Mucosal T cell activation pathways are upregulated by equine herpesvirus type 1 infection

Equine herpesvirus type 1 (EHV-1) is a high morbidity and mortality virus that impacts horse populations worldwide. As a respiratory virus, it enters through the upper respiratory tract (URT), where mucosal immunity plays a crucial role in preventing severe disease. In this study, flow cytometry was used to characterize the nasal leukocyte population during EHV-1 infection, and RNA sequencing of nasal secretions was employed to assess transcriptional markers of the mucosal immune response. Horses with distinct immune statuses were compared at four stages: pre-infection, early (day 1 and 3 post-infection [pi]), mid (days 8 and 10 pi) and late (day 18 pi) infection. Non-immune horses (n = 4), developed clinical signs, shed virus, and established viremia, whereas, immune horses (n = 4), possessing pre-existing immunity, showed no disease or detectable virus after EHV-1 infection. This comparison revealed differential expression of various T cell associated markers based on pre-existing immunity. In immune horses, upregulation of fewer genes occurred during early infection only, while in non-immune horses, a large number of genes were upregulated during mid and late infection. Collectively, these findings highlight a key role for mucosal T cells in mediating viral clearance and restoring homeostasis.