Utilization of feline ELISPOT for mapping vaccine epitopes

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.

Conserved epitopes on HIV-1, FIV and SIV p24 proteins are recognized by HIV-1 infected subjects

Cross-reactive peptides on HIV-1 and FIV p24 protein sequences were studied using peripheral blood mononuclear cells (PBMC) from untreated HIV-1-infected long-term survivors (LTS; >10 y of infection without antiretroviral therapy, ART), short-term HIV-1 infected subjects not on ART, and ART-treated HIV-1 infected subjects. IFNγ-ELISpot and CFSE-proliferation analyses were performed with PBMC using overlapping HIV-1 and FIV p24 peptides. Over half of the HIV-1 infected subjects tested (22/31 or 71%) responded to one or more FIV p24 peptide pools by either IFNγ or T-cell proliferation analysis. PBMC and T cells from infected subjects in all 3 HIV(+) groups predominantly recognized one FIV p24 peptide pool (Fp14) by IFNγ production and one additional FIV p24 peptide pool (Fp9) by T-cell proliferation analysis. Furthermore, evaluation of overlapping SIV p24 peptide sequences identified conserved epitope(s) on the Fp14/Hp15-counterpart of SIV, Sp14, but none on Fp9-counterpart of SIV, Sp9. The responses to these FIV peptide pools were highly reproducible and persisted throughout 2-4 y of monitoring. Intracellular staining analysis for cytotoxins and phenotyping for CD107a determined that peptide epitopes from Fp9 and Fp14 pools induced cytotoxic T lymphocyte-associated molecules including perforin, granzyme B, granzyme A, and/or expression of CD107a. Selected FIV and corresponding SIV epitopes recognized by HIV-1 infected patients indicate that these protein sequences are evolutionarily conserved on both SIV and HIV-1 (e.g., Hp15:Fp14:Sp14). These studies demonstrate that comparative immunogenicity analysis of HIV-1, FIV, and SIV can identify evolutionarily-conserved T cell-associated lentiviral epitopes, which could be used as a vaccine for prophylaxis or immunotherapy.

An initial examination of the potential role of T-cell immunity in protection against feline immunodeficiency virus (FIV) infection

The importance of vaccine-induced T-cell immunity in conferring protection with prototype and commercial FIV vaccines is still unclear. Current studies performed adoptive transfer of T cells from prototype FIV-vaccinated cats to partial-to-complete feline leukocyte antigen (FLA)-matched cats a day before either homologous FIVPet or heterologous-subtype pathogenic FIVFC1 challenge. Adoptive-transfer (A-T) conferred a protection rate of 87% (13 of 15, p < 0.001) against FIVPet using the FLA-matched T cells, whereas all 12 control cats were unprotected. Furthermore, A-T conferred protection rate of 50% (6 of 12, p<0.023) against FIVFC1 using FLA-matched T cells, whereas all 8 control cats were unprotected. Transfer of FLA-matched T and B cells demonstrated that T cells are needed to confer A-T protection. In addition, complete FLA-matching and addition of T-cell numbers > 13 × 10(6) cells were required for A-T protection against FIVFC1 strain, reported to be a highly pathogenic virus resistant to vaccine-induced neutralizing-antibodies. The addition of FLA-matched B cells alone was not protective. The poor quality of the anti-FIV T-cell immunity induced by the vaccine likely contributed to the lack of protection in an FLA-matched recipient against FIVFC1. The quality of the immune response was determined by the presence of high mRNA levels of cytolysin (perforin) and cytotoxins (granzymes A, B, and H) and T helper-1 cytokines (interferon-γ [IFNγ] and IL2). Increased cytokine, cytolysin and cytotoxin production was detected in the donors which conferred protection in A-T studies. In addition, the CD4(+) and CD8(+) T-cell proliferation and/or IFNγ responses to FIV p24 and reverse transcriptase increased with each year in cats receiving 1X-3X vaccine boosts over 4 years. These studies demonstrate that anti-FIV T-cell immunity induced by vaccination with a dual-subtype FIV vaccine is essential for prophylactic protection against AIDS lentiviruses such as FIV and potentially HIV-1.

The Significance of Interferon-γ in HIV-1 Pathogenesis, Therapy, and Prophylaxis

Interferon-γ (IFNγ) plays various roles in the pathogenesis of HIV/AIDS. In an HIV-1 infected individual, the production of IFNγ is detected as early as the acute phase and continually detected throughout the course of infection. Initially produced to clear the primary infection, IFNγ together with other inflammatory cytokines are involved in establishing a chronic immune activation that exacerbates clinical diseases associated with AIDS. Unlike Type 1 IFNs, IFNγ has no direct antiviral activity against HIV-1 in primary cultures, as supported by the in vivo findings of IFNγ therapy in infected subjects. Results from both in vitro and ex vivo studies show that IFNγ can instead enhance HIV-1 replication and its associated diseases, and therapies aimed at decreasing its production are under consideration. On the other hand, IFNγ has been shown to enhance cytotoxic T lymphocytes and NK cell activities against HIV-1 infected cells. These activities are important in controlling HIV-1 replication in an individual and will most likely play a role in the prophylaxis of an effective vaccine against HIV-1. Additionally, IFNγ has been used in combination with HIV-1 vaccine to augment antiviral immunity. Technological advancements have focused on using IFNγ as a biological marker to analyze the type(s) of immunity generated by candidate HIV vaccines and the levels of immunity restored by anti-retroviral drug therapies or novel immunotherapies. Hence, in addition to its valuable ancillary role as a biological marker for the development of effective HIV-1 prophylactic and therapeutic strategies, IFNγ has a vital role in promoting the pathogenesis of HIV.

Evolutionarily conserved epitopes on human immunodeficiency virus type 1 (HIV-1) and feline immunodeficiency virus reverse transcriptases detected by HIV-1-infected subjects

Anti-human immunodeficiency virus (HIV) cytotoxic T lymphocyte (CTL)-associated epitopes, evolutionarily conserved on both HIV type 1 (HIV-1) and feline immunodeficiency virus (FIV) reverse transcriptases (RT), were identified using gamma interferon (IFN-γ) enzyme-linked immunosorbent spot (ELISpot) and carboxyfluorescein diacetate succinimide ester (CFSE) proliferation assays followed by CTL-associated cytotoxin analysis. The peripheral blood mononuclear cells (PBMC) or T cells from HIV-1-seropositive (HIV(+)) subjects were stimulated with overlapping RT peptide pools. The PBMC from the HIV(+) subjects had more robust IFN-γ responses to the HIV-1 peptide pools than to the FIV peptide pools, except for peptide-pool F3. In contrast, much higher and more frequent CD8(+) T-cell proliferation responses were observed with the FIV peptide pools than with the HIV peptide pools. HIV-1-seronegative subjects had no proliferation or IFN-γ responses to the HIV and FIV peptide pools. A total of 24% (40 of 166) of the IFN-γ responses to HIV pools and 43% (23 of 53) of the CD8(+) T-cell proliferation responses also correlated to responses to their counterpart FIV pools. Thus, more evolutionarily conserved functional epitopes were identified by T-cell proliferation than by IFN-γ responses. In the HIV(+) subjects, peptide-pool F3, but not the HIV H3 counterpart, induced the most IFN-γ and proliferation responses. These reactions to peptide-pool F3 were highly reproducible and persisted over the 1 to 2 years of testing. All five individual peptides and epitopes of peptide-pool F3 induced IFN-γ and/or proliferation responses in addition to inducing CTL-associated cytotoxin responses (perforin, granzyme A, granzyme B). The epitopes inducing polyfunctional T-cell activities were highly conserved among human, simian, feline, and ungulate lentiviruses, which indicated that these epitopes are evolutionarily conserved. These results suggest that FIV peptides could be used in an HIV-1 vaccine.