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. 2015 Nov 20;10(11):e0143094.
doi: 10.1371/journal.pone.0143094. eCollection 2015.

Pathology of Equine Influenza virus (H3N8) in Murine Model

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Pathology of Equine Influenza virus (H3N8) in Murine Model

Selvaraj Pavulraj et al. PLoS One. .

Abstract

Equine influenza viruses (EIV)-H3N8 continue to circulate in equine population throughout the world. They evolve by the process of antigenic drift that leads to substantial change in the antigenicity of the virus, thereby necessitating substitution of virus strain in the vaccines. This requires frequent testing of the new vaccines in the in vivo system; however, lack of an appropriate laboratory animal challenge model for testing protective efficacy of equine influenza vaccine candidates hinders the screening of new vaccines and other therapeutic approaches. In the present investigation, BALB/c mouse were explored for suitability for conducting pathogenecity studies for EIV. The BALB/c mice were inoculated intranasally @ 2×106.24 EID50 with EIV (H3N8) belonging to Clade 2 of Florida sublineage and monitored for setting up of infection and associated parameters. All mice inoculated with EIV exhibited clinical signs viz. loss in body weights, lethargy, dyspnea, etc, between 3 and 5 days which commensurate with lesions observed in the respiratory tract including rhinitis, tracheitis, bronchitis, bronchiolitis, alveolitis and diffuse interstitial pneumonia. Transmission electron microscopy, immunohistochemistry, virus quantification through titration and qRT-PCR demonstrated active viral infection in the upper and lower respiratory tract. Serology revealed rise in serum lactate dehydrogenase levels along with sero-conversion. The pattern of disease progression, pathological lesions and virus recovery from nasal washings and lungs in the present investigations in mice were comparable to natural and experimental EIV infection in equines. The findings establish BALB/c mice as small animal model for studying EIV (H3N8) infection and will have immense potential for dissecting viral pathogenesis, vaccine efficacy studies, preliminary screening of vaccine candidates and antiviral therapeutics against EIV.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Pathogenicity of eq/J-K/08 H3N8 EIV in mice:
A:Percentage changes in body weight of EIV infected mice and negative control mice during the period of experiment (0–14 dpi). B:LDH level in pooled serum samples from EIV infected mice during the period of experiment. C:Humaral immune response following experimental EIV infection in BALB/c mice.
Fig 2
Fig 2. Pathogenicity of eq/J-K/08 H3N8 EIV in mice:
A:Focal area of consolidation (red hepatization—arrow) of lung with moderate congestion at 5 dpi (n = 6). B: Degeneration of lining epithelial cells with diffuse goblet cell hyperplasia at 2 dpi X400. C: Degeneration of lining epithelial cells, severe degeneration of submucosal glandular epithelium and impacted lumen with neutrophils and necrotic tissue debris 3 dpi X400. D: Loss of cilia, with degeneration and necrosis of lining epithelium (arrow) of nasal mucosa at 2 dpi. E: Immunohistochemical staining (Fig 2C section) for EIV antigens in cytoplasm and nucleus of degenerated and necrotic lining epithelial cells of nasal turbinate (arrow) at 2 dpi (IIPT) X400. F: Severe degeneration of lining epithelial cells with goblet cell hyperplasia and presence of denuded ciliated epithelium along with lymphocytes in lumen (arrow) at 5 dpi X400. G: Goblet cell hyperplasia of lining epithelial cells with infiltrations of macrophages (arrow) and lymphocytes in lamina propria at 5 dpi X600 (n = 6).
Fig 3
Fig 3. Pathogenicity of eq/J-K/08 H3N8 EIV in mice:Trachea (H&E):
A: Severe degeneration and necrosis of lining epithelial cells (arrow) at 2 dpi X400. B: Loss of cilia with necrosis of mucosal lining epithelium (arrow) at 2 dpi X1000. C: Tracheal lumen completely impacted with denuded epithelial cells with inflammatory exudates mixed with mucus at 2 dpi X100. D: Immunohistochemical staining for EIV positive antigens in cytoplasm of lining epithelial cells (arrow) at 3 dpi (IIPT) X600.E: Moderate degeneration of lining epithelial cells with severe infiltration of lymphocytes, macrophages and neutrophils in lamina propria at 5 dpi X400. F: Regeneration of lining epithelial cells with mild infiltrations of macrophages in lamina propria X400 at 7 dpi (n = 6).
Fig 4
Fig 4. Pathogenicity of eq/J-K/08 H3N8 EIV in mice:Lung (H&E):
A: Early interstitial thickening with mild perivascular cuffing by inflammatory cells (arrow) at 1 dpi X100. B: Majority of bronchial epithelial cells shows hydropic degeneration and necrosis of lining epithelial cells (arrow) at 12hpi. C: Degenerated and necrosed bronchial epithelial cells (Fig 4C section) reveals EIV positive antigens in cytoplasm (arrow) at 12 hr post infection (IIPT) X400 (n = 6). D: Moderate degeneration and denudation of bronchial lining epithelial cells (arrow) with macrophage infiltration in interstitial spaces at 2 dpi X400. E: Immunohistochemical staining for EIV positive antigens (Fig 4D section) in degenerated and denuded bronchiolar epithelial cells (arrow) and interstitial macrophages at 2 dpi (IIPT) X400 (n = 6).
Fig 5
Fig 5. Pathogenicity of eq/J—K/08 H3N8 EIV in mice: Lung (H&E):
A: Margination (arrow head), transmigration (arrow) and perivascular cuffing by neutrophils at 2 dpi X400. B: Necrotic bronchiolar epithelium with macrophages in bronchial lumen (arrow) at 5 dpi X400. C: Degeneration of lining epithelial cells (arrow) with mild peribronchiolar infiltration of lymphocytes at 5 dpi X400. Immunohistochemical localization of EIV antigens in mice lung. D: Distributions of EIV positive antigens in bronchiolar epithelial cells (arrow) at 5 dpi (Fig 5C section) (IIPT) X400 (n = 6). E: Severe consolidation of parenchyma with narrowing of bronchiolar lumen (arrow head) at 5 dpi X100 (n = 6). F: Resolution of lung lesion with mild thickening of alveolar septa and mild perivascular infiltrations at 14 dpi X100 (n = 6).
Fig 6
Fig 6. Pathogenicity of eq/J-K/08 H3N8 EIV in mice:
Immunohistochemical localization of EIV antigens in mice lung. A: Intra-cytoplasmic EIV positive antigens in bronchiolar epithelial cells (arrow) at 3 dpi X1000. B: EIV positive interstitial macrophages (arrow) at 3 dpi X400. C: EIV positive antigens in cytoplasm of interstitial macrophages at 3 dpi (arrow) X1000 (n = 6). D: EIV positivity in bronchial lining epithelial cells (arrow) at 5 dpi (IIPT) X600 (n = 6).
Fig 7
Fig 7. Pathogenicity of eq/J-K/08 H3N8 EIV in mice:
A: Intercellular space with influenza virions, budding and released from surface of the degenerating cells from trachea at 3 dpi. Irregularly shaped microvilli can be seen amongst the virions X10000. B: Ultra thin section of lung at 3 dpi, showing budding of influenza virions from degenerating cell along with disintegration of nuclear envelope and loss of organelle architecture X 10000. C: Budding and release of influenza virions from surface of a degenerating cell in lung X8000. D: Fragmentation of nucleus with degeneration of endoplasmic reticulum of epithelial cells of lung at 3 dpi X8000 (n = 6).
Fig 8
Fig 8. EIV replication kinetics.
A: Quantification and duration of EIV shedding through mice nostril (per ml of nasal washings) following experimental infection with EIV. B: Replication kinetics of EIV in mice lung (per gram of lung tissues) following experimental infection in BALB/c mice. C and D: Quantification of EIV genome copy numbers in nasal washings (140 μl) (n = 3) and lung tissues (25mg) (n = 6) following experimental EIV infection in BALB/c mice (n = 6).

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The present study was funded by Indian Council of Agricultural Research from institutional grant. Indian Veterinary Research Institute provided junior research fellowship to the first author. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

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