Abstract
Equine herpesvirus type 1 (EHV-1) is one of the most prevalent respiratory pathogens in horses with a high impact on animal health worldwide. Entry of the virus into epithelial cells of the upper respiratory tract and rapid local viral replication is followed by infection of local lymphoid tissues leading to cell-associated viremia and disease progression. Pre-existing mucosal immunity has previously been shown to reduce viral shedding and prevent viremia, consequently limiting severe disease manifestations. Here, nasopharyngeal transcriptomic profiling was used to identify differentially expressed genes following EHV-1 challenge in horses with different EHV-1 immune statuses. Immune horses (n = 4) did neither develop clinical disease nor viremia and did not shed virus after experimental infection, while non-immune horses (n = 4) did all the above. RNA sequencing was performed on nasopharyngeal samples pre- and 24 hours post-infection (24hpi). At 24hpi, 109 and 44 genes were upregulated in immune horses and non-immune horses, respectively, and three genes were explored in further detail. Antileukoproteinase (SLPI) gene expression increased 2.1-fold within 24 hours in immune horses in concert with protein secretion. Interferon (IFN)-induced proteins with tetratricopeptide repeats 2 (IFIT2) and 3 (IFIT3) were upregulated in non-immune horses, corresponding with nasal IFN-α secretion and viral replication. By contrast, neither IFIT expression nor IFN-α secretion was induced by EHV-1 infection of immune horses. Transcriptomic profiling offered a tool to identify, for the first time, the role of SLPI in innate immunity against EHV-1, and further emphasized the central role of the type I IFN response in the anti-viral defense of non-immune horses. Importance: Equine herpesvirus type 1 (EHV-1) remains a considerable concern in the equine industry, with yearly outbreaks resulting in morbidity, mortality, and economic losses. In addition to its importance in equine health, EHV-1 is a respiratory pathogen and an alphaherpesvirus, and it may serve as a model for other viruses with similar pathogenicity or phylogeny. Large animal models allow the collection of high-volume samples longitudinally, permitting in-depth investigation of immunological processes. This study was performed on bio-banked nasopharyngeal samples from an EHV-1 infection experiment, where clinical outcomes had previously been determined. Matched nucleic acid and protein samples throughout infection permitted longitudinal quantification of the protein or related proteins of selected differentially expressed genes detected during the transcriptomic screen. The results of this manuscript identified novel innate immune pathways of the upper respiratory tract during the first 24 hours of EHV-1 infection, offering a first look at the components of early mucosal immunity that are indicative of protection.