Development of Staphylococcus aureus vaccine designed to counter multiple bacterial virulence factors

Abstract

Staphylococcus aureus (S. aureus) is a common pathogen found in communities and hospitals. The most notable form which is resistant to many antibiotics is Methicillin-resistant S. aureus (MRSA) and has formed a major health concern. Emergence of such drug-resistant strains has prompted the search for alternative treatments such as immunotherapies. Several studies have explored surface proteins and virulence factors for S. aureus which can be used as potential vaccine targets. However, till date most of the clinical trials for vaccines and passive immunization against S. aureus have failed. Previous work by others suggests that two ESAT-6 like proteins: EsxA and EsxB may play an important role in establishing S. aureus infection in the host. My work therefore focuses on the vaccine potential of EsxA and EsxB. Mice vaccinated with these purified recombinant proteins elicited high titers of anti-EsxA and anti-EsxB antibodies and induced Th1- and Th17-biased immune responses. Most importantly, mice immunized with rEsxA and rEsxB had significantly improved survival rates when challenged with S. aureus compared with the controls. But these antibodies could not prevent S. aureus infection suggesting that EsxA and EsxB are two promising Th1 and Th17 candidate antigens, which could be developed into multivalent and serotype-independent vaccines against S. aureus infection. It was also reported that adenosine synthase A (AdsA), a S. aureus cell wall–anchored enzyme could serve as a possible target for immunotherapy. The mice were vaccinated with aluminium hydroxide-formulated rAdsA which induced a high level of anti-AdsA antibodies and also showed a consistency in protection of three mouse infection models when challenged with 2 clinical strains of S. aureus. Importance of anti-AdsA antibody in protection by passive transfer experiments was also shown. Altogether, the data obtained demonstrate that the AdsA protein is a promising target for vaccines and therapeutics development to alleviate severe Staphylococcal diseases. Further attempts were made to make an effective multicomponent serotype independent vaccine against S. aureus infection. Eight proteins (AdsA, EsxA, EsxB, EsaC, PmtA, PmtB, PmtC and PmtD) and a Th17 adjuvant (Uric acid) were selected to generate a multivalent vaccine named Sta-C9. The Sta-C9 formulated with aluminum hydroxide induced high levels of antibodies and provided consistent protection in three mouse models (Skin infection, Peritonitis and Bacteremia) when challenged with MRSA (USA300) strains. Overall, the data demonstrated the rational selection of mixtures of conserved proteins combined with Th17 adjuvants could lead to promising vaccines against S.aureus. AdsA acts as a potential regulator to balance the protective immunity and immunopathology during S. aureus infection. However, the underlying mechanisms are not clear. This thesis work also involves a mechanistic study to reveal the function of AdsA in the establishment of aureus persistent infection. It has revealed that a higher death rate was observed in the mice infected with adsA mutant of S. aureus and also there was an increased production of proinflammatory cytokines which might have enhanced the lethality in BALB/C infection model. Interestingly, the increased expression of selective proinflammatory cytokines of adsA mutant-infected mice is not associated with higher bacterial growth. This study provides a multi-component and serotype-independent vaccine candidate against S. aureus infection. It also provides evidences that AdsA is an important protein for the bacteria to counter host protective immunity and balance S. aureus infection-induced immunopathology to establish a persistent infection.