Abstract:
Staphylococcus aureus is a complex opportunistic human pathogen and the most commonly acquired nosocomial infection worldwide. Current treatment relies upon the use of antibiotic therapy, but due to the rapid emergence of resistant strains, future antibiotic treatment options are limited. Despite an extensive global effort, there is currently no protective vaccine against S. aureus. Staphylococcal Superantigen-like proteins (SSLs) are a family of secreted virulence factors used to evade host immune mechanisms. Neutralisation of these secreted SSL proteins using humoral immunity provides an innovative approach to an anti-staphylococcal vaccine. This project investigated the feasibility of creating a multivalent Poly-SSL vaccine consisting of tandem allelic variations of SSLs 3, 7 and 11 using a novel protein ligation technology. SSL antigens were expressed as protein subunits along with molecular scaffold (MOL) proteins, which act as a backbone for the construct. The SSL and MOL protein subunits were ligated together to form a single multivalent Poly-SSL vaccine construct. Several setbacks occurred during the vaccine construction phase, including solubility issues with the SSL3 antigen, resulting in inefficient purification and poor final yields. However, after progression through several vaccine versions and iterations, three vaccine constructs were produced containing repeating SSL7 and SSL11 antigens. There was a full 12-valent construct, a partial four-valent construct and finally, a vaccine control consisting of individual unligated SSL-MOL fusion proteins. These vaccines constructs, co-administered with the adjuvant AddaVax™, were used to immunise mice subcutaneously; and the subsequent antibody response was analysed. Although there was variation in the antibody responses between SSL alleles, there was no significant difference in total antibody titres between constructs after three immunisations. The neutralising ability of anti-SSL7 antibodies was further investigated using two functional assays; however, there was no difference in neutralising ability between the three constructs. In addition to the complications encountered using this protein ligation technology, there was no evidence to suggest any advantage of the full construct over the unligated components. Therefore, this project provides evidence that an alternate vaccine delivery system is required to continue pursuing a multivalent Poly-SSL vaccine.