Elsevier

Vaccine

Volume 28, Issue 2, 11 December 2009, Pages 474-483
Vaccine

Ovine atadenovirus, a novel and highly immunogenic vector in prime-boost studies of a candidate HIV-1 vaccine

https://doi.org/10.1016/j.vaccine.2009.09.136Get rights and content

Abstract

Ovine adenovirus type 7 (OAdV) is the prototype member of the genus Atadenovirus. No immunity to the virus has so far been detected in human sera. We describe the construction and evaluation of a candidate HIV-1 vaccine based on OAdV and its utilisation alone and in combination with plasmid-, human adenovirus type 5 (HAdV5; a Mastadenovirus)-, and modified vaccinia Ankara (MVA)-vectored vaccines. All vectors expressed HIVA, an immunogen consisting of HIV-1 clade A consensus Gag-derived protein coupled to a T cell polyepitope. OAdV.HIVA was genetically stable, grew well and expressed high levels of protein from the Rous sarcoma virus promoter. OAdV.HIVA was highly immunogenic in mice and efficiently primed and boosted HIV-1-specific T cell responses together with heterologous HIVA-expressing vectors. There were significant differences between OAdV and HAdV5 vectors in priming of naïve CD8+ T cell responses to HIVA and in the persistence of MHC class I-restricted epitope presentation in the local draining lymph nodes. OAdV.HIVA primed T cells more rapidly but was less persistent than AdV5.HIVA and thus induced a qualitatively distinct T cell response. Nevertheless, both vectors primed a response in mice that reduced viral titres in a surrogate challenge model by three to four orders of magnitude. Thus, OAdV is a novel, underexplored vaccine vector with potential for further development for HIV-1 and other vaccines. The data are discussed in the context of the latest HIV-1 vaccine developments.

Introduction

A vaccine against human immunodeficiency virus type 1 (HIV-1) is sorely needed. To maximize the control of diverse HIV-1 isolates and decrease the tissue damage, an effective vaccine should induce broadly neutralizing antibodies and T cells [1]. Because the challenges for induction of each of these arms of immunity are considerable and yet quite different, the two successful vaccine strategies may have to be developed and optimized separately before combining them for maximum protection. For induction of HIV-1-specific T cells, the most advanced vaccine modalities under development include plasmid DNA [2], non-replicating viruses [3], [4], [5], attenuated bacteria [6], [7], [8], virus-like particles [9], and adjuvanted proteins and peptides [10]. HIV-1 immunogens introduced by all these modalities reach the major histocompatibility complex (MHC) class I presentation pathways for induction of CD8+ T cells directly and/or through cross-priming. To induce high frequencies of T cells specific for the passenger antigen and likely improve T cell quality, these subunit vaccines are often delivered by heterologous prime-boost regimens so as to avoid anti-vector immunity.

Prior to Merck's phase IIb proof-of-concept trial known as the STEP study, non-replicating vaccine vectors based on human adenovirus serotype 5 (HAdV5) were perceived as the most promising vaccine modality. This was due to the relatively high frequencies and persistence of HIV-1-specific T cell responses induced in HAdV5-seronegative humans [11], [12], which overshadowed two well-recognized caveats. First, high pre-existing levels of anti-HAdV5 antibodies found in 30% and up to 95% of individuals in European/North American and African countries, respectively, decreased recombinant HAdV5 vaccine take and lowered induction of T cells against expressed HIV-1-derived immunogens [13]. Second, the relatively high ex vivo T cell responses were based on measurements of interferon (IFN)-γ-producing T cells. Although this remains a very useful means for enumerating vaccine-elicited T cells, the relevance of IFN-γ production to protection against HIV-1 infection is uncertain [14], [15]. Indeed, more recent studies questioned the functionality of HAdV5-induced CD8+ T cells [16]. In the Step study, the HAdV5 vaccine failed to decrease both transmission and the break-through virus load at set-point [15]. Moreover, a largely unexplained trend towards a greater risk of HIV-1 acquisition in individuals with pre-existing antibodies to HAdV5 was observed. The lack of protection and safety concerns caused the testing of HAdV5-vectored vaccine candidates in humans to be suspended and the pre-STEP study momentum has not yet been fully recovered.

In parallel to HAdV5 [15], rare serotypes of HAdVs, such as 6, 11, 26 or 35 have been under development as HIV-1 vaccine vectors [17], [18], [19], [20], [21] because low levels of neutralizing antibodies to these viruses are found more often in humans. However, tests that measure virus neutralization in vitro are not necessarily predictive of the outcome in vivo, particularly in relation to changes in the hexon protein upon which serotype classification is based [22]. Adenoviruses have also been isolated from a variety of different animal species and these, too, are open to exploitation as potential vaccine vectors. Thus, vectors developed from AdV that naturally infect chimpanzees and closely resemble HAdV have entered or are in the pipeline for human trials [13], [23], [24], [25]. Bovine, porcine and canine AdV are also under investigation [26], [27]. The above AdV are all members of the Mastadenovirus genus.

Another non-human AdV is the prototype of the genus Atadenovirus. OAdV serotype 7 (OAdV), which is of ovine origin, is structurally and biologically distinct from the Mastadenoviruses [28], [29]. It carries two structural and many non-structural genus-specific genes, uses a receptor that is distinct from HAdV5 (but unidentified) [30] and lacks an obvious integrin-binding domain in its penton base protein. Therefore, because of its different tropism, it may induce a unique profile of immune cells. OAdV also has a favourable safety profile: it is not targeted to the liver, no transforming genes have been identified in its genome and it does not replicate in any human cell types tested even in the presence of replicating HAdV5. Thus, OAdV in humans is expected to be an attenuated non-replicating virus. As a vaccine vector, OAdV should accommodate up to 6.3 kbp of foreign DNA and it promotes drive high level gene expression in mouse muscle [31]. Importantly, human sera that neutralized HAdV5 did not neutralize OAdV and recombinant OAdV induced humoral and cellular immune responses in animals that had been previously vaccinated with HAdV5 [32], [33]. A certified cell line, documentation and protocols used for Good Manufacturing Practice manufacture of clinical trial material are available for the future production of OAdV-vectored HIV-1 vaccines, if justified, and the vector has been approved and registered for use in a phase I trial for prostate cancer [34]. This places OAdV vector development well ahead of many other non-human AdV vectors. Here, we inserted the model immunogen HIVA [35] into the OAdV vector and, in comparing it with a similar non-replicating AdV5 vector, demonstrated several desirable features of this vaccine in terms of preparation, genetic stability, immunogenicity, and protective efficacy against a surrogate virus challenge. These data provide comparative insights into the properties of vectors from two different adenovirus genera.

Section snippets

Construction and preparation of OAdV.HIVA and OAdV.HIVANP virus stocks

The HIVA and HIVANP genes were excised from the pTH.HIVA [35] and pTH.HIVANP [36] plasmid DNA with HindIII (blunted by Klenow DNA polymerase) and XbaI restriction endonucleases and inserted between the Rouse sarcoma virus (RSV) promoter and BGH polyadenylation signal in plasmids OAdVshuttleR and OAdVshuttleL cut with EcoRV and XbaI. The expression cassettes were then excised using flanking AscI and RsrII sites and inserted in the leftward or rightward orientation into the modified plasmid

Construction and characterization of OAdV-vectored vaccines

Chimeric protein HIVA was used as an HIV-1-derived immunogen. HIVA consists of consensus HIV-1 clade A Gag p24 and p17 regions coupled to a string of CD8+ T cell epitopes (Fig. 1A) [35]. It focuses on induction of HIV-1-specific T cell responses and has been well characterized immunologically in mice [41], [42], Mamu-A*01+ rhesus monkeys [43], and healthy and HIV-1-infected humans [44]. As a candidate human vaccine, it has the theoretical advantage of focusing T cell responses on the Gag

Discussion

OAdV is the prototype member of the Atadenovirus genus and thus biologically distinct from Mastadenoviruses vectors such as HAdV5.OAdV, which does not replicate in human cells [28], was used to construct a novel candidate HIV-1 vaccine. The data presented here provide comparative insights into the properties of adenovirus vectors from two different genera that both express HIVA, an HIV-1-derived T cell immunogen [35]. OAdV.HIVA was genetically stable, expressed HIVA and was highly immunogenic

Acknowledgements

The work was supported by the MRC UK, Australian Research Council (DP0452362) and SRC. YR is supported by the Thai Government DPhil Scholarship.

Conflict of interest: The authors have no conflict of interest except for GWB, who is an inventor of the ovine atadenovirus vector system and the Chief Scientific Officer of Biotech Equity Partners Pty Ltd, which has licensed the vector from the CSIRO.

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