DNA vaccine delivery by densely-packed and short microprojection arrays to skin protects against vaginal HSV-2 challenge

Abstract

There is an unmet medical need for a prophylactic vaccine against herpes simplex virus (HSV). DNA vaccines and cutaneous vaccination have been tried for many applications, but few reports combine this vaccine composition and administration route. We compared DNA administration using the Nanopatch™, a solid microprojection device coated with vaccine comprised of thousands of short (110 μm) densly-packed projections (70 μm spacing), to standard intramuscular DNA vaccination in a mouse model of vaginal HSV-2 infection. A dose-response relationship was established for immunogenicity and survival in both vaccination routes. Appropriate doses administered by Nanopatch™ were highly immunogenic and enabled mouse survival. Vaginal HSV-2 DNA copy number day 1 post challenge correlated with survival, indicating that vaccine-elicited acquired immune responses can act quickly and locally. Solid, short, densely-packed arrays of microprojections applied to the skin are thus a promising route of administration for DNA vaccines.

Introduction

Infections due to herpes simplex virus (HSV) cause significant individual and public health burden [1]. Currently approved antiviral drug regimens can reduce but not eliminate symptomatic reactivations, asymptomatic shedding, and transmission, and have no influence over chronic latent infection in dorsal root ganglia (DRG) neurons [2]. For these reasons, considerable research effort focuses on prophylactic and therapeutic vaccines. The most advanced clinical candidate targets genital herpes due to HSV-2, and contains the extracellular domain of glycoprotein D of HSV-2 (gD2), administered intramuscularly with alum and a TLR4 agonist [3]. While HSV-2 is associated with genital herpes, and HSV-1 is more often associated with oral and ocular disease, evidence does exist for a substantial contribution of HSV-1 to clinical first episode of genital herpes. Moderate efficacy against HSV-2 infection and genital herpes disease was observed in phase III trials, but only in HSV-1/HSV-2 dually seronegative women. A replication study in this population is underway [4].

The rationale for gD2 as a vaccine antigen has been clear for almost 30 years, as anti-gD antibodies have potent neutralizing activity [5]. More recently, CD4 and CD8 epitopes in gD2 have also been defined [6], [7]. The amino acid sequences of gD1 and gD2 are highly homologous, as are the principle cellular receptors used by gD of HSV-1 and HSV-2 to facilitate viral entry [8], [9], [10]. Thus, an effective gD-based vaccine might thus provide cross-protection against both HSV-1 and HSV-2. HSV gD is a type I membrane protein with a single transmembrane domain. While full-length gD has been explored in animal models, most protein subunit development has used the extracellular domain to facilitate manufacturing in mammalian cell lines [1]. Candidate DNA vaccines reaching clinical trials have also encoded this portion of the molecule [11].

Intramuscular administration of plasmid DNA (pDNA) encoding an ORF or fragment of interest, with appropriate flanking sequences, will generate an integrated antibody and T-cell immune response. There are licensed veterinary DNA vaccines [12]. In contrast, immunogenicity in humans has generally been inadequate for clinical licensure [13], [14], [15], [16], [17], [18]. Several approaches are being investigated to improve immunogenicity, including adjuvants, microparticle ballistic delivery to the skin, and cutaneous vaccine delivery using microprojections [19], [20]. The skin is rich in antigen presenting cells (APC) including several dendritic cell (DC) subsets. In fact, mobile DC are thought to transport HSV antigen to draining lymph nodes (DLN) during primary herpetic infections [21].

In this report, we investigate the tolerability, immunogenicity, and efficacy of a plasmid DNA vaccine encoding the extracellular domain of gD2, using the Nanopatch™, a recently devised silicon microprojection device [22], [23], [24]. To significantly increase direct cell targeting, the Nanopatch™ was designed with 3364 microprojections per device in a 4 × 4 mm area and tailored to directly target thousands of skin APCs. The projections are slender, with much of their length below the diameter of cells, such that they induce low stresses to target cells. They are thus likely to induce a low incidence of cell death near the tips [25].

Our standard mouse model of intravaginal inoculation with a virulent HSV-2 strain differs in important ways from human infection. However, it allows for measurement of IgG immune responses, vaginal HSV-2 replication, survival, and the establishment of viral latency in the DRG. We observed that low doses of gD2-encoding DNA, delivered cutaneously via Nanopatch™, were as immunogenic and as efficacious as higher delivered doses of the vaccine administered intramuscularly. Correlative studies determined that low vaginal HSV-2 replication and seroconversion were predictive of survival, and that high dose vaccines could prevent latent HSV-2 infection of DRG.