Syphilis, caused by Treponema pallidum (Tp), continues to be a global public health concern. An ulcerated chancre unique to syphilis forms at the site of initial infection, and Tp undergoes rapid and widespread dissemination, causing the plethora of symptoms associated with syphilis. Recent focus has been on syphilis vaccine development, to synergize with public health measures aimed at controlling syphilis. In prior studies we identified the Tp proteins Tp0751, TprK, and TprC as promising vaccine candidates due to their critical roles in Tp attachment to endothelial cells (Tp0751) and Tp antigenic variation and persistence (TprK/TprC). Pre-clinical vaccine trials using a Tp0751/TprK/TprC tri-antigen cocktail and a custom adjuvant significantly reduced bacterial load, chancre formation, and Tp dissemination. In the current study, we describe a novel protein engineering approach to refine the tri-antigen cocktail into a singular, soluble, stable and high yield vaccine chimera by grafting essential Tpr epitopes into the Tp0751 scaffold.
Methods
Structure-guided approaches complemented with bioinformatic and biophysical studies were used to identify Tp0751 loop regions amenable to substitution. A soluble and stable multi-epitope Tp0751 chimera was recombinantly produced with endotoxin levels below the FDA-recommended threshold for clinical-grade therapeutic products. Pre-clinical protection experiments were performed by immunizing animals (n=8) with the vaccine chimera/custom adjuvant, followed by intradermal Tp challenge of the immunized animals and comparator unimmunized animals (n=8). Vaccine performance was assessed by comparing chancre development, induced cytokine profiles, bacterial burden, and Tp dissemination.
Results
Chimera immunization induced high Tp0751/TprK/TprC antibody titers, as well as a reduction in chancre progression and Tp dissemination to distant tissue sites compared to unimmunized animals. Ongoing optimization of the Tp0751 vaccine platform enabled production of a chimera construct containing 4 heterologous Tp protein epitopes grafted into the Tp0751 scaffold.
Conclusion
Our investigations show proof-of-concept for a multi-epitope syphilis vaccine chimera which provides partial protection while satisfying downstream GMP manufacturing requirements of high yield, solubility and stability. Ongoing efforts to optimize our Tp0751 vaccine platform technology will enable development of a syphilis vaccine that incorporates multiple protective B- and T-cell epitopes to enhance the protective efficacy of the vaccine.
