Immunogenicity of viral vector, prime-boost SIV vaccine regimens in infant rhesus macaques: attenuated vesicular stomatitis virus (VSV) and modified vaccinia Ankara (MVA) recombinant SIV vaccines compared to live-attenuated SIV

Abstract

In a previously developed infant macaque model mimicking HIV infection by breast-feeding, we demonstrated that intramuscular immunization with recombinant poxvirus vaccines expressing simian immunodeficiency virus (SIV) structural proteins provided partial protection against infection following oral inoculation with virulent SIV. In an attempt to further increase systemic but also local antiviral immune responses at the site of viral entry, we tested the immunogenicity of different orally administered, replicating vaccines. One group of newborn macaques received an oral prime immunization with a recombinant vesicular stomatitis virus expressing SIVmac239 gag, pol and env (VSV-SIVgpe), followed 2 weeks later by an intramuscular boost immunization with MVA-SIV. Another group received two immunizations with live-attenuated SIVmac1A11, administered each time both orally and intravenously. Control animals received mock immunizations or non-SIV VSV and MVA control vectors. Analysis of SIV-specific immune responses in blood and lymphoid tissues at 4 weeks of age demonstrated that both vaccine regimens induced systemic antibody responses and both systemic and local cell-mediated immune responses. The safety and immunogenicity of the VSV-SIVgpe+MVA-SIV immunization regimen described in this report provide the scientific incentive to explore the efficacy of this vaccine regimen against virulent SIV exposure in the infant macaque model.

Figure 1. Summary of infant vaccine groups

As described in the materials and methods, in two sets of experiments newborn macaques were immunized shortly after birth via the oral route with replicating SIV vaccines, either VSV-SIV (expressing SIV gag, pol, env) or live-attenuated SIVmac1A11. Booster immunizations on some groups consisted of intramuscular MVA-SIV. The 2^nd set of experiments had also a group that received control vectors expressing non-SIV antigens (VSV-HA, expressing influenza hemagglutinine, and MVA-MV expressing measles virus).

Figure 2. Humoral immune responses

Vector-specific antibody responses in the immunized groups (see Figure 1) were evaluated by measuring VSV neutralizing antibodies (graph A) or MVA-binding antibodies by ELISA (graph B). SIV-specific IgG was measured via SIV gag/pol or SIV gp130 env-specific ELISA s. All titers are endpoint titers (i.e. highest dilution above cut-off value of the respective assays). For each graph, p values refer to comparison of 2 groups via the Mann-Whitney test

Figure 3. SIV-specific cell-mediated immune responses in CD4+ T and CD8+ lymphocytes

In experiment 2, SIV specific immune responses in PBMC and lymphoid tissues were measured for the two SIV vaccine arms (groups G and H) via intra-cellular cytokine flow cytometry for interferon-γ, TNF-α and IL-2. Values are expressed as percentages of cytokine-expressing cells per CD4+CD3+ T lymphocytes (panel A) or CD8+CD3+ T lymphocytes (panel B) for the 2 vaccine groups. Abbreviations: LN: lymphnode; R.: retropharyngeal; S.: submandibular; Mes.: mesenteric; Ax.: axillary; n.t. indicates not tested.

Figure 3. SIV-specific cell-mediated immune responses in CD4+ T and CD8+ lymphocytes

In experiment 2, SIV specific immune responses in PBMC and lymphoid tissues were measured for the two SIV vaccine arms (groups G and H) via intra-cellular cytokine flow cytometry for interferon-γ, TNF-α and IL-2. Values are expressed as percentages of cytokine-expressing cells per CD4+CD3+ T lymphocytes (panel A) or CD8+CD3+ T lymphocytes (panel B) for the 2 vaccine groups. Abbreviations: LN: lymphnode; R.: retropharyngeal; S.: submandibular; Mes.: mesenteric; Ax.: axillary; n.t. indicates not tested.

Figure 4. Proportion of animals with SIV-specific cell-mediated immune responses

In graph A, using the data presented in Figure 3, the number of animals (out of a total of 8 animals for each of the 2 vaccine groups) with a detectable SIV-specific cell-mediated immune response for any of the 3 cytokines measured (IFN-γ, TNF-α and IL-2) was tabulated for 3 groups of tissues: the oral lymphoid tissues (tonsil, retropharyngeal lymph node (R. LN) and submandibular lymph node (S. LN)); Mesenteric LN; and the peripheral lymphoid system consisting of axillary lymph node (Ax. LN) and PBMC. In graph B, the number of animals with a detectable SIV-specific cell-mediated immune response in any of the tissues was tabulated for each of the 3 cytokines that were measured.