The sequence of non-structural protein NS1 of bluetongue virus (BTV), which contains immunodominant CD8+ T cell epitopes, is highly conserved among BTV serotypes, and has therefore become a major tool in the development of a universal BTV vaccine. In this work, we have engineered multiserotype BTV vaccine candidates based on recombinant chimpanzee adenovirus (ChAdOx1) and modified vaccinia virus Ankara (MVA) vectors expressing the NS1 protein of BTV-4 or its truncated form NS1-Nt. A single dose of ChAdOx1-NS1 or ChAdOx1-NS1-Nt induced a moderate CD8+ T cell response and protected IFNAR(-/-) mice against a lethal dose of BTV-4/MOR09, a reassortant strain between BTV-1 and BTV-4, although the animals showed low viremia after infection. Furthermore, IFNAR(-/-) mice immunized with a single dose of ChAdOx1-NS1 were protected after challenge with a lethal dose of BTV-8 in absence of viremia nor clinical signs. Additionally, the heterologous prime-boost ChAdOx1/MVA expressing NS1 or NS1-Nt elicited a robust NS1 specific CD8+ T cell response and protected the animals against BTV-4/MOR09 even 16 weeks after immunization, with undetectable levels of viremia at any time after challenge. Subsequently, the best immunization strategy based on ChAdOx1/MVA-NS1 was assayed in sheep. Non-immunized animals presented fever and viremia levels up to 10⁴ PFU/mL after infection. In contrast, although viremia was detected in immunized sheep, the level of virus in blood was 100 times lower than in non-immunized animals in absence of clinical signs.

The aim of this work was to evaluate the immunogenicity and efficacy of DNA and MVA vaccines encoding the RVFV glycoproteins Gn and Gc in an ovine model of RVFV infection. Adult sheep of both sexes were challenged 12 weeks after the last immunization and clinical, virological, biochemical and immunological consequences, were analyzed. Strategies based on immunization with homologous DNA or heterologous DNA/MVA prime-boost were able to induce a rapid in vitro neutralizing antibody response as well as IFNγ production after in vitro virus specific re-stimulation. In these animals we observed reduced viremia levels and less clinical signs when compared with mock-immunized controls. In contrast, sheep inoculated with a homologous MVA prime-boost showed increased viremia correlating with the absence of detectable neutralizing antibody responses, despite of inducing cellular responses after the last immunization. However, faster induction of neutralizing antibodies and IFNγ production after challenge were found in this group when compared to the mock vaccinated group, indicative of a primed immune response. In conclusion, these results suggest that vaccination strategies based on DNA priming were able to mount and maintain specific anti-RVFV glycoprotein immune responses upon homologous or heterologous booster doses, warranting further optimization in large animal models of infection.

In vitro neutralizing antibodies have been often correlated with protection against Rift Valley fever virus (RVFV) infection. We have reported previously that a single inoculation of sucrose-purified modified vaccinia Ankara (MVA) encoding RVFV glycoproteins (rMVAGnGc) was sufficient to induce a protective immune response in mice after a lethal RVFV challenge. Protection was related to the presence of glycoprotein specific CD8+ cells, with a low-level detection of in vitro neutralizing antibodies. In this work we extended those observations aimed to explore the role of humoral responses after MVA vaccination and to study the contribution of each glycoprotein antigen to the protective efficacy. Thus, we tested the efficacy and immune responses in BALB/c mice of recombinant MVA viruses expressing either glycoprotein Gn (rMVAGn) or Gc (rMVAGc). In the absence of serum neutralizing antibodies, our data strongly suggest that protection of vaccinated mice upon the RVFV challenge can be achieved by the activation of cellular responses mainly directed against Gc epitopes. The involvement of cellular immunity was stressed by the fact that protection of mice was strain dependent. Furthermore, our data suggest that the rMVA based single dose vaccination elicits suboptimal humoral immune responses against Gn antigen since disease in mice was exacerbated upon virus challenge in the presence of rMVAGnGc or rMVAGn immune serum. Thus, Gc-specific cellular immunity could be an important component in the protection after the challenge observed in BALB/c mice, contributing to the elimination of infected cells reducing morbidity and mortality and counteracting the deleterious effect of a subneutralizing antibody immune response.