New mRNA Vaccine May Shield Against C difficile Infections

A group of researchers from the University of Pennsylvania, Philadelphia, has developed a messenger RNA (mRNA) vaccine, delivered via lipid nanoparticles (LNPs) — the same type as the COVID-19 vaccine produced by Moderna and Pfizer — targeting Clostridioides difficile (formerly Clostridium difficile). According to the authors, the results of their preclinical study, published in Science, demonstrated this technology as a promising platform for C difficile vaccine development and could be the starting point for curbing intestinal infections that, in their most severe forms (pseudomembranous colitis, toxic megacolon), can be fatal.

An Increasingly Pressing Issue

C difficile is the leading cause of infectious diarrhea acquired in healthcare settings. In recent years, community-acquired C difficile infections have also become more frequent. The increase in infections has been attributed to the emergence of highly virulent and antibiotic-resistant strains.

A 2019 study reported a global incidence of C difficile infections at 2.2 per 1000 hospital admissions per year and 3.5 per 10,000 patient-days per year.

The Vaccine Candidate

Vaccine candidates tested so far have used toxoids or recombinant proteins targeting the combined repetitive oligopeptide (CROP) or receptor-binding domain (RBD) of the two primary C difficile toxins, TcdA and TcdB. The US researchers are now exploring the mRNA-LNP vaccine approach to target multiple antigens simultaneously. They developed a bivalent vaccine (including the CROP and RBD domains of both toxins) and a trivalent vaccine (with an additional virulence factor, the metalloprotease Pro-Pro endopeptidase-1).

Mice vaccinated with the bivalent and trivalent vaccines produced immunoglobulin G antibody titers two to four times higher than those elicited by recombinant protein with an adjuvant. The vaccination stimulated the proliferation of follicular T helper cells and the antigen-specific response of B lymphocytes, laying the foundation for a strong and long-lasting humoral response. The vaccines were also immunogenic in hamsters.

Vaccinated mice not only survived a toxin dose five times higher than the 100% lethal dose but also demonstrated the vaccine’s protective effect through serum transfer; unvaccinated mice given serum from vaccinated mice survived the lethal challenge. More importantly, when exposed to a lethal dose of the bacterium itself, all vaccinated mice survived.

To demonstrate the vaccine’s efficacy in patients with a history of C difficile infection and high recurrence risk — ideal candidates for vaccination — the researchers vaccinated mice that had previously survived a sublethal infection. Six months after the initial infection and vaccination, these mice remained protected against mortality when reexposed to the bacterium.

Additionally, a quadrivalent vaccine that included an immunogen targeting C difficile spores — key agents in transmission — also proved effective. Low levels of bacteria and toxins in the feces of mice vaccinated in this way suggested that spore vaccination could limit initial colonization.

In tests with nonhuman primates, two doses of the vaccines targeting either the vegetative form or the spores elicited strong immune responses against bacterial toxins and virulence factors. Human trials may indeed be on the horizon.

This story was translated from Univadis Italy using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.