Immune Responses and Viral Replication in Long-Term Inapparent Carrier Ponies Inoculated with Equine Infectious Anemia Virus

Abstract

Persistent infection of equids by equine infectious anemia virus (EIAV)is typically characterized by a progression during the first year postinfection from chronic disease with recurring disease cycles to a long-term asymptomatic infection that is maintained indefinitely. The goal of the current study was to perform a comprehensive longitudinal analysis of the course of virus infection and development of host immunity in experimentally infected horses as they progressed from chronic disease to long-term inapparent carriage. We previously described the evolution of EIAV genomic quasispecies (C. Equine infectious anemia virus (EIAV) infection of horses provides a novel system in which to examine the natural immunological control of lentivirus replication and disease (reviewed in reference 27). Horses infected in the field or experimentally with EIAV typically develop within the first month postinfection acute disease (fever, diarrhea, lethargy, anemia, and thrombocytopenia) and an associated high level of infectious plasma viremia. Following this initial clinical episode that lasts 3 to 5 days, most infected horses experience recurring disease episodes and associated waves of viremia at irregular intervals. This cyclic disease is designated chronic EIA. The frequency of disease episodes and the severity of clinical symptoms typically decrease with time and are usually completely resolved by 1 year postinfection. At this time, persistently infected horses become clinically asymptomatic for EIA and negative for infectious plasma viremia, indicating a highly effective control of virus replication and disease. In fact, most horses infected by EIAV are inapparent carriers that will remain asymptomatic for the remainder of their life span of up to 20 years. Thus, the EIAV system offers a uniquely dynamic model in which to examine changes in viral replication and host immune responses during the clearly demarcated progression from chronic disease to a long-term inapparent infection.LerouxA number of studies indicate that the eventual control of EIAV replication and disease in horses is mediated by host immune responses that control virus infection to subclinical levels and not by the attenuation of the virus during persistent infection. For example, transfer of whole blood from long-term inapparent carriers to naive horses reproducibly causes infection and disease (11), and experimental immune suppression of inapparent carriers can cause recrudescence of disease and associated viremia (19,43). Recent analyses of EIAV infection in long-term apparent carriers by genetic (8, 40) and in situ (29) methods demonstrate persistent low levels of virus infection and replication predominantly in tissue macrophages, with negligible virus detectable in plasma or peripheral blood cells. These studies indicate that the progression from chronic EIA to inapparent infection is associated with the evolution of highly effective and enduring host immune responses that are able to suppress EIAV replication, despite the array of persis-* Corre...

Immune suppression of challenged vaccinates as a rigorous assessment of sterile protection by lentiviral vaccines

“…Serum samples were also evaluated for seroreactivity by the standard agar gel immunodiffusion (AGID) procedure [36] diagnostic assay for EIA. Serum IgG antibody reactivity to EIAV envelope glycoproteins was assayed quantitatively (end point titer) and qualitatively (avidity index, conformation ratio) using our standard concanavalin A (ConA) ELISA procedures as described previously [15]. Statistical significance of the differences between vaccine groups was determined through a nonparametric One-way ANOVA (KruskalWallis Test) with Dunnett's post test (GraphPad InStat version 3.0, San Diego, CA).…”

“…inoculations, at two day intervals, of 10 median horse infectious doses (HID 50 ) of the virulent challenge virus, EIAV PV ; 1 HID 50 is the equivalent of approximately 0.1 TCID 50 . The horses were monitored daily for clinical symptoms of EIA, and blood was drawn at regular intervals (weekly, daily if febrile) for assays of platelets, viral replication, and virus-specific immune responses [15,22]. The horses were observed for a total of 310 days, at which time they were euthanized.…”

“…EIA is characterized during its acute and chronic stages by defined episodes of clinical disease triggered by waves of viremia and distinguished by fever, anemia, thrombocytopenia, edema, and various wasting signs. By 8-12 months post-infection horses typically progress to life-long inapparent carriers, but continue to harbor various steady state levels of viral replication in monocyte-rich tissue reservoirs [15][16][17]. Stress or immune suppression of EIAV inapparent carriers can induce an increase in viral replication and potentially a recrudescence of disease [17,18].…”

Envelope variation as a primary determinant of lentiviral vaccine efficacy

“…To develop challenge strains of increasing divergence from the parental vaccine strain, we used EIAV variant strains from a quasispecies population previously isolated and characterized as part of various studies on EIAV genetic and antigenic evolution during persistent infections with the reference EIAV PV strain (21,(28)(29)(30). The EIAV D9 attenuated vaccine strain was derived from molecular clones generated from the EIAV PV strain (25,31).…”

“…The acute and chronic stages are defined by episodes of clinical disease, triggered by waves of viremia, and distinguished by fever, anemia, thrombocytopenia, edema, and various wasting signs. By 1 year after infection, animals typically progress to life-long inapparent carriers, continuing to harbor steady-state levels of viral replication in monocyte-rich tissue reservoirs (19)(20)(21). Stress or immune suppression of inapparent carriers can induce increases in viral replication and, potentially, a recrudescence of disease (19,22).…”