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Posted: 8 Feb 2012
Journal Club: Giving DNA vaccines a boost

In this week's Journal Club, Dr. Denise Hsu reviews two recent attempts to improve immune responses to DNA vaccines using plasmids that code for immune-modulatory cytokines.


Of the three HIV vaccine candidates that have completed human efficacy trials, two used viral vectors to deliver the HIV antigen. The STEP trial used an adenovirus type 5 vector, and the RV-144 trial used a canarypox vector.

One issue with vaccines using the recombinant viral vector platform is that naturally acquired immunity to these viruses, both humoral and cellular, can reduce immune responses to the insert antigens. This was evident in the STEP trial (Buchbinder et al., Frahm et al.).

DNA vaccines can be delivered without a vector and are able to induce T cell and humoral immunity in animal studies. The approach involves the delivery of the gene of the antigen of interest on a plasmid. Once the plasmid enters the host cell, the gene is transcripted, translated and expressed by the host cell and thus triggers an immune response.

Unfortunately, DNA vaccines have shown a reduced level of immunogenicity in human studies compared to subunit or vector vaccines. However, their immunogenicity can be improved through a variety of techniques, such as optimizing the transcriptional elements, the protein expression, the formulation, the delivery system and complementing with immune modulatory adjuvants (Kutzler and Weiner). The use of immune modulatory adjuvants gave promising results in animal studies (Lai et al., Schadeck et al.).

Two studies have been published recently, which assessed the safety and immunogenicity of DNA vaccines complemented with cytokine adjuvants in humans.  The first study, by Kalams et al., described the phase 1 randomized, doubled blinded, placebo controlled HVTN 060 and HVTN 063 trials. They were conducted in the USA, Brazil and Thailand, involving 264 participants.

The gag DNA vaccine candidate tested was adjuvanted with plasmids coding for IL-12 (important for the induction of cellular immune response) or IL-15 (important for stimulation and proliferation of activated T and B cells and maintenance of NK cells).

The study found that the vaccines were well tolerated and there were no significant differences in either reactogenicity or rates of adverse events between the placebo, the vaccine alone or the vaccine in combination with cytokine adjuvants. There were no Grade 3, 4 or severe adverse events.

The T cells responses generated after vaccinations with all regimens (either a three or a five dose regimen) were minimal and none of the participants assayed post 5th vaccination had a response. There was also no statistically significant augmentation of response with either the IL-12 or IL-15 plasmid adjuvants. No participants developed a vaccine-induced humoral anti-gag response.

The results from this study were disappointing. It may be that DNA vaccination is not very immunogenic in humans. An explanation for the lack of immune response maybe the choice of antigen, as Gag is less immunogenic than Env and the Gag used in this vaccine was truncated, so that not all immuno-dominant epitopes were included.

The second study, by Baden et al., was also a randomized, double blinded, placebo controlled phase 1 trial that evaluated the safety and immunogenicity of a DNA vaccine (Gag-Pol-Nef- multiclade Env [clades A, B, and C]) alone, in combination with escalating doses of IL-2/Ig fusion plasmid adjuvant or DNA vaccine followed by IL-2/Ig fusion plasmid 48 hours later.

IL-2 is a pleiotropic cytokine with multiple effects. It is a potent T cell growth factor and is crucial for antigen specific immune responses. In this study, IL-2 was fused with the Fc portion of immunoglobulin G in order to prolong IL-2’s half life. The IL-2/Ig plasmid was either administered simultaneously with the DNA vaccine or given 48 hours post vaccine. Administration of IL-2/Ig 48 hours later was associated with augmentation of response in mice. The hypothesis was that simultaneous administration of IL-2 and antigen may stimulate a broad cellular response whereas IL-2 administered 48 hours later would mainly stimulate cells that are reacting to the vaccine antigens.

The study involved 70 participants at 7 clinical sites within the HIV Vaccine Trials Network (HVTN). It found that both vaccine and IL-2/Ig were well tolerated and there were no significant differences in local or systemic reactogenicity among the groups.

T cell responses to Env A, B and C were significantly higher when the IL-2/Ig plasmid was given 48 hrs after the HIV DNA vaccine than when it was given simultaneously (see Figure 1). However, there was only a trend of an augmentation of T cell responses to Env A or B by IL-2/Ig plasmid when given 48 hours later compared to the vaccine alone. A greater IFNγ response to Env C was detected in those who had IL-2/ Ig plamsid 48 hours later when compared to vaccine alone. Almost no responses were identified to Gag, Pol or Nef in all participants. Antibody response rate and magnitude were also low.

Figure 1. The magnitude of IFNγ Elispot responses to vaccine-matched Env A, B and C peptide antigens. Reproduced from Baden et al. Timing of plasmid cytokine (IL-2/Ig) administration affects HIV-1 vaccine immunogenicity in HIV- seronegative subjects. JID. 2011;204:1541-9 by permission of Oxford University Press.

Results from this study are interesting in that alteration of time of administration of cytokine adjuvant can lead to augmentation of DNA vaccine responses. Thus future studies on cytokine plasmid adjuvant would need to carefully consider the timing of cytokine adjuvant administration in addition to which cytokine to use.

The immunogenicity of DNA vaccines in human is still sub optimal. The above two studies demonstrated the safety of immune-modulatory plasmids. However, further work is required to demonstrate their efficacy in augmenting vaccine response in humans. The use of these immune-modulatory plasmids maybe applied to other vaccine strategies, e.g., prime-boost vaccines to increase immunogenicity. Thus these findings will likely be important for future vaccine studies.

About the Author:
Dr. Denise Hsu, BscMBBS, FRACP, FRCPA
Kirby Institute for infection and immunity in society, UNSW, Sydney, Australia
HIVNAT, Bangkok, Thailand


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