Immunogenic screening of novel diphtheria vaccine candidates effective against the toxin variants and nontoxigenic Corynebacterium diphtheriae strains causing invasive infections

Summary 

Diphtheria, a disease caused by the bacterium Corynebacterium diphtheriae, causes significant illness and death across the world. The numbers of cases are constantly rising, with a 5-fold increase between 2015 and 2019.

The current diphtheria toxoid vaccine induces antibodies that only neutralise the toxin but do not eliminate the pathogen. However, new toxin variants C. diphtheriae strains are in global circulation including some with high-impact mutations, introducing structural modifications to the toxin that may result in vaccine escape and failure of anti-toxin treatment. In addition, non-toxigenic strains are causing severe invasive infections which is further exaggerated by emerging multidrug resistance (MDR), limiting the choice of treatment. Therefore, a more effective vaccine is needed to address these challenges.

We have identified three proteins that are highly conserved within the species and were present in diphtheria vaccines. These proteins are essential for the bacterium’s survival and are either membrane-associated or secreted which makes them highly suitable vaccine candidates. Furthermore, they contain peptides which we have predicted to be highly immunogenic, both for antibody and T-cell immunity.

In this study, we will perform immunogenic screening of these peptides for antibody and T-cell response. The findings will help selecting highly immunogenic peptides for successfully designing a multiepitope vaccine that will be able to neutralise the pathogen and eliminate cells infected by emerging nontoxigenic, MDR and toxin variant C. diphtheriae strains.

Project Outcomes

Reverse vaccinology approach is highly effective in identification of promising vaccine targets. We screened overlapping peptides spanning three proteins that were identified using reverse vaccinology approach for humoral (antibody) and cell-mediated immune response. The results are quite promising as several peptides from two of these proteins induced significant humoral and/or cell-mediated responses, some of which were comparable to the response induced by the toxin peptides. Please note that current toxoid vaccine is inactivated toxin that has been highly successful in protecting from toxin-mediated disease, diphtheria. However, it is not effective against multidrug-resistant nontoxigenic strains of Corynebacterium diphtheriae that are causing severe infections around the world. Also, mutations in the toxin encoding gene that are capable of introducing high-impact structural changes have been reported, which may influence the efficacy of current vaccine against these mutants. We found antibodies against some of the peptides were cross-reactive to the whole proteins, i.e., they have potential to eliminate the pathogen. These findings are very exciting, providing proof-of-concept for further characterisation of these peptides/proteins that may result into development of a highly effective diphtheria vaccine to address challenges posed by emerging nontoxigenic, MDR and toxin variant C. diphtheriae strains.