Study Identifies First Structure of C. diff Toxin B, Paving the Way for Next Generation Therapeutics

Investigators found that the therapeutic mechanism for bezlotoxumab, the only FDA-approved anti-TcdB antibody, is sensitive to escaping mutations in some bacterial strains.

New research has revealed the first 3D structure of the Clostridioides difficile toxin B (TcdB) in complex with chondroitin sulfate proteoglycan 4 (CSPG4), a human receptor. According to researchers from the University of California, Irvine School of Medicine, this could help future efforts to develop new therapeutics for the prevention and treatment of C. difficile.

C. difficile infections are now the most common cause of antibiotic-resistant diarrhea and gastroenteritis-associated death in developed countries, accounting for approximately 223,900 infections, 12,800 deaths, and $1 billion in health care costs in the United States alone in 2017. It is classified as one of the top 5 urgent threats by the CDC, in addition to being a growing global concern regarding the emergence of rapidly spreading, hyper-virulent C. difficile strains.

“TcdB is one of two homologous C. difficile exotoxins, which are major virulence factors responsible for the spread of C. difficile infections,” said senior author Rongsheng Jin, PhD, a professor in the University of California–Irvine department of Physiology and Biophysics, in the press release. “TcdB alone is capable of causing the full spectrum of diseases associated with [Clostridioides difficile infection] in humans.

Earlier research had identified CSPG4 as a potential receptor for TcdB, although the pathophysiological relevance and molecular details remained unclear. The new study results, published in Nature Communications, identified a unique binding site involving TcdB and CSPG4, and also showed that CSPG4-binding residues are highly conserved across most known TcdB variants.

“What these new findings tell us is that a rationally designed CSPG4-mimicking decoy could neutralize major TcdB vairants, providing a unique therapeutic avenue for combating some of the hyper-virulent C. difficile strains,” Jin said in the press release.

In contrast, the investigators found that the therapeutic mechanism for bezlotoxumab, the only FDA-approved anti-TcdB antibody, is sensitive to escaping mutations in some bacterial strains. The current standard of care for C. difficile infections includes treatments that use broad spectrum antibiotics, which frequently lead to recurrence. Although bezlotoxumab could reduce the recurrence rate among some patients, results from this and earlier studies suggest that it has weaker potency against some variants.

“We have designed a CSPG4-mimicking decoy based on the 3D structure we observed,which could neutralize major TcdB variants and is superior to bezlotoxumab on a major TcdB variant from a hyper-virulent strain (TcdB2) in our studies,” Jin explained in the press release. “As a highly conserved cellular receptor of TcdB, a CSPG4 decoy molecule would be difficult for TcdB to escape, since any mutations that disrupt toxin binding to the decoy would also disrupt binding to its native receptors.”

In addition to this decoy, the team has developed a family of recombinant protein therapeutics based on their new findings and on an earlier discovery of how TcdB recognizes the human receptor Frizzled. They have filed a patent on these neutralizing molecules.

“We are now examining the therapeutic features of these novel antitoxin molecules, and we believe they could provide broad-spectrum protection and neutralization against most known TcdB variants, thus improving existing antibody therapeutics for [Clostridioides difficile infection],” Jin concluded.

REFERENCE

Study reveals new therapeutic target for C. difficile infection. News release. University of California, Irvine School of Medicine. June 18, 2021. Accessed June 22, 2021. https://som.uci.edu/news_releases/Therapeutic-target-for-C-difficile-infection.asp