Author ORCID Identifier

https://orcid.org/0000-0001-5050-0183

Date Available

9-14-2019

Year of Publication

2019

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Agriculture, Food and Environment

Department/School/Program

Veterinary Science

First Advisor

Dr. Thomas M. Chambers

Abstract

Equine herpesvirus-1 (EHV-1) is one of the most important and prevalent viral pathogens of horses causing a major threat to the equine industry throughout most of the world. EHV-1 primarily causes respiratory disease but viral spread to distant organs enables the development of more severe sequelae; abortion and neurologic disease. In order to produce disease, EHV-1 has to overcome the innate barrier of the type-I interferon (IFN) system in host cells. However, the underlying mechanisms employed by EHV-1 to circumvent the type-I IFN response in host cells are not well understood. In this project study, using molecular techniques, we explored how EHV-1 is able to escape the type-I IFN response in host cells during infection. We also investigated whether EHV-4, a closely related but less pathogenic virus, has similar effects on type-I IFN as a clue to understanding how widespread IFN suppressive function is found among equine alphaherpesviruses.

Our data showed that inhibition of the type-I IFN response in host cells is not a function of neuropathogenicity of EHV-1 strains. However, a reduced type-I IFN response correlated with pathogenicity as EHV-4, unlike EHV-1, was unable to down-regulate the type-I IFN response in equine endothelial cells (EECs). Investigation of the mechanisms employed by EHV-1 to suppress type-I IFN revealed that the virus sequentially prevented outside-in signaling events that lead to type-I IFN production. Specifically, EHV-1 blocked the expression of Toll-like receptors (TLR) 3 and TLR4 at 6 hours post-infection (hpi) and 12 hpi. EHV-1 also prevented the transcription of IRF7 and IRF9 at different time-points during infection. The virus also perturbed the JAK-STAT signaling pathway by negatively regulating the cellular levels of TYK2 and phosphorylation-mediated activation of STAT2 molecules. Immunofluorescence data revealed that during infection, EHV-1 was able to sequester STAT2 molecules from nuclear translocation. This may be a limiting step preventing the formation of interferon- stimulated gene factor 3 (ISGF3) whose nuclear translocation is required to transactivate interferon-stimulated genes (ISGs) including IRF7.

Further investigation showed that unlike EHV-1, EHV-4 only interfered with phosphorylation-mediated activated STAT1 and STAT2 molecules at 3 and 6 hpi. EHV-4 was unable to block TLR3/4 and IRF7/9 mRNA expression at any time-point. Intriguingly, while viral late gene of EHV-1 mediates inhibition of STAT phosphorylation, our data showed that for EHV-4, a virus late gene did not mediate the inhibition of STAT phosphorylation. The findings from this study help illuminate how EHV-1 strategically interferes with limiting steps required for type-I IFN response in host cells to promote pathology. Our data also strengthen the hypothesis that the ability to shut off host factors required for type-I IFN production might be directly related to the degree of pathogenicity of the EHV subtypes.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2019.374

Funding Information

This work was supported by the USDA National Institute of Food and Agriculture, Hatch project no. 1011380. It is a project of the Kentucky Agricultural Experiment Station (project no. KY014053), supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Animal Health Program. I was also supported by grant from the American Quarter Horse Association Young Investigator Award and by a fellowship from the Geoffrey C. Hughes Foundation.

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