Abstract:
Staphylococcus aureus is one of the common causes of surgical site infections. Its capability to adapt to diverse environments, acquire antimicrobial resistance and plethora of immune evasion mechanisms has made it a healthcare burden. The most noteworthy example of this pathogen,
that has gained worldwide attention, is methicillin-resistant S. aureus (MRSA). Specifically, due
to their variability, MRSA strains are catalogued into specific clonal complex (CC) groups that
have been identified worldwide, with six CCs predominating within New Zealand.
An alternative to prevent staphylococcal infections involves creating a universal vaccine that
includes factors common amongst strains. In this regard, the Staphylococcal Superantigen-Like
(SSL) proteins are secreted proteins conserved across S. aureus strains that target the host
mechanisms pivotal in the establishment of an infection. These are SSL3, SSL7 and SSL11.
Theoretically, a construct with multiple SSLs would be able to induce the production of broad
coverage cross-neutralising antibodies. Previously, a model system using peptide tags fused to
portions of a Clostridium perfringens adhesin domain allowed the creation of 2-valent constructs.
This project represents a proof of concept test by which five domains from C. perfringens were
fused to different SSL7 variants to create a 5-valent construct. It was possible to observe that
each SSL7 variant could be efficiently produced as a fusion protein with different scaffolding
domains. However, during construct assembly it was possible to observe that additional
interactions occurred which affected purification of the desired multivalent construct. These
results may provide guidance in the use of superglue technology. Regardless, the project was
discontinued.
Further work involved assessing antibodies produced by a vaccine candidate including proteins
SSL3, 7 and 11 fused en tandem as antigens (polySSL). Rabbits were immunised with polySSL
in Incomplete Freund’s Adjuvant, while mice were immunised with polySSL in AddaVax. These
antisera were tested to observe the development of cross-reactive antibodies towards SSL
proteins from six CC groups. It was observed that the antibodies generated recognised all SSL
variants with SSL7 showing the highest IgG titres. Subsequently, both antisera´s neutralising
potential towards the IgA and C5 binding mechanisms of SSL7 were assessed for each variant.
In both assays rabbit antisera lacked neutralising activity, while mouse antisera presented high
neutralising potential towards the SSL7 variant present in the polySSL vaccine. The neutralising
potential of each polySSL antisera towards other variants varied. However, it was possible to
observe an association between the antisera´s functional activity and the homology of the SSL7 variants. This finding may contribute to developing universal vaccine strategies against multiple
S. aureus strains.