Utilization of Feline ELISpot to Evaluate the Immunogenicity of a T Cell-Based FIV MAP Vaccine

The prototype and the commercial dual-subtype feline immunodeficiency virus (FIV) vaccines conferred protection against homologous FIV strains as well as heterologous FIV strains from the vaccine subtypes with closely related envelope (Env) sequences. Such protection was mediated by the FIV neutralizing antibodies (NAbs) induced by the vaccines. Remarkably, both prototype and commercial FIV vaccines also conferred protection against heterologous FIV subtypes with highly divergent Env sequences from the vaccine strains. Such protection was not mediated by the vaccine-induced NAbs but was mediated by a potent FIV-specific T-cell immunity generated by the vaccines (Aranyos et al., Vaccine 34: 1480-1488, 2016). The protective epitopes on the FIV vaccine antigen were identified using feline interleukin-2 (IL-2) and interferon-γ (IFNγ) ELISpot assays with overlapping FIV peptide stimulation of the peripheral blood mononuclear cells (PBMC) from cats immunized with prototype FIV vaccine. Two of the protective FIV peptide epitopes were identified on FIV p24 protein and another two protective peptide epitopes were found on FIV reverse transcriptase. In the current study, the multiple antigenic peptides (MAPs) of the four protective FIV peptides were combined with an adjuvant as the FIV MAP vaccine. The laboratory cats were immunized with the MAP vaccine to evaluate whether significant levels of vaccine-specific cytokine responses can be generated to the FIV MAPs and their peptides at post-second and post-third vaccinations. The PBMC from vaccinated cats and non-vaccinated control cats were tested for IL-2, IFNγ, and IL-10 ELISpot responses to the FIV MAPs and peptides. These results were compared to the results from CD4⁺ and CD8⁺ T-cell proliferation to the FIV MAPs and peptides. Current study demonstrates that IL-2 and IFNγ ELISpot responses can be used to detect memory responses of the T cells from vaccinated cats after the second and third vaccinations.