Replicating Rather than Nonreplicating Adenovirus-Human Immunodeficiency Virus Recombinant Vaccines Are Better at Eliciting Potent Cellular Immunity and Priming High-Titer Antibodies

The overall immune responses elicited naturally by human immunodeficiency virus (HIV) infection are not effective at controlling viral replication or disease progression. HIV establishes persistence by immune evasion strategies (13), inherently resisting neutralizing antibodies, repeatedly selecting mutants that escape antibody and T-cell immune responses, and avoiding cytotoxic T-lymphocyte (CTL) killing and impairing CD4 T-cell function by downregulation of major histocompatibility complex class I and CD4 molecules from the surface of infected cells. Since HIV is exquisitely adapted for pathogenesis, an efficacious HIV vaccine will need to induce broader, more potent cellular and humoral immune responses than those elicited by natural infection (7). Live viral vectors, such as adenovirus (Ad), as vaccine vehicles present one option for inducing more potent immunity. Ads are advantageous because they target epithelial cells of the upper respiratory tract and gut, inducing mucosal immunity critical for preventing HIV infection at genital and/or rectal sites. Ads infect immature dendritic cells (DC), leading to DC maturation and efficient antigen presentation of inserted viral gene products (49, 50). Ads are highly immunogenic and engage both arms of the immune system, eliciting long-lasting cellular and humoral immunity to inserted gene products. Replication-competent (replicating) Ad-HIV recombinants exploit the potential of Ad vectors for eliciting persistent immune responses. In replicating Ad recombinants, expression of the encoded HIV antigen is incorporated into the Ad replication cycle, so lower immunization doses can achieve longer and higher expression levels of HIV gene product in vivo than replication-defective Ad recombinants. In vivo replication of Ad recombinants stimulates production of proinflammatory cytokines that can augment immune responses. Apoptotic cells arising from Ad replication can provide DC with exogenous antigens for initiation of T-cell responses through cross-presentation (12). Although vaccine vectors may compete with transgenes for induction of immune responses (see below), strong immune responses to Ad antigens may paradoxically enhance immunity to transgene-encoded HIV antigens via CD8-T-cell-mediated autocrine help (39), whereby CD8+ T cells can provide help for other responding CD8+ T cells if present in sufficient numbers (43). A combination vaccine regimen involving priming with replicating Ad-HIV or simian immunodeficiency virus (SIV) recombinants and boosting with HIV or SIV envelope proteins has elicited strong cellular, humoral, and mucosal immune responses to inserted HIV and SIV gene products in both chimpanzee and macaque models (22, 23, 32, 51). Chimpanzees immunized with this regimen exhibited long-lasting protection against HIV challenges (22, 34), whereas macaques have shown significant protection against a highly pathogenic SIVmac251 challenge (33, 48). Recently, such priming with multigenic Ad-SIV recombinants and boosting with envelope protein subunits induced potent protection against SIVmac251 intrarectal challenge. A total of 39% of immunized macaques remained aviremic after challenge or cleared or controlled plasma viremia to the threshold of detection (33). Protection during the chronic phase of infection was correlated with vaccine-induced cellular immunity and during the acute phase of infection with anti-envelope binding antibodies. The latter antibodies have been recently shown to mediate antibody-dependent cellular cytotoxicity (ADCC), and the activity was significantly correlated with reduced acute-phase viremia (15). Replication-defective (nonreplicating) Ad recombinants lacking E1 genes required for replication are also being developed (6, 11, 20, 35). In macaques, a nonreplicating Ad5-SIVgag recombinant combined with SIVgag DNA priming very effectively induced high frequencies of SIV-specific T cells and significantly reduced viral burden after a SHIV89.6P challenge (35). Although these animal model results are encouraging, some obstacles need to be overcome before using Ad vaccines in humans. Preexisting immunity to Ad vectors can impede induction of effective immunity to encoded immunogens. Anti-Ad5 immunity has suppressed nonreplicating Ad/HIV vaccines in mice (4, 38, 46) and rhesus macaques (6). Since Ad-neutralizing antibodies are serotype specific, anti-Ad immunity may be surmounted to a large extent by use of alternative Ad vectors not prevalent in humans (10, 42) or by sequential immunization with Ad vectors of different serotype (22). Use of high-dose nonreplicating Ad vaccines (>1010 PFU) to overcome prior immunity, however, can result in significant toxicity and immunopathology (21, 28, 41). Lower doses of replication-defective Ad vector may avoid toxicity, but at the expense of an adequate immune response as seen in a phase I trial in which only 40% of the volunteers exhibited a positive response to the vaccine (8). Thus, circumventing anti-Ad immunity while using an immunization dose that avoids immunopathology and yet achieves sufficient antigen expression for induction of potent immune responses is critical. We hypothesized that sequential immunization with low doses of replicating Ad recombinants based in different serotypes would better induce potent persistent cellular immunity to the encoded antigen and more effectively prime strong antibody responses to an envelope protein boost, while circumventing anti-Ad immunity than a nonreplicating Ad recombinant. Therefore, we compared the priming ability and immune responses elicited by low-dose replicating and higher-dose nonreplicating Ad recombinants in chimpanzees, permissive for replication of human Ad, to allow evaluation of both vectors. To evaluate priming of antibody responses, we included boosts with oligomeric HIVSF162 gp140ΔV2, an HIV envelope subunit now being tested in phase I clinical trials. This optimally modified HIV envelope immunogen possesses a trimeric structure, has critical neutralizing epitopes exposed, and elicits antibodies able to neutralize a spectrum of clade B primary isolates (2, 37). In parallel, we assessed induction of humoral and cellular immune responses to the Ad vectors themselves.