Equine infectious anemia virus genomic evolution in progressor and nonprogressor ponies

A live attenuated equine infectious anemia virus proviral vaccine with a modified S2 gene provides protection from detectable infection by intravenous virulent virus challenge of experimentally inoculated horses

Previous evaluations of inactivated whole-virus and envelope subunit vaccines to equine infectious anemia virus (EIAV) have revealed a broad spectrum of efficacy ranging from highly type-specific protection to severe enhancement of viral replication and disease in experimentally immunized equids. Among experimental animal lentivirus vaccines, immunizations with live attenuated viral strains have proven most effective, but the vaccine efficacy has been shown to be highly dependent on the nature and severity of the vaccine virus attenuation. We describe here for the first time the characterization of an experimental attenuated proviral vaccine, EIAV(UK)deltaS2, based on inactivation of the S2 accessory gene to down regulate in vivo replication without affecting in vitro growth properties. The results of these studies demonstrated that immunization with EIAV(UK)deltaS2 elicited mature virus-specific immune responses by 6 months and that this vaccine immunity provided protection from disease and detectable infection by intravenous challenge with a reference virulent biological clone, EIAV(PV). This level of protection was observed in each of the six experimental horses challenged with the reference virulent EIAV(PV) by using a low-dose multiple-exposure protocol (three administrations of 10 median horse infectious doses [HID(50)], intravenous) designed to mimic field exposures and in all three experimentally immunized ponies challenged intravenously with a single inoculation of 3,000 HID(50). In contrast, naïve equids subjected to the low- or high-dose challenge develop a detectable infection of challenge virus and acute disease within several weeks. Thus, these data demonstrate that the EIAV S2 gene provides an optimal site for modification to achieve the necessary balance between attenuation to suppress virulence and replication potential to sufficiently drive host immune responses to produce vaccine immunity to viral exposure.

Characterization of EIAV LTR variability and compartmentalization in various reservoir tissues of long-term inapparent carrier ponies

Dynamic genomic variation resulting in changes in envelope antigenicity has been established as a fundamental mechanism of persistence by equine infectious anemia virus (EIAV), as observed with other lentiviruses, including HIV-1. In addition to the reported changes in envelope sequences, however, certain studies indicate the viral LTR as a second variable EIAV gene, with the enhancer region being designated as hypervariable. These observations have lead to the suggestion that LTR variation may alter viral replication properties to optimize to the microenvironment of particular tissue reservoirs. To test this hypothesis directly, we examined the population of LTR quasispecies contained in various tissues of two inapparent carrier ponies experimentally infected with a reference EIAV biological clone for 18 months. The results of these studies demonstrated that the EIAV LTR is in fact highly conserved with respect to the infecting LTR species after 1.5 years of persistent infection and regardless of the tissue reservoir. Thus, these comprehensive analyses demonstrate for the first time that the EIAV LTR is highly conserved during long-term persistent infection and that the observed variations in viral LTR are associated more with in vitro adaptation to replication in cultured cells rather than in vivo replication in natural target cells.

Equine infectious anemia virus envelope evolution in vivo during persistent infection progressively increases resistance to in vitro serum antibody neutralization as a dominant phenotype

Equine infectious anemia virus (EIAV) infection of horses is characterized by well-defined waves of viremia associated with the sequential evolution of distinct viral populations displaying extensive envelope gp90 variation; however, a correlation of in vivo envelope evolution with in vitro serum neutralization phenotype remains undefined. Therefore, the goal of the present study was to utilize a previously defined panel of natural variant EIAV envelope isolates from sequential febrile episodes to characterize the effects of envelope variation during persistent infection on viral neutralization phenotypes and to define the determinants of EIAV envelope neutralization specificity. To assess the neutralization phenotypes of the sequential EIAV envelope variants, we determined the sensitivity of five variant envelopes to neutralization by a longitudinal panel of immune serum from the source infected pony. The results indicated that the evolution of the EIAV envelope sequences observed during sequential febrile episodes produced an increasingly neutralization-resistant phenotype. To further define the envelope determinants of EIAV neutralization specificity, we examined the neutralization properties of a panel of chimeric envelope constructs derived from reciprocal envelope domain exchanges between selected neutralization-sensitive and neutralization-resistant envelope variants. These results indicated that the EIAV gp90 V3 and V4 domains individually conferred serum neutralization resistance while other envelope segments in addition to V3 and V4 were evidently required for conferring total serum neutralization sensitivity. These data clearly demonstrate for the first time the influence of sequential gp90 variation during persistent infection in increasing envelope neutralization resistance, identify the gp90 V3 and V4 domains as the principal determinants of antibody neutralization resistance, and indicate distinct complex cooperative envelope domain interactions in defining sensitivity to serum antibody neutralization.

Transient immune suppression of inapparent carriers infected with a principal neutralizing domain-deficient equine infectious anaemia virus induces neutralizing antibodies and lowers steady-state virus replication

The genetic variation of equine infectious anaemia virus (EIAV) clearly affects the antigenic properties of the viral envelope; however, effects on immunogenicity remain undefined, although widely assumed. Here, the immunogenicity is reported of a novel, neutralization-resistant, pony-isolate envelope EIAV(PV564DeltaPND) that contains a 14-residue deletion in the designated principal neutralizing domain (PND) of the gp90 protein. Two ponies inoculated with a chimeric virus, EIAV(DeltaPND), containing the EIAV(PV564DeltaPND) envelope in a reference provirus strain, remained asymptomatic through 14 months post-inoculation, producing high steady-state levels of envelope-specific antibodies but no detectable serum-neutralizing antibodies. Consequent dexamethasone-induced immune suppression produced characteristic EIA that resolved concomitantly with the development of high-titre, strain-specific, neutralizing antibodies and a 100-fold reduction in steady-state virus loads. These results demonstrate: natural variations in the EIAV envelope have profound effects on both antigenic and immunogenic properties; the PND is not required for neutralizing antibody responses; and transient immune suppression can enhance established host immunity to achieve more effective control of steady-state lentivirus replication.