Equine herpesvirus type 1 infection induces procoagulant activity in equine monocytes

Abstract

The alphaherpesvirus, equine herpesvirus type 1 (EHV-1), is a highly prevalent cause of equine infectious abortion and encephalomyelopathy. These syndromes have been attributed to ischemic necrosis from thrombosis in placental and neural vessels, although the mechanisms underlying thrombosis are unknown. After inhalation, EHV-1 establishes a peripheral blood mononuclear cell-associated viremia, with monocytes being a target of infection. Monocytes are also the main source of tissue factor (TF) in diseased states. Since TF is the primary activator of coagulation, increased monocyte TF expression could be involved in EHV-1-associated thrombosis. We hypothesized that EHV-1 infection would induce TF-dependent procoagulant activity in equine monocytes. Monocyte-enriched fractions of blood were infected with abortigenic (RacL11, NY03) and neuropathogenic (Ab4) EHV-1 strains. All strains induced procoagulant activity, to variable degrees, within 1 to 4 h, with maximal activity at 24 h, after infection. Virus-induced procoagulant activity was similar to that seen with lipopolysaccharide, a known stimulant of TF-mediated procoagulant responses. Virus-induced procoagulant activity was factor VIIa-dependent and temporally associated with TF gene transcription, implicating TF as the main driver of the activity. Procoagulant activity was mildly decreased (30-40%) when virus was inactivated by ultraviolet light or when infected cells were treated with aphidicolin, a virus DNA polymerase inhibitor, suggesting early events of virus infection (attachment, entry or intracellular trafficking) are the primary stimulus of procoagulant activity. Our results indicate that EHV-1 rapidly stimulates procoagulant activity in equine monocytes in vitro. The EHV-1-induced procoagulant activity in monocytes may contribute to clinical thrombosis in horses with EHV-1 infection.

Coagulation parameters following equine herpesvirus type 1 infection in horses

“…In addition, Ebola virus infection induces TF expression in lymphoid macrophages and circulating blood cells, which is associated with Ebola haemorrhagic fever [21,24]. In this study, we confirm that in vitro, EHV-1 infection of PBMCs increases TF procoagulant activity, as was shown previously [11]. Our studies are the first to examine this relationship in infected horses, and the results support that PBMCs isolated from EHV-1 infected horses show increased TF procoagulant activity during acute infection (Fig 3).…”

“…Stimulation of PBMCs with either EHV-1 at a MOI of 5 for 24 h (Fig 1) or 100 ng/mL of LPS for 6 h (Fig 1) significantly increased FXa activity (P<0.05) indicating that in vitro exposure of PBMCs to either EHV-1 or LPS results in measurable increases in TF procoagulant activity, confirming previous results by Stokol et al and Yeo et al [10,11].…”

Unfractionated and Low-Molecular-Weight Heparin and the Phosphodiesterase Inhibitors, IBMX and Cilostazol, Block Ex Vivo Equid Herpesvirus Type-1-Induced Platelet Activation

“…The cause of the documented hypercoagulability and thrombosis in EHV-1-infected horses is unknown, but is likely multifactorial, involving release or upregulation of procoagulants, such as tissue factor, with endothelial injury or vasculitis. We have also previously shown that EHV-1 induces tissue factor expression in equine monocytes (9). Because tissue factor is the main activator of coagulation in vivo (10), upregulated expression of tissue factor on monocytes could be one trigger of the hypercoagulable state.…”

Plasma D‐Dimer Concentrations during ExperimentalEHV‐1 Infection of Horses

“…An alternative possible explanation of DD‐production during EHV‐1 viremia has been recently suggested by Yeo et al. They describe significantly increased release of tissue factor from EHV‐1‐infected equine monocytes in vitro when compared to uninfected monocytes . As the percentage of infected PBMC is very low during EHV‐1 viremia and the monocyte is not the predominant virus carrier during viremia, these interesting findings warrant further investigation …”