The Mechanism Of Equine Tetherin- Mediated Restriction Of Equine Infectious Anemia Virus And Its Antagonist

Equine infectious anemia virus (EIAV) is a macrophage-tropic lenti virus. The genome structure, replication cycle, and antigenic drift of EIAV are similar with other lentiviruses. However, among the lentiviruses, the EIAV genome is the smallest and genetically the simplest. EIAV uses gag, pol and env genes to encode structural proteins which are common to all retroviruses. In addition, four regulatory accessory proteins are encoded by virus. These proteins are Tat, Rev, S2 and Ttm. This makes EIAV a useful model system to examine the contribution of specific viral genes in lentivirus replication and pathogenesis. However, the interaction between these viral proteins and host cellular factor during the course of EIAV infection has not been fully determined. EIAV causes a persistent infection which is typically classified in three defined stages: acute, chronic, and long-term asymptomatic. Greater than 90% of horses infected with EIAV undergo transition into the inapparent or subclinical state and appear clinically normal with no signs of disease. The inapparent stage can be due to the establishment of a strict immunological control over virus replication by host immune response. This makes EIAV a practical model to examine the natural immunological control of lentivirus replication and disease.During viral infection, cellular pattern recognition receptors (PRRs) that are located on cellular membranes or within the cytosol could recognize viral components, and then trigger an IFN response that induces a program of interferon-stimulated genes (ISGs) with broad-ranging effects on cell growth and metabolism. Many of these ISGs are restriction factors that specifically inhibit viral growth within the infected cell. These restriction factors intervene against the virus at every step of replication. However, viruses have evolved mechanisms that antagonize restriction factors. For example, Trim5a specifically binds and multimerizes around incoming retroviral capsid proteins to cause an accelerated uncoating of the viral core, thereby inhibiting the viral replication. APOBEC3 proteins could be packaged into the nascent virion particles and restricts the virus through its cytidine deaminase enzymatic activity up its next round of infection. Meanwhile, the Vif protein of human immunodeficiency virus 1 (HIV-1) acts as an antagonist to overcome the antiviral activity of APOBEC3. Tetherin protein blocks HIV-1 virion particles release from host cells. HIV-1 escapes from the antiviral activity of tetherin by its Vpu protein. So far, it is not clear about the relationship between EIAV and these restriction factors. Equine APOBEC3 was the only equine restriction factor that has been characterized.In present study, we hypothesize that,1) whether equine macrophage cells encode an ortholog of tetherin and how tetherin transcripts are expressed in equine cells permissive for EIAV replication.2) whether equine tetherin inhibit EIAV replication in equine macrophage cells.3) whether equine tetherin blocks retroviral virus-like particles (VLPs) release.4) how EIAV evades this host restriction. Based on these hypotheses, we isolated the equine tetherin gene and investigated its antiviral activity and how this activity is counteracted by EIAV. Equine tetherin shares 53%,40%,36%, and 34% amino acid sequence identity with feline, human, simian, or murine tetherins, respectively. Equine tetherin has a shorter N-terminal domain than human tetherin. Equine tetherin is localized on cell surface, and strongly blocks HIV-1, SIV and EIAV release from virus producing cells. The antiviral activity of equine tetherin is neutralized by EIAV envelope protein, but not by the HIV-1 accessory protein Vpu, which is a human tetherin antagonist; and EIAV envelope protein does not counteract human tetherin. These results shed new light on our understanding of the species-specific tetherin antiviral mechanism.