The failure of a DNA prime/protein boost regime and CTLA-4 mediated targeting to improve the potency of a DNA vaccine encoding Fasciola hepatica phosphoglycerate kinase in sheep

Abstract

DNA vaccination in large animals has often been associated with poor immunogenicity, consequently several approaches have been evaluated to enhance its efficacy. Here, we tested a cDNA encoding a phosphoglycerate kinase from Fasciola hepatica (cDNA-FhPGK/pCMV) as a vaccine against ovine fasciolosis and investigated whether a DNA prime/protein boost regime or CTLA-4 (cytotoxic lymphocyte antigen 4) mediated targeting improved DNA vaccine efficacy. No statistically significant differences in the cellular responses were seen in either vaccine trial when compared with the respective control groups. However, specific antibody responses were considerably enhanced in DNA primed/protein boosted sheep, but not among CTLA-4 targeted cDNA-FhPGK/pCMV vaccinated animals. Nevertheless, increased titers of specific IgG1 did not contribute to protection against infection, with no differences in liver fluke recoveries reported. If DNA vaccines against fasciolosis in target species are to reach the market one day, more research in this area is needed.

Introduction

Fasciola hepatica infections pose a considerable threat to the health and economic value of livestock, mainly sheep and cattle (Cwiklinski et al., 2016). There is currently no approved vaccine against ovine or bovine fasciolosis and drugs are the only means of disease control. However, the emergence of drug resistance and the issue of drug residues entering the food chain are reasons to focus research on vaccine development. To date several vaccine studies in both laboratory and natural F. hepatica hosts have been conducted with some antigens showing potential (Toet et al., 2014; Molina-Hernández et al., 2015). Recently, genetic vaccination has provided excellent promise in the control of different diseases and a few DNA vaccines have already been approved for commercial use in companion and food animals (Redding and Weiner, 2009). Unique advantages of DNA vaccines, including their simplicity, safety, scalability, ease of transport and potentially cost-effective production, make this immunization approach a very attractive strategy. DNA-based vaccines encoding F. hepatica antigens have already shown their potential (Smooker et al., 1999, 2001; Wędrychowicz et al., 2003; Kennedy et al., 2006; Espino et al., 2010; Jaros et al., 2010; Jayaraj et al., 2012; Wesołowska et al., 2013). We have previously demonstrated that rats vaccinated with cDNA encoding a phosphoglycerate kinase of F. hepatica (FhPGK) and subsequently exposed to liver fluke infection showed 48% and 54% reductions in worm burdens in two trials (Wesołowska et al., 2016; Wesołowska et al., 2018). However, cDNA-FhPGK/pCMV vaccinated sheep were not protected from experimental challenge (Wesołowska et al., 2016). The poor immunogenicity of genetic vaccines in large animals is still a major obstacle with this vaccination approach. Here, our aim was to investigate if a heterologous prime/boost strategy or use of a targeting molecule (cytotoxic lymphocyte antigen 4, CTLA-4) could enhance vaccine efficacy in sheep. The former approach applies to priming the immune response with the genetic form of the vaccine, and then boosting with the protein form commonly co-delivered with an adjuvant (McShane, 2002). The latter strategy is based on trafficking vaccine antigens directly to immune cells through the use of antigen-CTLA-4 fusion. CTLA-4 is known to bind with high affinity to B7 molecules present on the surface of antigen presenting cells (APCs), thus CTLA-4 fusion antigens are directly targeted to APCs. Both approaches for enhancing specific immune responses to genetic vaccines have already proven their beneficial potential in large animal models (Rothel et al., 1997; Kennedy et al., 2006).