Tubules composed of non-structural protein NS1 of african horsesickness virus as system for the immune display of foreign peptides

Abstract

Non-structural protein, NS1 of African horse sickness virus is a hydrophobic protein of 63 kDa that spontaneously assembles into highly distinct tubular structures when expressed in mammalian or insect cells. The spontaneous assembly of these proteins into a predictable multimeric structure, high levels of expression and ease of purification make this protein an ideal candidate for the immune display of foreign peptides. The potential of such a display system has been investigated for BTV NS1 that is able to successfully elicit both a humoral and a cellular immune response against inserted peptides. The aims of this study were to investigate both the stability of the AHSV NS1 particulate structure after insertion of peptides as well as the antigenicity and immunogenicity of the peptides presented in this system. Two overlapping regions consisting of 40 and 150 amino acids, and which correspond to a neutralising region identified within the AHSV major neutralising protein VP2, were inserted into an internal site in NS1. This site offered the best surface display of inserted peptides on the tubular structures. An enhanced green fluorescent protein, 240 amino acids long, was also inserted into the NS1 protein. Sucrose gradient analysis of the recombinant proteins indicated that the majority of the baculovirus expressed chimeric proteins formed particulate structures with a sedimentation value similar to that of the native NS1 protein. This was confirmed by transmission electron microscopic analysis, which clearly showed that all the chimeric proteins assembled into tubular structures similar to those observed for AHSV NS1 proteins. Furthermore, fluorescence analysis of sucrose gradients of NS1/eGFP also showed high levels of fluorescence that corresponded directly to particle formation. Not only do the inserts remain functional but are also presented successfully on the surface of the intact NS1 tubule structure. The potential of the NS1 vector to efficiently present peptides to the immune system was subsequently investigated. The serums generated against these chimeric proteins in guinea pigs were tested against chimeric constructs, the baculovirus expressed inserts (for eGFP) and the inserts presented on other presentation vectors. Western blot analysis showed that most of the serums generated against the chimeric proteins contained antibodies not only against the chimeric proteins but antibodies that reacted specifically with the inserted peptides on their own or on another presentation system. Preliminary immune studies seem to indicate that the humoral immune response elicited by the chimeric NS1 proteins is predominantly against the inserts. The inserts are successfully presented to the immune system on the surface of the NS1 vector and are able to elicit the production of antibodies with the potential to provide a protective immune response.