Lipopeptide immunization to study cytotoxic T cell responses using the equine infectious anemia lentiviral animal model

The purpose of this study was to test the hypothesis that cytotoxic T lymphocyte (CTL) epitopes recognized by horses chronically infected with equine infectious anemia virus (EIAV) will induce a protective immune response in immunized horses. In the first experiment, the ability of lipopeptides containing previously identified CTL epitopes to stimulate memory cytotoxic T lymphocytes (CTLm) in horses was demonstrated. In the second experiment, lipopeptide containing the ELA-A1-restricted CTL epitope from the envelope surface unit (SU) protein of the EIAV_WSU5 strain was used to immunize three horses having the ELA-A1 haplotype. Peptide-specific ELA-A1-restricted CTL were induced in all three horses, although these were present transiently. Horses were further immunized with lipopeptide containing the corresponding CTL epitope from the EIAVpv strain, which differs from the EIAV_WSU5 CTL epitope by two amino acids. Following immunization with lipopeptide containing the EIAV_PV CTL epitope, horses were challenged by intravenous inoculation with 300 TCID₅₀ EIAV_PV. Following EIAV _PV challenge, all horses developed cell free viremia, fever and thrombocytopenia. However, there was a statistically lower fever and thrombocytopenia severity score in the immunized group. There was no statistical difference between plasma viral load in immunized and non-immunized horses. Compared to the control group, two of the immunized horses developed titratable plasma viremia later and cleared the initial plasma viremia earlier than non-immunized horses. This shorter duration of plasma viral load in two immunized horses likely explains the significantly less severe clinical disease in this group. Results indicate that immunization with a lipopeptide containing a CTL epitope induced CTL and had a demonstrable protective effect against development of clinical disease following virus challenge.