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Structural Vaccinology to drive the design and optimization of protein antigens for a multicomponent vaccine against Staphylococcus aureus

Final Report Summary - SV-STAPH-VAX (Structural Vaccinology to drive the design and optimization of protein antigens for a multicomponent vaccine against Staphylococcus aureus)

Introduction & Project Objectives
Staphylococcus aureus is one of the most important bacterial pathogens causing human death and disease on a global scale. The rapid generation of antibiotic resistance, coupled with the frequency and severity of staphylococcal disease, underlie the increasing medical need to combat S. aureus infections. As an alternative to the current inadequate antibiotic therapies, a preventative vaccine against S. aureus is highly desired.
In this project we aimed to use structure-based design to develop novel antigens for a staphylococcal vaccine. The project focused primarily on the conserved staphylococcal antigen (CSA)–family proteins and on the leukocidin family of leukotoxins (luk). Both families are composed of multiple members with likely partially overlapping functions. Therefore, a highly effective vaccine antigen should be able to elicit antibodies that can bind to optimally all members of the family. Despite the relatively high sequence similarity within the families, none of the CSAs or luks tested so far have been able to elicit a fully cross-reactive antibody response. We hypothesized that structure-based rationally designed mutations on selected members of these families could be used to build antigens with improved capabilities to induce cross-reactive antibodies. We aimed to design and produce such recombinant antigens. We further aimed to study the structures and biophysical properties of the new antigens in comparison to wild-type proteins, focusing primarily on the leukocidins, in order to validate the new designs. The most promising candidates were tested in mouse immunization and challenge models to determine if the mutants are able to outperform the wild-type proteins by inducing better protection against infection, showing better cross-reactivity and toxin neutralization profiles of the immune sera, or by showing improved safety.

Results & Conclusions
We designed and produced several constructs of a number of antigens selected from the CSA and leukocidin families. By targeted mutations we were able to obtain new antigens with thermostability profiles improved when compared to the wild-type proteins. Using a combination of X-ray crystallography and 3D electron microscopy, we determined the structures of several mutated antigen constructs and confirmed that the mutations did not alter the structures in undesired ways. A number of luk mutants were used for immunizing mice to analyze if the altered proteins were able to provide protection against infection. Four mutant antigen formulations were found to provide statistically significant protection against S. aureus in the mouse models tested. Cross-reactivity and toxin neutralization analyses are currently underway to determine the cross-reaction and cross-neutralization capability of the sera obtained from the immunized mice. Preliminary biochemical experiments with the sera from mice immunized with a mutant leukocidin demonstrated an enhanced cross-reactivity profile compared to the sera from mice vaccinated with the corresponding wild-type protein.
Our results demonstrate that structure-based design can be used to generate stabilized CSA proteins and leukotoxins to be used as staphylococcal vaccine antigens. Ongoing serological experiments will reveal whether the modified antigens have potential as broadly-protective immunogens for vaccine formulations against S. aureus.

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