Genome Analysis and Animal Model Development of Fowl Adenovirus 8b

Inclusion body hepatitis (IBH), hydropericardium syndrome, and gizzard erosion associated with fowl adenovirus (FAdV) infection have caused notable economic losses worldwide. In 2020, severe IBH was observed in a layer chicken farm in Hebei Province, China. Liver samples were collected from layer chickens with severe IBH and virus isolation was performed in LMH cells. DNA sequence and bioinformatics analyses were conducted to determine the phylogenetic relationship and the pathogenicity assay was conducted in specific-pathogen-free (SPF) chickens. HeB20 strain was isolated and identified as FAdV-8b, and the complete genome was successfully sequenced (GenBank No. OK188966). Although widespread recombination in clinical strains has been reported within FAdVs, HeB20 showed some novel characteristics, and did not show any recombination, highlighting that recombinant and non-recombinant FAdV-8b coexist in the clinic poultry industry. Finally, pathogenicity animal model of HeB20 was developed and showed severe IBH and 10% mortality. Collectively, a new FAdV-8b strain (HeB20) was isolated and responsible for the severe IBH in layer chickens. Complete genome of HeB20 was sequenced and valuable for future epidemiological investigations. HeB20 was capable of inducing severe IBH and 10% mortality in SPF chickens; this animal model provides a powerful tool for the future vaccine development.

Epidemiological survey of avian adenovirus in China from 2015 to 2021 and the genetic variability of highly pathogenic Fadv-4 isolates

The prevalence of poultry adenovirus in China is determined using clinical diagnosis, molecular biological testing, serological testing, and LMH cell virus isolation. These methods can track and test key poultry and waterfowl breeding areas across the country. From 2015 to 2021, 9613 suspected adenovirus samples were collected from 28 provinces. After the first generation of gene sequencing, a total of 2210 hexo gene fragments were obtained. Among them, FAdV-1 type accounted for 7.65%, FAdV-2 type accounted for 5.34%, FAdV-3 type accounted for 2.04%, FAdV-4 type accounted for 38.24%, FAdV-5 type accounted for 2.17%, FAdV-6 type accounted for 0.32%, FAdV-7 type accounted for 0.77%, FAdV-8a type accounted for 10.63%, FAdV-8b type accounted for 11.58%, FAdV-9 type accounted for 0.50%, FAdV-10 type accounted for 8.10%, and FAdV-11 type accounted for 12.67%. A total of 877 FAdV strains were isolated from FAdV suspected samples by seeding LMH cells, and there were 475 FAdV-4 strains among them. A total of 473 isolates were highly pathogenic FAdV-4, and the percentage of amino acid homology with the highly pathogenic FAdV-4 reference strains was >99.1%. Two isolates were non-pathogenic, and the amino acid homology with the ON1 reference strain was >99.6%. Part of the amino acid positions of the hexon gene have mutations, including positions 188, 193, 195, 238, and 240.

Advances in Vaccine Development of the Emerging Novel Genotype Fowl Adenovirus 4

Fowl adenovirus (FAdV) was first reported in Angara Goth, Pakistan, in 1987. For this reason, it is also known as “Angara disease.” It was later reported in China, Japan, South Korea, India, the United States, Canada, and other countries and regions, causing huge economic losses in the poultry industry worldwide. Notably, since June 2015, a natural outbreak of severe hydropericardium hepatitis syndrome (HHS), associated with a hypervirulent novel genotype FAdV-4 infection, has emerged in most provinces of China. The novel virus FAdV-4 spread rapidly and induced a 30-100% mortality rate, causing huge economic losses and threatening the green and healthy poultry breeding industry. Vaccines against FAdV-4, especially the emerging novel genotype, play a critical role and will be the most efficient tool for preventing and controlling HHS. Various types of FAdV-4 vaccines have been developed and evaluated, such as inactivated, live-attenuated, subunit, and combined vaccines. They have made great contributions to the control of HHS, but the details of cross-protection within FAdVs and the immunogenicity of different vaccines require further investigation. This review highlights the recent advances in developing the FAdV-4 vaccine and promising new vaccines for future research.

The Role of Hexon Amino Acid 188 Varies in Fowl Adenovirus Serotype 4 Strains with Different Virulence

Hepatitis-hydropericardium syndrome (HHS) induced by fowl adenovirus serotype 4 (FAdV-4) has caused huge economic losses to poultry industries. The key genes responsible for different virulence of FAdV-4 strains are not fully elucidated. Previous studies indicated that hexon of pathogenic FAdV-4 has a conserved arginine (R) at position 188, and a conserved isoleucine (I) is present at this position in reported nonpathogenic FAdV-4. Recently, it was reported that R188 of hexon is the determinant site for pathogenicity of the emerging Chinese FAdV-4 strain. However, the role of hexon amino acid 188 (aa188) has not been examined in the nonpathogenic FAdV-4 strain. In this study, three recombinant FAdV-4 viruses, H/H/R188I, O/O/I188R, and H/O/I188R, were constructed by mutating hexon aa188 of FAdV-4 pathogenic strain CH/HNJZ/2015 (H) and nonpathogenic strain ON1 (O), and pathogenicity was assessed in specific-pathogen-free (SPF) chickens. Consistent with previous findings, H/O/I188R exhibited pathogenicity similar to that of CH/HNJZ/2015, yet H/H/R188I induced no mortality. Unexpectedly, all chickens infected with O/O/I188R survived. Postmortem examination of O/O/I188R-infected chickens showed typical lesions of inclusion body hepatitis rather than HHS. Expression of proinflammatory cytokines in CH/HNJZ/2015- and H/O/I188R-infected chickens was significantly higher than that in H/H/R188I-, ON1-, and O/O/I188R-infected chickens. Analysis of predicted hexon protein structures indicated that aa188 mutation leads to conformational changes in the L1 loop of HNJZ-hexon but not in ON1-hexon. In summary, the present study demonstrated that the role of hexon aa188 in the virulence of FAdV-4 varies between different strains. Induction of HHS requires factors aside from hexon aa188 in the emerging Chinese FAdV-4 strain.

IMPORTANCE HHS induced by FAdV-4 has caused huge economic losses to the poultry industry. The key determinants for the different virulence of FAdV-4 have not been fully elucidated. Here, we investigated the role of hexon aa188 in FAdV-4 strains with different virulence and showed that the role of hexon aa188 varies in FAdV-4 strains with different genetic contents. The hexon R188 may be the key amino acid for causing inclusion body hepatitis by the pathogenic FAdV-4 strain, and induction of HHS by FAdV-4 may need other viral cofactors. Moreover, the hexon R188I mutation greatly affected the expression of proinflammatory cytokines induced by the pathogenic strain CH/HNJZ/2015, but no significant difference was observed between the nonpathogenic strain ON1 and ON1 with hexon I188R mutation. We found that hexon aa188 mutation induced conformational changes to hexon protein in CH/HNJZ/2015 but not in ON1, which might be the underlying reason for the changing virulence.

Gga-miR-30c-5p Enhances Apoptosis in Fowl Adenovirus Serotype 4-Infected Leghorn Male Hepatocellular Cells and Facilitates Viral Replication through Myeloid Cell Leukemia-1

Fowl adenovirus serotype 4 (FAdV-4) is the primary causative agent responsible for the hepatitis-hydropericardium syndrome (HHS) in chickens, leading to considerable economic losses to stakeholders. Although the pathogenesis of FAdV-4 infection has gained attention, the underlying molecular mechanism is still unknown. Here, we showed that the ectopic expression of gga-miR-30c-5p in leghorn male hepatocellular (LMH) cells enhanced apoptosis in FAdV-4-infected LMH cells by directly targeting the myeloid cell leukemia-1 (Mcl-1), facilitating viral replication. On the contrary, the inhibition of endogenous gga-miR-30c-5p markedly suppressed apoptosis and viral replication in LMH cells. Importantly, the overexpression of Mcl-1 inhibited gga-miR-30c-5p or FAdV-4-induced apoptosis in LMH cells, reducing FAdV-4 replication, while the knockdown of Mcl-1 by RNAi enhanced apoptosis in LMH cells. Furthermore, transfection of LMH cells with gga-miR-30c-5p mimics enhanced FAdV-4-induced apoptosis associated with increased cytochrome c release and caspase-3 activation. Thus, gga-miR-30c-5p enhances FAdV-4-induced apoptosis by directly targeting Mcl-1, a cellular anti-apoptotic protein, facilitating FAdV-4 replication in host cells. These findings could help to unravel the mechanism of how a host responds against FAdV-4 infection at an RNA level.

Cellular protein HSC70 promotes fowl adenovirus serotype 4 replication in LMH cells via interacting with viral 100K protein

Fowl adenovirus serotype 4 (FAdV-4), the predominant causative agent of hepatitis-hydropericardium syndrome (HHS), has caused severe economic losses to poultry industry since 2015. Although fiber2 and hexon have been confirmed to be the virulence-related factors, the roles of nonstructural viral proteins in pathogenicity of FAdV-4 remain poorly understood. Here, a tandem mass spectrometry (MS) was used to identify host factors interacted with 100K protein of hypervirulent FAdV-4 isolate (CH/HNJZ/2015), and 2595 cellular proteins associated with many biological processes and pathways were identified according to Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses. Among the proteins, HSC70 was verified to interact with 100K through co-immunoprecipitation assay. Notably, overexpression of HSC70 promoted the replication of FAdV-4 in LMH cells, whereas blocking HSC70 with inhibitor ver-155008 markedly suppressed viral replication. Collectively, these findings suggested that many cellular proteins involved in FAdV-4 infection through interacting with 100K and HSC70 positively regulated virus replication.

Pathology and Molecular Epidemiology of Fowl Adenovirus Serotype 4 Outbreaks in Broiler Chicken in Abu Dhabi Emirate, UAE

BACKGROUND

Fowl adenovirus serotype 4 (FAdV-4), causing inclusion body hepatitis (IBH) and hydropericardium hepatitis syndrome (HPS), is responsible for the significant economic losses in poultry industry worldwide. This study describes FAdV disease and molecular characteristics of the virus as the first report in UAE.

METHODOLOGY

Clinical, necropsy, histopathology, qPCR and phylogenetic analysis of hexon gene were used to diagnose and characterize the virus.

RESULTS

The age of the infected broiler chicken was 2-4 weeks. The morbidity and mortality rates ranged between 50 and 100% and 44 and 100%, respectively. Clinically, sudden onset, diarrhea, anemia and general weakness were recorded. At necropsy, acute necrotic hepatitis, with swollen, yellowish discoloration, enlarged and friable liver; hydropericarditis with hydropericardium effusions; and enlarged mottled spleen were observed. Histopathology examination revealed degeneration and necrosis, lymphocytic infiltration and inclusion bodies. The qPCR analysis detected the virus in all samples tested. Hexon gene sequence analysis identified FAdV serotype 4, species C as the major cause of FAdV infections in UAE in 2020, and this strain was closely related to FAdV-4 circulating in Saudi Arabia, Pakistan, Nepal and China.

CONCLUSION

The serotype 4, species C, was the common FAdV strain causing IBH and HPS episodes in the region. This result may help design effective vaccination programs that rely on field serotypes.

Genomic characterization of a fowl adenovirus serotype 4 strain isolated from a chicken with hydropericardium syndrome in Japan

Here, we report the genomic characterization of a fowl adenovirus serotype 4 strain isolated from a chicken with hydropericardium syndrome in Japan. The viral genome of FAdV-4 strain JP/LVP-1/96 was found to be 45,688 bp long. Amino acid substitutions at position 219 (G to D) in the fiber-2 protein and at position 188 (I to R) in the hexon protein, which are commonly found in virulent FAdV-4 strains, were also found in the JP/LVP-1/96 strain. Additional specific amino acid substitutions commonly found in virulent FAdV-4 strains were found in ORFs 4 and 43, which are present only in members of the species Fowl adenovirus C. Phylogenetic analysis based on complete hexon protein gene sequences showed that strain JP/LVP-1/96 belongs to a different genetic cluster from the strains circulating in neighboring countries.

High Phenotypic Variation between an In Vitro-Passaged Fowl Adenovirus Serotype 1 (FAdV-1) and Its Virulent Progenitor Strain despite Almost Complete Sequence Identity of the Whole Genomes

Adenoviral gizzard erosion is an emerging disease with negative impact on health and production of chickens. In this study, we compared in vitro and in vivo characteristics of a fowl adenovirus serotype 1 (FAdV-1), attenuated by 53 consecutive passages in primary chicken embryo liver (CEL) cell cultures (11/7127-AT), with the virulent strain (11/7127-VT). Whole genome analysis revealed near-complete sequence identity between the strains. However, a length polymorphism in a non-coding adenine repeat sequence (11/7127-AT: 11 instead of 9) immediately downstream of the hexon open reading frame was revealed. One-step growth kinetics showed delayed multiplication of 11/7127-AT together with significantly lower titers in cell culture (up to 4 log₁₀ difference), indicating reduced replication efficiency in vitro. In vivo pathogenicity and immunogenicity were determined in day-old specific pathogen-free layer chicks inoculated orally with the respective viruses. In contrast to birds infected with 11/7127-VT, birds infected with 11/7127-AT did not exhibit body weight loss or severe pathological lesions in the gizzard. Virus detection rates, viral load in organs and virus excretion were significantly lower in birds inoculated with 11/7127-AT. Throughout the experimental period, these birds did not develop measurable neutralizing antibodies, prevalent in birds in response to 11/7127-VT infection. Differences in pathogenicity between the virulent FAdV-1 and the attenuated strain could not be correlated to prominently discriminate genomic features. We conclude that differential in vitro growth profiles indicate that attenuation is linked to modulation of viral replication during interaction of the virus with the host cells. Thus, hosts would be unable to prevent the rapid replication of virulent FAdV leading to severe tissue damage, a phenomenon broadly applicable to further FAdV serotypes, considering the substantial intra-serotype virulence differences of FAdVs and the variation of diseases.

Importin alpha 1 is required for the nucleus entry of Fowl Adenovirus serotype 4 Fiber-1 protein

Fiber-1 protein (F1) is the structural protein of Fowl Adenovirus serotype 4 (FAdV-4), which could recondite the receptors of host cytomembrane. In this study, we firstly determined that F1 protein located in nucleus of LMH cells after infection with FAdV-4. We additionally revealed that F1 protein had a classic NLS, and the NLS was required for F1 nucleus entry, which was intently associated to the 26th Pro in NLS. The time rule result indicated that some F1 proteins firstly positioned in the nucleus 6 h posttranfection, and it entirely located in the nucleus 12 h posttranfection, then it ordinarily placed in cytoplasm 18 h posttranfection by means of microscopic fluorescence observation and Western Blotting. Then after transfection with pCI-neo-flag-F1 or infection with FAdV-4, the importin alpha 1 was once investigated whether or not it was required for F1 protein nucleus entry through immunofluorescence and/or Co-IP, results demonstrated that the F1 protein and importin alpha 1 co-located in the nucleus 6 h and 12 h posttranfection. The tiers of F1 protein vicinity in nucleus have been additionally investigated after knockdown expression or overexpression of importin alpha 1, and the results further revealed that importin alpha 1 used to be required for F1 protein nucleus entry. Finally, the function of F1 protein nucleus entry was investigated by qPCR, RT-PCR and Western Blotting, and the results revealed that F1 protein nucleus location was conducive to the proliferation of FAdV-4. In summary, we firstly reveal that the F1 protein of FAdV-4 locates in nucleus infected with FAdV-4, and confirm that importin alpha 1 binds to the NLS of F1 protein to nucleus localization, which promotes the proliferation of FAdV-4.

Development of a Novel Avian Vaccine Vector Derived From the Emerging Fowl Adenovirus 4

Severe hepatitis-hydropericardium syndrome (HHS) associated with a novel viral genotype, fowl adenovirus 4 (FAdV-4), has emerged and widely spread in China since 2015, causing severe economic losses to the poultry industry. We previously reported that the hexon gene is responsible for pathogenicity and obtained a non-pathogenic hexon-replacement rHN20 strain; however, the lack of information about the non-essential regions for virus replication limits the development of a FAdV-4 vector. This study first established an enhanced green fluorescent protein (EGFP)-indicator virus based on the FAdV-4 reverse genetic technique, effective for batch operations in the virus genome. Based on this, 10 open reading frames (ORFs) at the left end and 13 ORFs at the right end of the novel FAdV-4 genome were deleted separately and identified as non-essential genes for viral replication, providing preliminary insertion sites for foreign genes. To further improve its feasibility as a vaccine vector, seven combinations of ORFs were successfully replaced with EGFP without affecting the immunogenicity of the vector backbone. Finally, a recombinant rHN20-vvIBDV-VP2 strain, expressing the VP2 protein of very virulent infectious bursa disease virus (vvIBDV), was rescued and showed complete protection against FAdV-4 and vvIBDV. Thus, the novel FAdV-4 vector could provide sufficient protection for HHS and efficient exogenous gene delivery. Overall, our findings systemically identified 23 non-essential ORFs for FAdV-4 replication and seven foreign gene insertion regions, providing valuable information for an in-depth understanding of the novel FAdV-4 pathogenesis and development of multivalent vaccines.

An inactivated vaccine based on artificial non-pathogenic fowl adenovirus 4 protects chickens against hepatitis-hydropericardium syndrome

Hepatitis-hydropericardium syndrome (HHS) in birds is mainly caused by virulent fowl adenovirus 4 (FAdV-4). A novel genotype, hypervirulent FAdV-4, emerged in 2015 with a high mortality rate ranging from 30 % to 100 % in chickens. Vaccination is an economically feasible method to control HHS. Although there have been various reports of inactivated vaccines from virulent wild-type FAdV-4 against HHS, biosafety threats of inactivated vaccines from potential pathogenic components have been presented to the poultry industry, and safer vaccines are urgently needed. A non-pathogenic recombinant FAdV-4 strain, designated as rHN20, was generated based on the hypervirulent strain in our previous study. Here, we developed a novel inactivated oil-adjuvanted vaccine derived from rHN20 strain and evaluated its immunogenicity in specific-pathogen-free chickens. Chickens subcutaneously or intramuscularly immunized with the inactivated vaccine produced high titers of neutralizing antibodies and were protected from a lethal dose of virulent wild-type FAdV-4 challenge. Collectively, an inactivated vaccine was developed, which was capable of providing full protection for chickens against HHS, and significantly reduced the potential biosafety threats.

Immunogenicity of Novel Live Vaccine Based on an Artificial rHN20 Strain against Emerging Fowl Adenovirus 4

In recent years, hepatitis-hydropericardium syndrome (HHS), caused by novel fowl adenovirus 4 (FAdV-4), has caused serious economic losses to the poultry industry. Vaccines are important for preventing and controlling HHS. Current FAdV-4 vaccine research and development are mainly focuses on inactivated vaccines and relatively fewer live vaccines. We previously demonstrated that the hexon gene is the key gene responsible for the high pathogenicity of FAdV-4 and constructed a non-pathogenic chimeric virus rHN20 strain based on the emerging FAdV-4. In this study, the immunogenicity of artificially rescued rHN20 was evaluated in chickens using different routes and doses as a live vaccine. The live rHN20 vaccine induced high titers of neutralizing antibodies against FAdV-4 and fully protected the immunized chickens against a lethal dose of FAdV-4. Furthermore, immunized chickens showed no clinical symptoms or histopathological changes in the FAdV-4-targeted liver, and the viral load in the tissues of immunized chickens was significantly lower than that of chickens in the challenge control group. Collectively, the live rHN20 vaccine effectively protected our sample against FAdV-4 infection and can be considered a live vaccine candidate for preventing HHS in the poultry industry.