The authors report that the complement proteins C1q and C3 are accessible within the healthy ileal mucosa. Exposure of intestinal tissue to the major C. difficile toxin TcdB differentially remodeled the spatial availability of these complement components: accessible C1q decreased while accessible C3 increased at higher concentrations of TcdB. The study frames these observations as evidence that toxin-mediated epithelial injury alters the local complement landscape in ways that could influence host–spore interactions.
Using in vivo and in vitro approaches, the investigators found that C. difficile spores associated with both C1q and C3. Direct binding was demonstrated with purified human C1q and C3 in vitro. In infected intestinal samples, spores showed association with these complement proteins, indicating that complement–spore interactions occur during natural host exposure.
Immunogold electron microscopy localized the interactions of C1q and C3 to the outer exosporium of C. difficile spores. The collagen-like BclA family proteins modulated complement binding under some experimental conditions but were not required for the association of either C1q or C3 with spores. Thus, while BclA members influence the interaction, other exosporium components mediate primary binding.
Far-Western analysis followed by mass spectrometry identified exosporium proteins CotE and CdeM as candidate complement-interacting proteins. The study further tested CdeM by purifying inclusion bodies of the protein; these purified CdeM preparations directly associated with both C1q and C3 in vitro. The authors therefore nominate CdeM (and potentially CotE) as an exosporium ligand for complement components, providing a molecular link between spore surface structure and complement binding.
Functional consequences of complement binding were evaluated using human serum and epithelial cell assays. Pre-exposure of spores to human serum promoted their interaction with intestinal epithelial cells. When either C1q or C3 was depleted from serum, spore internalization by epithelial cells was markedly reduced. Reconstitution of depleted serum with the corresponding purified protein partially restored spore entry. These results indicate that both C1q and C3 in serum contribute to spore uptake into intestinal epithelial cells.
The investigators detected C3-derived species, including C3dg, selectively associated with spores but not with vegetative cells. Importantly, the presence of these C3-derived fragments on spores did not reduce spore viability, indicating that complement tagging of spores does not necessarily lead to immediate loss of spore viability under the conditions tested.
In vivo studies evaluating the role of C1q in disease course showed that C1q deficiency did not alter the initial onset of C. difficile infection. However, C1q-deficient animals exhibited accelerated recovery and a marked attenuation of recurrent disease following vancomycin treatment. These findings suggest that C1q contributes to persistence and recurrence mechanisms rather than early disease initiation.
Collectively, the data identify intestinal complement as a previously unrecognized component of the C. difficile spore–host interface. The study provides multiple lines of evidence that C1q and C3 bind to the spore exosporium—interactions that are mediated in part by exosporium proteins such as CdeM—and that these complement interactions promote spore internalization into epithelial cells and influence persistence and recurrent disease. The selective association of C3-derived fragments with spores, without loss of viability, further supports a role for complement in modulating spore behavior rather than clearing spores outright.
These findings implicate local intestinal complement as a modulator of key spore phenotypes relevant to pathogenesis and recurrence, and they nominate exosporium ligands (notably CdeM) as potential targets for interventions aimed at preventing spore uptake, persistence, or recurrence.
Note on methods and details: The source provides results and key experimental observations but does not include detailed protocols, sample sizes, or full methodological parameters in the presented summary. For experimental specifics, full methods, and raw data, consult the original preprint document referenced in the source.