Improved Protection of Rhesus Macaques against Intrarectal Simian Immunodeficiency Virus SIVmac251 Challenge by a Replication-Competent Ad5hr-SIVenv/rev and Ad5hr-SIVgag Recombinant Priming/gp120 Boosting Regimen

Despite extensive efforts made to combat human immunodeficiency virus (HIV) infection and AIDS since the discovery of the virus, the number of people infected with HIV and developing the disease worldwide is still increasing rapidly. The need for a vaccine against HIV is now one of the world's greatest public health problems; however, development of a safe and effective HIV vaccine has proved difficult due to several unique challenges presented by the virus. These include difficulty in eliciting broadly reactive neutralizing antibodies, the high variability of the virus, and integration of HIV proviral DNA into the host genome, resulting in latent infection and making achievement of sterilizing immunity nearly impossible (44). Considering recent reports associating either humoral or cellular immune responses with protection against HIV infection or disease progression, it is difficult to define requirements for protective immunity against HIV (1, 4, 9, 23, 24, 32, 38, 40). Accumulating evidence indicates that an ideal HIV vaccine should induce broad humoral, cellular, and mucosal immunity against multiple viral antigens in order to combat infectious viral particles and HIV-infected cells at any point during infection (19, 25, 33, 50). To achieve this goal, many strategies are being investigated, including recombinant viral proteins and peptides, naked DNA, live viral and bacterial vectors, and prime-boost combinations (19). Adenovirus (Ad) is one of the live viral vectors being developed for use as an HIV vaccine. Ad infects a broad spectrum of human cells, including immature dendritic cells, leading to efficient antigen presentation and causing their maturation without polarizing the T-helper response (22, 53, 54). Because AIDS is mainly a sexually transmitted disease, vaccine-elicited mucosal immunity against HIV is critical. Ad vectors are therefore highly attractive, because they target epithelial cells at mucosal surfaces and can be administered orally and intranasally. Both replication-competent and replication-defective Ad recombinants have been investigated as potential AIDS vaccines. Replication-defective Ad vectors, long used in gene therapy applications, have been adapted for use as HIV vaccines (5, 46, 51). Recent studies with an E1- and E3-deleted Ad5-SIVgag recombinant to immunize rhesus macaques elicited high-frequency SIV p11C-tetramer-positive cells. Following challenge with pathogenic SHIV89.6P the monkeys exhibited significantly reduced viral burdens and were protected against SHIV-induced disease (46). We have taken a different approach, using replication-competent Ad recombinants with deletions only in the E3 region. Because of the inability of human Ad to replicate in most mammalian species, our studies initially were carried out with chimpanzees, which are permissive for Ad replication. Replication-competent, E3 region-deleted Ad-HIVenv and -HIVgag/pro recombinants were investigated and shown to elicit cellular immune responses, antibody responses in mucosal secretions, high-titer serum antibodies able to neutralize both T-cell-line-adapted and primary HIV isolates, and significant protective efficacy (20, 21, 30, 31, 43, 55). Chimpanzees immunized with an Ad-HIVenv priming/gp120 boosting regimen were protected against both low- and high-dose HIV challenges, including challenge with a heterologous primary HIV isolate. The protection elicited was shown to be long lasting. To further develop this approach in a macaque model, we took advantage of an Ad5 host range mutant (Ad5hr) (41) and carried out experiments by using an Ad5hr-SIVsmH4env/rev recombinant shown to replicate in monkey cells in vitro (8). Again by using a recombinant priming/gp120-boosting regimen, we demonstrated that the Ad5hr recombinant also replicated in vivo and elicited SIV-specific cellular immunity and humoral immune responses in serum and secretory fluids (6, 7). This solely envelope-based vaccine achieved a reduction in acute-phase viral burden following intravaginal challenge with pathogenic SIVmac251; however, the viral load began increasing by 8 weeks postchallenge. Accumulating data have shown the potential benefit of incorporating additional viral antigens that elicit strong cellular immune responses into vaccine regimens. Therefore, we added an SIV gag component to the vaccination regimen. We have reported that coadministration of the Ad5hr-SIVsmH4env/rev plus Ad5hr-SIVgag recombinant elicits cellular immune responses to each of the encoded SIV gene products, Env, Gag, and Rev, and that the responses are boosted by a second administration of the same recombinants (35, 52). These recombinant immunizations also prime strong neutralizing antibodies and both IgG and IgA SIV-specific antibodies in secretory fluids (J. Pinczewski, J. Zhao, L. J. Patterson, N. Malkevitch, G. Alvord, and M. Robert-Guroff, unpublished results). Here we have investigated the ability of the dual immunization regimen to protect against an intrarectal challenge with pathogenic SIVmac251 and have explored whether particular immune responses are correlated with the protective outcome. We report improved protective efficacy against a mucosal SIVmac251 challenge, with viremia reduced at both the acute phase and set point of infection. Several immune parameters contributed to the protective outcome.