The bacterium Clostridioides difficile is named “difficult” for a reason. Originally, it was hard to grow in the lab, and, now, it’s the source of gut infections that are tough to treat. About half a million people in the U.S. contract C. diff every year, often from hospitals—and, even once treated, it’s common for the microbe to reinfect patients again and again.
Now, researchers have developed an mRNA vaccine that protected mice from succumbing to C. diff infections and prevented recurring cases. The results suggest the vaccine could become a flexible tool to target different strains of C. diff and prevent the pesky bacteria from trapping patients in a cycle of disease. The results were published in Science on Oct. 3.
“It's a really beautiful, well-done piece of work,” Dena Lyras, Ph.D., a microbiologist at Monash University in Melbourne who wasn’t involved with the study, told Fierce Biotech in an interview. “They've asked all the relevant questions in terms of which antigens are really important to get the protective response.”
C. diff is a vexing pathogen, able to take advantage of disrupted gut microbiomes to colonize the colon and secrete toxins that can cause fever, diarrhea and, in rare cases, sepsis and death. The microbe forms long-lasting spores that are challenging to kill and can persist in the environment for months or years—and can even survive being put through an industrial washing machine.
Further complicating matters, different C. diff strains can have vastly divergent features. For example, some C. diff cells have flagella, long wiggly appendages that help them move around, while others don’t. This makes choosing a target for a vaccine tricky.
“Because C. diff’s surface is naturally very variable, what we were look looking for was something that was involved with C. diff’s surface transiently, but might also be highly conserved,” Joseph Zackular, Ph.D., a microbiologist at the University of Pennsylvania, told Fierce in an interview. “That's where we came up with PPEP.”
PPEP is Pro-Pro endopeptidase 1, an enzyme that C. diff uses to control its attachment and detachment to the gut. Because of the vital role it serves, PPEP-1 is common across C. diff strains, but it also isn’t seen in other bacteria, which could limit collateral damage to the gut microbiome if the protein is targeted by a vaccine.
In addition to PPEP-1, the researchers’ vaccine also targets two of the toxins C. diff releases, TcdA and TcdB, and another protein specific to the protective coat that encases C. diff spores, called CdeM. Because it’s relatively easy to add new targets to mRNA vaccines, the team is actively searching for even more C. diff antigens to go after.
“We're looking at this conservation, but also at understanding what are other molecules, either secreted on the surface or internally, that can be used as novel targets,” Mohamad-Gabriel Alameh, Ph.D., a biomedical engineer at Penn and lead author of the paper, told Fierce in a joint interview with Zackular. Another member of the research team includes Drew Weissman, M.D., Ph.D., who won a Nobel Prize in 2023 for his work on mRNA vaccines.
Mice given the four-pronged vaccine got sick from C. diff infection, but didn’t die, and developed antibodies that target PPEP-1, signifying that their immune systems learned how to fight off C. diff. Mice were similarly protected from serious disease if they were reinfected six months after.
By analyzing the microbes found in the mice’s feces, the researchers confirmed that this protective effect also didn’t come at the expense of the healthy gut microbiome.
“Where I think this will have a lot of value is for people that have recurrent infections,” Lyras said. “Having had the infection and getting this vaccination on top of it all gives fantastic protective immunity.”
“A lot of the people that have the most trouble already had C. diff recently, or they're stuck in a cycle of recurrence,” Zackular said. “One of the things that's promising about this vaccine platform is that there might be a potential to then vaccinate those patients retrospectively.”
Zackular, Alameh and colleagues are now working to test the vaccine in nonhuman primates and have already seen that it produces an immune response in hamsters. Others attempts to make C. diff vaccines haven’t fared well; Pfizer’s candidate recently failed to meet the primary endpoint of preventing initial infections in a phase 3 trial, and Sanofi stopped developing its candidate in 2017 after an independent data monitoring committee found it was unlikely the phase 3 program would meet its primary objective.
“Our data, whether it's in mice, hamster or nonhuman primate, are better than the benchmarks,” Alameh said. “If you look at the data from other companies that went into clinical trials, we are either at par or even better, and that's without really dosing and optimizing.”
“The development of effective vaccines against C. diff could further public health efforts to prevent C. diff infection,” the Centers for Disease Control and Prevention (CDC) told Fierce in an emailed statement. “Once new vaccines are licensed, CDC uses its robust surveillance systems to monitor their safety and track their impact.”
The researchers are now planning how to best move forward into phase 1 trials but have also received interest in their vaccine from another group of medical professionals: veterinarians. Many animals, including horses, pigs and cows, can catch C. diff, and the bacterium can readily spread between humans and animals.
“That's one of the really attractive things about an mRNA vaccine like this one,” Lyras said, “because the low cost and the ability to change it rapidly means that you can deploy it in lots of different ways,” including in animals. “I think this sort of vaccine could have tremendous One Health potential.”