Recombinant vaccine-induced protection against the highly pathogenic simian immunodeficiency virus SIV(mac251): dependence on route of challenge exposure

Abstract

Vaccine protection from infection and/or disease induced by highly pathogenic simian immunodeficiency virus (SIV) strain SIV(mac251) in the rhesus macaque model is a challenging task. Thus far, the only approach that has been reported to protect a fraction of macaques from infection following intravenous challenge with SIV(mac251) was the use of a live attenuated SIV vaccine. In the present study, the gag, pol, and env genes of SIV(K6W) were expressed in the NYVAC vector, a genetically engineered derivative of the vaccinia virus Copenhagen strain that displays a highly attenuated phenotype in humans. In addition, the genes for the alpha and beta chains of interleukin-12 (IL-12), as well as the IL-2 gene, were expressed in separate NYVAC vectors and inoculated intramuscularly, in conjunction with or separate from the NYVAC-SIV vaccine, in 40 macaques. The overall cytotoxic T-lymphocyte (CTL) response was greater, at the expense of proliferative and humoral responses, in animals immunized with NYVAC-SIV and NYVAC-IL-12 than in animals immunized with the NYVAC-SIV vaccine alone. At the end of the immunization regimen, half of the animals were challenged with SIV(mac251) by the intravenous route and the other half were exposed to SIV(mac251) intrarectally. Significantly, five of the eleven vaccinees exposed mucosally to SIV(mac251) showed a transient peak of viremia 1 week after viral challenge and subsequently appeared to clear viral infection. In contrast, all 12 animals inoculated intravenously became infected, but 5 to 6 months after viral challenge, 4 animals were able to control viral expression and appeared to progress to disease more slowly than control animals. Protection did not appear to be associated with any of the measured immunological parameters. Further modulation of immune responses by coadministration of NYVAC-cytokine recombinants did not appear to influence the outcome of viral challenge. The fact that the NYVAC-SIV recombinant vaccine appears to be effective per se in the animal model that best mirrors human AIDS supports the idea that the development of a highly attenuated poxvirus-based vaccine candidate can be a valuable approach to significantly decrease the spread of human immunodeficiency virus (HIV) infection by the mucosal route.

FIG. 1

Immunological parameters measured for vaccinated control animals. (A) Mean titers of neutralizing antibodies (on H9 cells), measured for four animals from each group, at different time points before viral challenge. The vertical bar represents the standard error within each column. Under each column, the week in which the samples were collected is indicated. The months (6 and 12) are the times of vaccine inoculation. Ab, antibody. (B) The mean values of CTL activity at various effector-to-target ratios are shown for four animals in each experimental group. The values of weeks and months correspond to the times of sample collection and of vaccine inoculation, respectively. (C) Mean values of IL-2 production, following in vitro stimulation with native SIV_mac251 gp120, by effector cells from four animals in each experimental group. Values of weeks and months correspond to times of sample collection and vaccine inoculation, respectively.

FIG. 2

Plasma virus load measurement and absolute CD4⁺ T-cell counts for immunized and control macaques following i.v. challenge exposure to SIV_mac251. The top panels show the results of virus load measurements for the plasma of control animals (A) and vaccinees (B and C). The bottom panels show absolute CD4⁺ T-cell counts for controls (D) and vaccinees (E and F).

FIG. 3

Histological staining and in situ RT-PCR results for lymph nodes from controls and vaccinees. (A) Results obtained from the lymph nodes of slow-progressor animals 269, 273, 274, and 276 at 6 months after viral exposure. (B) Three representative lymph nodes from control animals 279, 282, and 286 collected 6 months after viral exposure.

FIG. 3

Histological staining and in situ RT-PCR results for lymph nodes from controls and vaccinees. (A) Results obtained from the lymph nodes of slow-progressor animals 269, 273, 274, and 276 at 6 months after viral exposure. (B) Three representative lymph nodes from control animals 279, 282, and 286 collected 6 months after viral exposure.

FIG. 4

Virus load in control and vaccinated animals following SIV_mac251 i.r. challenge. •, viral RNA copies per milliliter of plasma; ○, neutralizing antibody titers against laboratory-adapted SIV_K1W. Data are presented for all vaccinated animals (A and B) before and after viral challenge. In the case of control animals (C), neutralizing antibodies were positive only after viral challenge, as expected. On the left side of each figure are results of in situ RT-PCR on viral RNA and histological staining of the lymph nodes of some of the animals in the study. The cross sign indicates the death of the animal.

FIG. 4

Virus load in control and vaccinated animals following SIV_mac251 i.r. challenge. •, viral RNA copies per milliliter of plasma; ○, neutralizing antibody titers against laboratory-adapted SIV_K1W. Data are presented for all vaccinated animals (A and B) before and after viral challenge. In the case of control animals (C), neutralizing antibodies were positive only after viral challenge, as expected. On the left side of each figure are results of in situ RT-PCR on viral RNA and histological staining of the lymph nodes of some of the animals in the study. The cross sign indicates the death of the animal.

FIG. 4

Virus load in control and vaccinated animals following SIV_mac251 i.r. challenge. •, viral RNA copies per milliliter of plasma; ○, neutralizing antibody titers against laboratory-adapted SIV_K1W. Data are presented for all vaccinated animals (A and B) before and after viral challenge. In the case of control animals (C), neutralizing antibodies were positive only after viral challenge, as expected. On the left side of each figure are results of in situ RT-PCR on viral RNA and histological staining of the lymph nodes of some of the animals in the study. The cross sign indicates the death of the animal.

FIG. 4

Virus load in control and vaccinated animals following SIV_mac251 i.r. challenge. •, viral RNA copies per milliliter of plasma; ○, neutralizing antibody titers against laboratory-adapted SIV_K1W. Data are presented for all vaccinated animals (A and B) before and after viral challenge. In the case of control animals (C), neutralizing antibodies were positive only after viral challenge, as expected. On the left side of each figure are results of in situ RT-PCR on viral RNA and histological staining of the lymph nodes of some of the animals in the study. The cross sign indicates the death of the animal.

FIG. 4

Virus load in control and vaccinated animals following SIV_mac251 i.r. challenge. •, viral RNA copies per milliliter of plasma; ○, neutralizing antibody titers against laboratory-adapted SIV_K1W. Data are presented for all vaccinated animals (A and B) before and after viral challenge. In the case of control animals (C), neutralizing antibodies were positive only after viral challenge, as expected. On the left side of each figure are results of in situ RT-PCR on viral RNA and histological staining of the lymph nodes of some of the animals in the study. The cross sign indicates the death of the animal.

FIG. 4

Virus load in control and vaccinated animals following SIV_mac251 i.r. challenge. •, viral RNA copies per milliliter of plasma; ○, neutralizing antibody titers against laboratory-adapted SIV_K1W. Data are presented for all vaccinated animals (A and B) before and after viral challenge. In the case of control animals (C), neutralizing antibodies were positive only after viral challenge, as expected. On the left side of each figure are results of in situ RT-PCR on viral RNA and histological staining of the lymph nodes of some of the animals in the study. The cross sign indicates the death of the animal.