Developing broad-spectrum therapeutics against C. difficile toxins

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Project Summary Clostridioides difficile (C. difficile) is a major opportunistic pathogen that colonizes the colon when normal gut microbiota is disrupted. The large protein toxin TcdB is a major virulence factor responsible for diseases associated with C. difficile infection (CDI). However, prior efforts to develop neutralizing monoclonal antibodies and vaccines against TcdB have yielded unexpectedly low efficacy or even failure. We believe that a key weakness of these previous studies might be the complexity of toxin variations seen clinically. While a single toxin sequence from a reference strain has been widely used in all previous therapeutic development, sequencing TcdB in clinical isolates in recent years has revealed a growing number of C. difficile strains as well as variations in toxin sequences. This may account for the reduced neutralization efficacy of the only FDA- approved monoclonal antibody, bezlotoxumab, against some TcdB variants such as the one produced by a hypervirulent strain (ribotype 027). The sequence variation and the toxin’s large size (~270 kDa) also pose daunting challenges to develop effective vaccines using the traditional toxoid approach. Building on our recent progress in identification of toxin receptors and understanding the structure and function of TcdB, here we propose to develop receptor-decoy-based therapeutic proteins as broad-spectrum antitoxins and a new generation of epitope-focused fragment-based vaccines, which could provide effective protection against most of the known TcdB variants. Frizzled proteins (FZDs) and CSPG4 are two major host receptors for TcdB, and we previously have revealed the mechanism by which TcdB recognizes FZDs. The first aim in this project is to establish a structural understanding of TcdB binding to CSPG4. Our second aim will focus on design and characterization of a family of bi-specific receptor-decoy proteins, which are composed of the optimized TcdB- binding fragments of CSPG4 and FZDs. In the third aim, we will take advantage of our knowledge of the structures of TcdB holotoxin, TcdB–antibody complexes, and TcdB–receptor complexes to design candidate vaccines based on the selected highly conserved and functionally critical TcdB fragments. This project is built on long-standing productive collaborations between the Jin lab and the Dong lab, combining their highly complementary expertise in structural biology and protein engineering (Jin lab) and TcdB receptors/CDI pathogenesis/animal models (Dong lab). Successful completion of this project will provide prototypes of antitoxins for immunoprophylactic therapy and broad-spectrum candidate vaccines that offer prophylactic and long-lasting protection.