The development of vaccine delivery systems based on presenting peptides on the surface of core protein VP7 of African horse sickness virus

Abstract

Novel vaccine strategies for the presentation of immunologically important epitopes to the immune system are continuously being developed. Two such systems include the particulate protein and live viral vector delivery systems. In his study the long-term objective is to explore the African horsesickness virus (AHSV) serotype 9 viral protein 7 (VP7) and the Lumpy skin disease (LSDV) viral vector as two different vaccine strategies, particularly in view of the development of an HIV-1 vaccine. Consequently two very specific objectives were outlined in this study. The first was to express the HIV¬1 subtype C strain Du 151 gp41 epitopes ALDSWK and RVLAIERYLKD on the surface of the AHSV-9 VP7 particulate protein crystalline structures. A longer-term aim is to synthesise large quantities of these chimeric VP7 crystals in order to assess the immune response against the inserted epitopes. Secondly, the efficiency of the LSDV bi-directional promoter pA7LA8R in expressing chimeric VP7 proteins was to be evaluated by utilising the late element of this promoter to determine expression levels. Nucleotide sequences encoding the ALDSWK and RVLAIERYLKD epitopes were amplified from the HIV-1 subtype C strain Du 151 gp160 gene utilising PCR. These sequences were cloned individually as well as in combination into a multiple cloning site (549-566bp) present in the AHSV-9 VP7 gene. Recombinant pFASTBAC vectors PFASTBAC-VP7-MT 177-RVLAIERYLKD, PFASTBAC-VP7-MT 177-ALDSWK AND PFASTBAC-VP7-MT-177-RVLAIERYLKD-ALDSWK were identified, sequenced and used in the generation of recombinant baculoviruses utilising the BAC-to-BAC™ Baculovirus expression system. Expression of all three chimeric proteins, VP7-ALDSWK, VP7-RVLAIERYLKD and VP7- RVLAIERYLKD-ALDSWK was detected in infected Sf9 insect cells utilising SDS-PAGE. Further investigations will involve high-level expression of these proteins, which in turn will allow their characterisation as well as solubility, scanning electron and immunogenicity studies. In order to evaluate the efficiency of the LSDV bi-directional promoter, the AHSV-9 VP7 gene was cloned under the control of the late element (pA7L) of this promoter. The recombinant pHSsgpt-VP7 transfer vector was subsequently transfected into lamb testis cells infected with wild type LSDV in order to generate recombinant LSDV-VP7. Several rounds of recombinant virus selection in the presence of mycophenolic acid resulted in the loss of the LSDV-VP7 recombinant. Due to this unforeseen result, the expression of the VP7 protein from the late element of the pA7LA8R bi¬directional promoter could not be quantified and the efficiency of this promoter was not determined. The loss of LSDV recombinants, which contain a gene under the control of the late promoter element pA7L, has occurred previously and is suspected to be because of the instability of these recombinants. Due to the difficulties inherent in working with the LSDV viral vector delivery system, it has subsequently been decided to explore an alternate poxviral vector system. The focus in this study is now being shifted onto the promising Modified Vaccinia Ankara (MVA) viral vector system.