In vitro antibody-dependent enhancement assays are insensitive indicators of in vivo vaccine enhancement of equine infectious anemia virus

We have previously demonstrated a high propensity for enhancement of virus replication and disease resulting from experimental immunization of ponies with a baculovirus recombinant envelope (rgp90) vaccine from equine infecti ous anemia virus (EIAV). The current studies were undertaken to examine the correlation between the observed in vivo vaccine enhancement and in vitro assays for antibody-dependent enhancement (ADE) of EIAV replication. Toward this goal an optimized EIAV in vitro enhancement assay was developed using primary equine macrophage cells and used to evaluate the enhancement prope rties of immune serum taken from rgp90 immunized ponies that displayed vari ous levels of vaccine enhancement after experimental challenge with EIAV. F or comparison, we analyzed in parallel immune serum samples from a group of ponies immunized with a viral envelope subunit vaccine (LL-gp) that produc ed sterile protection from EIAV challenge. The results of these assays demo nstrated that the rgp90 immune serum had a greater propensity for in vitro enhancement of EIAV replication than serum from the protected LL-gp immuniz ed ponies; in vitro enhancement levels for the rgp90 immune sera averaged a bout 1.5, with a maximum enhancement value of about 2.0. While distinguishi ng between immune serum produced by the rgp90 and LL-gp immunizations, the in vitro enhancement assay failed to reliably correlate with the severity o f in vivo enhancement observed among the rgp90 vaccine recipients. Vaccinat ed ponies that experienced moderate to no disease signs displayed levels of in vitro enhancement similar to those of ponies that experienced severe an d fatal enhancement of disease after viral challenge. The observed in vitro enhancement was demonstrated to be dependent on serum immunoglobulin, but independent of complement. These studies demonstrate in the EIAV system tha t in vitro ADE assays appear to be relatively insensitive indicators of the severity of in vivo enhancement and that relatively low levels of in vitro ADE can be associated with severe to fatal enhancement of virus replicatio n and disease in vivo. These observations suggest that relatively low level s of serum ADE observed in other lentivirus systems, including HIV-1, may h ave more profound effects on in vivo virus replication and disease than pre viously recognized. (C) 1999 Academic Press.