Paeniclostridium sordellii uterine infection is dependent on the estrous cycle

Human infections caused by the toxin-producing, anaerobic and spore-forming bacterium Paeniclostridium sordellii are associated with a treatment-refractory toxic shock syndrome (TSS). Reproductive-age women are at increased risk for P. sordellii infection (PSI) because this organism can cause intrauterine infection following childbirth, stillbirth, or abortion. PSI-induced TSS in this setting is nearly 100% fatal, and there are no effective treatments. TcsL, or lethal toxin, is the primary virulence factor in PSI and shares 70% sequence identity with Clostridioides difficile toxin B (TcdB). We therefore reasoned that a neutralizing monoclonal antibody (mAB) against TcdB might also provide protection against TcsL and PSI. We characterized two anti-TcdB mABs: PA41, which binds and prevents translocation of the TcdB glucosyltransferase domain into the cell, and CDB1, a biosimilar of bezlotoxumab, which prevents TcdB binding to a cell surface receptor. Both mABs could neutralize the cytotoxic activity of recombinant TcsL on Vero cells. To determine the efficacy of PA41 and CDB1 in vivo, we developed a transcervical inoculation method for modeling uterine PSI in mice. In the process, we discovered that the stage of the mouse reproductive cycle was a key variable in establishing symptoms of disease. By synchronizing the mice in diestrus with progesterone prior to transcervical inoculation with TcsL or vegetative P. sordellii, we observed highly reproducible intoxication and infection dynamics. PA41 showed efficacy in protecting against toxin in our transcervical in vivo model, but CDB1 did not. Furthermore, PA41 could provide protection following P. sordellii bacterial and spore infections, suggesting a path for further optimization and clinical translation in the effort to advance treatment options for PSI infection. Author summary P. sordellii infection (PSI) in humans is rare but typically fatal. It is most frequently observed as a uterine infection in postpartum women following childbirth, stillbirth, or abortion. Once identified, antibiotics can be used to eradicate the bacteria, but these are not effective at neutralizing the secreted TcsL lethal toxin that can cause a treatment-refractory toxic shock syndrome. A neutralizing antibody against TcsL would address this problem in concept but has never been directly tested. TcsL shares high sequence identity with C. difficile TcdB, the primary virulence factor in C. difficile infection (CDI). We characterized two C. difficile TcdB antibodies, CDB1, a biosimilar to a clinically available drug, and PA41, in the neutralization capabilities of TcsL. In the process, we established a non-surgical, uterine infection model and made the discovery that disease symptoms varied with the reproductive cycle of the animals. This opens the door for new research questions at the interface of bacterial spore and toxin biology with reproductive health. While CDB1 did not provide protection against PSI in our animal model, PA41 did show protection. If developed for CDI, this antibody could have an added therapeutic utility in the life-threatening instances of human PSI.

Automated summary

Infections caused by Paeniclostridium sordellii, a bacterium that produces toxins and spores anaerobically, are related to treatment-resistant toxic shock syndrome. A neutralizing monoclonal antibody against Clostridioides difficile toxin B (TcdB) has been found to potentially protect against P. sordellii infection. A mouse model was used to demonstrate the efficacy of the antibody and identify the impact of the mouse's reproductive cycle on disease symptoms.