The induction of significant T cell and antibody responses in nonhuman primates by use of a multiclade rAd5 prototype and the conferment of protection from SHIV and SIV challenge provided the rationale for the present clinical trial. rAd5 vectors have several attractive features, including (1) scalable manufacturing in stable cell lines approved for other human biologics; (2) efficient gene delivery to antigen-presenting cells [21
]; (3) potent transient protein expression; (4) induction of innate immune responses; (5) induction of antibody responses; (6) intracellular production of antigen, allowing induction of both CD8+
T cell responses [22
]; and (7) induction of immunity after a single injection.
The present study identified a delivery approach and dose of a candidate multiclade HIV-1 rAd5 vaccine that was assessed as safe and immunogenic. Vaccination did not cause severe adverse reactions. The highest dose of rAd5 caused a moderate, short-lived syndrome of headache, myalgia, malaise, and fever that began within 24 h after vaccination. Therefore, it is expected that dose-dependent systemic reactogenicity may follow rAd5 vaccination.
The induction of HIV-specific cellular immunity is a major goal for HIV vaccine development. HIV-specific CD8+
T cell responses clear virus-infected cells and appear during the declining viremia that follows acute infection [23
]. High levels of CD8+
T cells present during chronic HIV-1 infection suggest their importance in the control of viremia [25
]. HIV-specific CD4+
T cell responses also peak early during infection; however, this response diminishes soon after seroconversion [35
]. Maintenance of a functional HIV-specific CD4+
T cell response correlates with long-term nonprogression of HIV disease [37
]. In addition, nonhuman primate models of lentivirus infection have shown that either CD8+
T cell depletion or mutation of key CD8+
T cell epitopes within the circulating virus results in increased levels of viremia and disease progression [32
]. These observations suggest that HIV-1–specific CD4+
T cells are important for controlling HIV replication and preventing disease progression.
The present study demonstrates that a single rAd5 immunization can induce HIV-specific CD4+
T cell responses in the majority of vaccinated subjects (response rates by week 24 were 28/30 [93.3%] and 20/30 [60%], respectively). The peak cellular response occurred 4 weeks after vaccination, and the frequency of detectable responses diminished during the 24-week follow-up period. The peak cellular immune response was not dose related, although there was a greater frequency of responses in the higher dose groups and greater frequency of detectable cellular responses for 24 weeks in the 1011
-PU group. The frequency and magnitude of both CD4+
T cell responses to rAd5 immunization as measured by the ICS and ELISpot assays were similar to those induced by 3 doses of plasmid DNA vaccine expressing matching vaccine antigens [17
]. Defining the duration of response more precisely will require larger studies, which are currently under way.
Generating a neutralizing antibody response to HIV-1 may be the greatest challenge to the development of a protective HIV vaccine [39
]. Although rAd5 vaccination induced modest levels of HIV-specific antibodies as measured by ELISA (~10-fold higher magnitude than in DNA-primed subjects [17
]), it did not induce significant HIV-1 neutralizing antibodies to HXB2 or SF162. Typically, neutralizing activity is not detected in the serum of HIV-1–infected humans or SIV-infected macaques until the ELISA titer is >1:100,000. By ELISA, there was a greater frequency and titer of HIV-specific antibodies in the 1011
-PU group () than in the 109
- or 1010
-PU group, suggesting a threshold effect for antibody induction. The magnitude of antibody responses to EnvC was greater than that to EnvA (P
= .022). However, it is not known whether this was related to the immunogenicity of EnvC in the vaccine or to enhanced antigenicity of the EnvC protein used in the ELISA. The rAd5 vaccine induced HIV-1 Env–specific antibodies to multiple clades, but the induction of neutralizing antibodies remains an elusive goal.
The existence of neutralizing antibodies to Ad5 in adults has led to questions regarding the utility of rAd5-based vaccines in humans. Nonhuman primate studies have suggested that preexisting Ad5 immunity diminishes the cellular response to vaccine antigens [40
] and that higher doses of rAd5 are required to overcome the immune inhibiting effects of preexisting Ad5 immunity. Selected vaccine recipients in the present study with high levels of Ad5 neutralizing antibody mounted significant HIV-specific humoral and cellular immune responses, indicating that preexisting neutralizing antibodies do not preclude the induction of an immune response to the recombinant antigens expressed by rAd5. However, overall the subjects with preexisting Ad5 immunity had T cell responses that were ~3-fold lower in magnitude than those of the Ad5-seronegative subjects.
The present clinical study identified 1010
PUs as a well-tolerated rAd5 dose that can stimulate a multiclade HIV-1 immune response in subjects with a range of preexisting Ad5 antibody titers for further evaluation as a booster vaccine to be used in combination with a multiclade HIV-1 DNA vaccine candidate [17
]. Although some vaccine recipients at the 1011
-PU dose experienced a short-lived and self-limited syndrome of headache, myalgia, malaise, and fever, they also had better induction of HIV-1–specific antibody and a trend toward better maintenance of T cell responses. Therefore, additional studies evaluating dose are in progress concomitantly with the expanded studies evaluating the 1010
-PU dose as a booster vaccine. The present clinical trial demonstrates the safety and immunogenicity of a rAd-based multigene, multi-clade HIV-1 vaccine in humans and may represent a step toward the development of a globally relevant vaccine regimen.