ABSTRACTEquine herpesvirus type 1 (EHV-1) is a main cause of respiratory disease, abortion, and encephalomyelopathy in horses. Monocytic cells (CD172a+) are the main carrier cells of EHV-1 during primary infection and are proposed to serve as a “Trojan horse” to facilitate the dissemination of EHV-1 to target organs. However, the mechanism by which EHV-1 is transferred from CD172a+cells to endothelial cells (EC) remains unclear. The aim of this study was to investigate EHV-1 transmission between these two cell types. We hypothesized that EHV-1 employs specific strategies to promote the adhesion of infected CD172a+cells to EC to facilitate EHV-1 spread. Here, we demonstrated that EHV-1 infection of CD172a+cells resulted in a 3- to 5-fold increase in adhesion to EC. Antibody blocking experiments indicated that α4β1, αLβ2, and αVβ3integrins mediated adhesion of infected CD172a+cells to EC. We showed that integrin-mediated phosphatidylinositol 3-kinase (PI3K) and ERK/MAPK signaling pathways were involved in EHV-1-induced CD172a+cell adhesion at early times of infection. EHV-1 replication was enhanced in adherent CD172a+cells, which correlates with the production of tumor necrosis factor alpha (TNF-α). In the presence of neutralizing antibodies, approximately 20% of infected CD172a+cells transferred cytoplasmic material to uninfected EC and 0.01% of infected CD172a+cells transmitted infectious virus to neighboring cells. Our results demonstrated that EHV-1 infection induces adhesion of CD172a+cells to EC, which enhances viral replication, but that transfer of viral material from CD172a+cells to EC is a very specific and rare event. These findings give new insights into the complex pathogenesis of EHV-1.IMPORTANCEEquine herpesvirus type 1 (EHV-1) is a highly prevalent pathogen worldwide, causing frequent outbreaks of abortion and myeloencephalopathy, even in vaccinated horses. After primary replication in the respiratory tract, EHV-1 disseminates via cell-associated viremia in peripheral blood mononuclear cells (PBMC) and subsequently infects the endothelial cells (EC) of the pregnant uterus or central nervous system, leading in some cases to abortion and/or neurological disorders. Recently, we demonstrated that CD172a+monocytic carrier cells serve as a “Trojan horse” to facilitate EHV-1 spread from blood to target organs. Here, we investigated the mechanism underlying the transmission of EHV-1 from CD172a+cells to EC. We demonstrated that EHV-1 infection induces cellular changes in CD172a+cells, promoting their adhesion to EC. We found that both cell-to-cell contacts and the secretion of soluble factors by EC activate EHV-1 replication in CD172a+cells. This facilitates transfer of cytoplasmic viral material to EC, resulting mainly in a nonproductive infection. Our findings give new insights into how EHV-1 may spread to EC of target organs in vaccinated horses.
Equine herpesvirus type 1 replication is delayed in CD172a+ monocytic cells and controlled by histone deacetylases
