Experimental infection of dogs with H3N2 canine influenza virus from China

The biological characteristics and infection dynamics of a novel H3N2 canine influenza virus genotype in beagles

BACKGROUND

The canine influenza virus (CIV) outbreak has garnered considerable attention as it poses a significant threat to dog health. During the H3N2 CIV evolution in beagles, the virus formed a new clade after 2019 and gradually became more adaptable to other mammals. Therefore, successfully elucidating the biological characteristics and constructing a canine influenza infection model is required for CIV characterization.

METHODS

We performed genetic analyses to examine the biological characteristics and infection dynamics of CIV.

RESULTS

The genotype of our H3N2 CIV strain (from 2019 in Shanghai) belonged to the 5.1 clade, which is now prevalent in China. Using MDCK cells, we investigated viral cytopathic effects. Virus size and morphology were observed using transmission electron microscopy. Beagles were also infected with 104, 105, and 106 50% egg-infectious doses (EID50). When compared with the other groups, the 106 EID50 group showed the most obvious clinical symptoms, the highest virus titers, and typical lung pathological changes. Our results suggested that the other two treatments caused mild clinical manifestations and pathological changes. Subsequently, CIV distribution in the 106 EID50 group was detected by hematoxylin and eosin (H&E) and immunofluorescence (IF) staining, which indicated that CIV primarily infected the lungs.

CONCLUSIONS

The framework established in this study will guide further CIV prevention strategies.

Comparison of Pathogenicity of Different Infectious Doses of H3N2 Canine Influenza Virus in Dogs

The canine influenza virus (CIV) outbreaks have raised concerns as they pose a threat to the health of dogs. The successful construction of a canine influenza (CI) infection model is essential to study the CIV. Here we investigated the pathogenicity of different infectious doses of H3N2 CIV in Beagle dogs. Thirty-seven healthy Beagle dogs were used in the experiment and were infected with 10³, 10⁴, 10⁵, and 10⁶ 50% egg-infectious doses (EID₅₀). Compared to the dogs in the other three groups, those in the 10⁶ EID₅₀ group presented with obvious clinical symptoms, high virus titer, and typical pathological changes. Considering the ensemble of clinical scores, body temperature, virus shedding, lung lesions, pathological section scores, and visceral virus titers, we determined that 10⁶ EID₅₀ is the minimum infectious dose for the Beagle infection model. The other three infectious doses had almost no clinical symptoms. These results indicate that 10⁶ EID₅₀ is the minimum infectious dose of H3N2 CIV that can cause obvious clinical manifestations and pathological changes associated with CI in Beagle dogs. The theoretical framework developed in this research will guide the establishment of an infection model of CIV for future research.

Comparative pathogenesis of H3N2 canine influenza virus in beagle dogs challenged by intranasal and intratracheal inoculation

As important companion animals, dogs may serve as intermediate hosts for transmitting influenza virus to humans. However, knowledge regarding H3N2 canine influenza virus (CIV) pathogenicity is not comprehensive, which directly affects the animal models of pathogenicity in H3N2 CIV vaccine research. Here, to assess H3N2 CIV pathogenicity, we utilized 30 ten-week-old purpose-bred beagles intratracheally or intranasally inoculated with 10⁶ 50% egg-infectious dose. Intratracheal inoculation was more virulent to dogs than intranasal inoculation as shown by lung pathology score, histopathological changes, clinical symptoms, and body temperature. More intense virus replication was observed in the upper and lower respiratory tracts by intratracheal than intranasal inoculation according to nasal swabs, various organ virus titers, and antigen expression. These results may enhance the H3N2 CIV infection model, providing a more complete experimental basis for studying intrinsic H3N2 CIV pathogenic mechanism, and also serving a reference role for CIV prevention and treatment.

Comparison of the virulence of three H3N2 canine influenza virus isolates from Korea and China in mouse and Guinea pig models

Background

Avian-origin H3N2 canine influenza virus (CIV) has been the most common subtype in Korea and China since 2007. Here, we compared the pathogenicity and transmissibility of three H3N2 CIV strains [Chinese CIV (JS/10), Korean CIV (KR/07), and Korean recombinant CIV between the classic H3N2 CIV and the pandemic H1N1 virus (MV/12)] in BALB/c mouse and guinea pig models. The pandemic H1N1 (CA/09) strain served as the control.

Results

BALB/c mice infected with H1N1 had high mortality and obvious body weight loss, whereas no overt disease symptoms were observed in mice inoculated with H3N2 CIV strains. The viral titers were higher in the group MV/12 than those in groups JS/10 and KR/07, while the mice infected with JS/10 showed higher viral titers in all tissues (except for the lung) than the mice infected with KR/07. The data obtained in guinea pigs also demonstrated that group MV/12 presented the highest loads in most of the tissues, followed by group JS/10 and KR/07. Also, direct contact transmissions of all the three CIV strains could be observed in guinea pigs, and for the inoculated and the contact groups, the viral titer of group MV/12 and KR/07 was higher than that of group JS/10 in nasal swabs. These findings indicated that the matrix (M) gene obtained from the pandemic H1N1 may enhance viral replication of classic H3N2 CIV; JS/10 has stronger viral replication ability in tissues as compared to KR/07, whereas KR/07 infected guinea pigs have more viral shedding than JS/10 infected guinea pigs.

Conclusions

There exists a discrepancy in pathobiology among CIV isolates. Reverse genetics regarding the genomes of CIV isolates will be helpful to further explain the virus characteristics.

Comparison of the virulence and transmissibility of canine H3N2 influenza viruses and characterization of their canine adaptation factors

Recent canine influenza outbreaks have raised concerns about the generation of pathogenic variants that may pose a threat to public health. Here, we examine avian-like H3N2 canine influenza viruses (CIVs) isolated from 2009 to 2013 in South Korea from dogs. Phylogenetic analysis revealed that these viruses are closely related to strains previously isolated from dogs in Korea and China. However, molecular characterization demonstrated non-synonymous mutations between the canine viruses, particularly in the putative H3 antigenic sites, NA stalk regions, and in the internal genes of the 2012–2013 isolates compared with the 2009 isolate. Animal experiments showed that three representative isolates (A/canine/Korea/AS-01/2009(AS-01/09), A/canine/Korea/AS-05/2012(AS-05/12) and A/canine/Korea/AS-11/2013(AS-11/13), were readily droplet transmitted between dogs, whereas AS-05/12 induced more severe clinical disease and was lethal in dogs compared with AS-01/09. Although all viruses were able to infect ferrets, AS-05/12 consistently yielded higher nasal wash titers and was transmissible to ferrets via airborne droplets. Using reverse genetics, we show that the NA, NP, and M genes of CIV are critical for the adaptation of avian H3N2 viruses, and the resulting reassortant genotypes promote viral growth in dogs in a manner similar to that of the wild-type AS-01/09 virus. Taken together, these results demonstrate that CIVs continuously evolve in dogs thereby allowing them to gain a foothold in mammalian hosts. Importantly, we elucidated the genetic contributions of the NA, NP, and M genes to the adaptability of CIVs derived from the avian H3N2 virus.

Establishment and characterization of a telomerase-immortalized canine bronchiolar epithelial cell line

Dogs are susceptible to infectious diseases that occur primarily in the respiratory tract. The airway epithelium acts as a first line of defense and is constantly exposed to microorganisms present in the environment. Respiratory epithelial cells have recently gained wide use as a cell model for studying the pathogenesis of human, murine or swine respiratory pathogen infections. However, studies of the pathogenic mechanisms of canine pathogens have been hindered by the lack of reliable respiratory cell lines. Here, we cultured primary canine bronchiolar epithelial cells (CBECs), whose characteristics were confirmed by their expression of the epithelial cell-specific marker cytokeratin 18, and have provided protocols for their isolation and ex vivo expansion. Further, we established immortalized CBECs containing the human telomerase reverse transcriptase (hTERT) gene via transfection of primary CBECs with the recombinant plasmid pEGFP-hTERT. Immortalized bronchiolar epithelial cells (hTERT-CBECs) retain the morphological and functional features of primary CBECs, as indicated by reverse transcriptase polymerase chain reaction, proliferation assays, karyotype analysis, telomerase activity assay, and Western blotting, which demonstrate that hTERT-CBECs have higher telomerase activity, an extended proliferative lifespan, and a diploid complement of chromosomes, even after Passage 50. Moreover, this cell line is not transformed, as evaluated using soft agar assays and tumorigenicity analysis in nude mice, and can therefore be safely used in future studies. The isolation and establishment of stable hTERT-CBECs is of great importance for use as an in vitro model for mechanistic studies of canine pathogenic infections.

Equine and Canine Influenza H3N8 Viruses Show Minimal Biological Differences Despite Phylogenetic Divergence

The A/H3N8 canine influenza virus (CIV) emerged from A/H3N8 equine influenza virus (EIV) around the year 2000 through the transfer of a single virus from horses to dogs. We defined and compared the biological properties of EIV and CIV by examining their genetic variation, infection, and growth in different cell cultures, receptor specificity, hemagglutinin (HA) cleavage, and infection and growth in horse and dog tracheal explant cultures. Comparison of sequences of viruses from horses and dogs revealed mutations that may be linked to host adaptation and tropism. We prepared infectious clones of representative EIV and CIV strains that were similar to the consensus sequences of viruses from each host. The rescued viruses, including HA and neuraminidase (NA) double reassortants, exhibited similar degrees of long-term growth in MDCK cells. Different host cells showed various levels of susceptibility to infection, but no differences in infectivity were seen when comparing viruses. All viruses preferred α2-3- over α2-6-linked sialic acids for infections, and glycan microarray analysis showed that EIV and CIV HA-Fc fusion proteins bound only to α2-3-linked sialic acids. Cleavage assays showed that EIV and CIV HA proteins required trypsin for efficient cleavage, and no differences in cleavage efficiency were seen. Inoculation of the viruses into tracheal explants revealed similar levels of infection and replication by each virus in dog trachea, although EIV was more infectious in horse trachea than CIV.

IMPORTANCE

Influenza A viruses can cross species barriers and cause severe disease in their new hosts. Infections with highly pathogenic avian H5N1 virus and, more recently, avian H7N9 virus have resulted in high rates of lethality in humans. Unfortunately, our current understanding of how influenza viruses jump species barriers is limited. Our aim was to provide an overview and biological characterization of H3N8 equine and canine influenza viruses using various experimental approaches, since the canine virus emerged from horses approximately 15 years ago. We showed that although there were numerous genetic differences between the equine and canine viruses, this variation did not result in dramatic biological differences between the viruses from the two hosts, and the viruses appeared phenotypically equivalent in most assays we conducted. These findings suggest that the cross-species transmission and adaptation of influenza viruses may be mediated by subtle changes in virus biology.

Monoclonal antibody specific to HA2 glycopeptide protects mice from H3N2 influenza virus infection

Canine influenza virus (CIV) subtype H3N2 is a newly identified, highly contagious respiratory pathogen that causes cough, pneumonia and other respiratory symptoms in dogs. Data indicate that the virus is responsible for recent clinical cases of dog disease in China. However, therapeutic options for this disease are very limited. In this study, seven monoclonal antibodies (mAbs) against CIV JS/10 (an H3N2 subtype virus) were produced and characterized. Among them, mAb D7, which is specific for the HA2 glycopeptide (gp), induced the highest neutralization titers. The protection provided by mAb D7 was evaluated in BALB/c mice challenged with homologous or heterologous strains of H3N2 influenza virus, including two strains of CIV and one strain of swine influenza virus (SIV). The data show that mAb D7 protected the mice from infection with the three viral strains, especially the homologous strain, which was indicated by the recovery of body weight, reduction of viral load, and reduction of tissue damage. Moreover, the levels of IFN-γ and TNF-α in the lungs, as detected by ELISA, were reduced in the infected mice treated with the mAb D7 compared with those without mAb D7 treatment. Thus, our findings demonstrate, for the first time, that a mAb could reduce the release of IFN-γ and TNF-α associated with tissue damage by CIV infection and that the mAb might be of great therapeutic value for CIV infection.