Multigene DNA Priming-Boosting Vaccines Protect Macaques from Acute CD4+-T-Cell Depletion after Simian-Human Immunodeficiency Virus SHIV89.6P Mucosal Challenge

An effective vaccine to reduce human immunodeficiency virus (HIV) infection and subsequent disease is not yet available. In 2002 alone, an estimated five million people became infected with HIV type 1 (HIV-1) (66). Ninety-five percent of new infections occur in developing countries. Although effective treatments are available, they have a high failure rate and their long-term effects are unknown and, more important, they are unaffordable in the countries hit hardest by the epidemic. Vaccines are likely to be the only viable long-term solution. A wide variety of HIV vaccine candidates are being tested in small clinical trials with humans; however, to date only one has been evaluated in a phase III efficacy trial (31). As new vaccine concepts are developed, the nonhuman primate models of AIDS allow direct testing of a vaccine followed by a viral challenge. The macaque monkey can be infected with viruses related to HIV. Several strains of simian immunodeficiency virus (SIV), and chimeras of HIV and SIV called simian HIV (SHIV), infect macaques and cause a disease similar to human AIDS (43). Both SIV and SHIV have been used to evaluate protection from infection or from disease. Use of SHIV further allows testing of vaccines that contain HIV Envelope, which could be moved directly into formulations for human trials. Importantly, a number of approaches have shown promise in these models, supporting further development and testing in humans. The predictive value of nonhuman primate models will only be determined after successful human trials. DNA vaccination is a promising strategy that has yet to be fully exploited. In principle, DNA vaccines share major advantages with live attenuated vaccines: the vaccine antigen is made by transfected host cells, will thus be folded and modified in its native conformation, and can be presented to the immune system by major histocompatibility complex (MHC) class I and II molecules. DNA vaccines are safer than live attenuated viruses because they can be made replication incompetent by carrying only single genes or genomes with large deletions, and a number of animal and human studies have demonstrated their safety. Both cellular and humoral immunity can be generated, and protection from disease has been achieved against a variety of pathogens in animal models (16, 54). Human trials of DNA vaccines for AIDS show induction of antibodies and cellular immune responses to HIV-1 (13, 31, 39), while several macaque studies have shown reduced SIV or SHIV viral load in animals vaccinated with DNA alone (10, 20, 28, 38, 42, 55). However, the relatively low levels of immunity elicited by current methods of DNA vaccine delivery have failed to provide sterilizing immunity against the primate lentiviruses. Thus, efforts have been directed toward enhancing the low-level immune responses elicited by DNA vaccines by combining them with a different type of vaccine modality to boost the priming response (29, 42, 55). The priming-boosting concept showed early promise for SIV and HIV vaccines with recombinant vaccinia virus and subunit protein boosting (33). Several recent studies have demonstrated good protection of macaques from disease induced by SHIV89.6P after priming-boosting vaccines including a DNA priming vaccine (2, 60). There is increasing evidence that inclusion of multiple immune targets in vaccines is more effective than using a single antigen. Many candidate AIDS vaccines have been made against Env only (reviewed in reference 64), Gag only (20, 60), or Tat only (24). While Env is the sole target of neutralizing antibody, all of the viral genes may be targets for cytotoxic T lymphocyte, and individual infected patients and animals develop cytotoxic-T-lymphocyte responses to multiple viral gene products (12, 44). Different MHC haplotypes are able to present epitopes from different genes; and even in related animals which share some MHC alleles, epitopes from different genes are targeted (23). Given the variable breadth of responses in individuals, single-antigen vaccines may not be effective in all recipients. Furthermore, HIV sequences vary greatly; several antigens in a vaccine increase the chance of providing at least one that is cross-reactive with a virus to which a vaccinee is exposed. Therefore, it is compelling to use multiple antigens in an AIDS vaccine. Indeed, recent studies suggest that multiantigen vaccines are superior to comparable Gag-only or Env-only formulations (1, 47). We have evaluated the immunogenicity and protective efficacy of several priming-boosting vaccine regimens for SHIV89.6P, a highly virulent challenge virus that causes rapid CD4+-T-cell depletion in unvaccinated Macaca mulatta (rhesus macaques) and in Macaca nemestrina (pigtailed macaques). Each regimen included a multigene DNA vaccine delivered by gene gun. We found significant protection from disease in M. nemestrina macaques given the combination of DNA and recombinant vaccinia virus, with DNA either as the priming vaccine or as the boosting vaccine.