Inhibition of bacterial protein toxins, in particular the toxins of Clostridioides difficile, by factors and cleavage products of the human complement system

• Applicants: Professor Dr. Holger Barth; Professor Dr. Markus Huber-Lang
• Subject Area: Public Health, Healthcare Research, Social and Occupational Medicine
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 450938962

Many pathogenic bacteria produce protein toxins, which upon uptake by human cells act as potent enzymes resulting in cellular damage. The toxin-induced cell injury causes severe diseases such as tetanus, diphtheria, cholera or anthrax. Of particular importance is the life-threatening pseudomembraneous colitis, which is caused by toxins (enterotoxins) TcdA, TcdB and CDT, which are produced by Clostridioides (C.) difficile. C. difficile associated diseases are increasing in incidence and severity. However, there are no sufficient therapeutic strategies against these toxins. Furthermore, antibiotic treatment is rather limited, especially in the case of C. difficile. Therefore, development of toxin inhibitors and novel therapeutic strategies poses an important unmet medical need. Our own data indicate that human peptides (e.g. defensins) and complement proteins may represent a promising approach in that context. Thus, in the proposed project, we will characterize in detail the effects of central human complement factors and their activation products in regard to their effects on bacterial toxins with a focus set on enterotoxins from C. difficile. The complement cascade functions as a “master alarm and defence system” of innate immunity against danger molecules. However, its particular role against bacterial toxins is unknown. In preliminary data, we could define the cellular uptake and mode of action of clostridial enterotoxins. We especially found that central complement factors (e.g. C3) and activation products (e.g. C3a, C3a(desArg), C5a) were capable to significantly impair intoxication of human intestinal epithelial cells by TcdA, TcdB, CDT, and Clostridium botulinum C2-Toxin, and furthermore, reduced toxin-induced substrate modification in the cells. These findings suggest that the biological toxin activity was decreased in presence of complement factors. However, the reason for this effect is not known so far. Thus, we have combined our toxin and complement research expertise and synergistically plan to pursue the overarching aim of deciphering the molecular mechanisms by which human complement factors and corresponding cleavage products inhibit clinically relevant bacterial toxins. Initially, we characterize those complement proteins which have revealed clostridial enterotoxin-neutralizing effects. The methodical platform uses human intestinal epithelial cells, human intestinal organoids, and human peptide banks established at Ulm University. We will then search for further toxin-inhibiting complement proteins and peptides and evaluate whether further toxins (e.g. diphtheria, anthrax) can be inhibited by complement factors. We expect to elucidate the underlying mechanisms of the so far unknown interactions between human complement and bacterial toxins and thereby reveal novel pharmacological strategies for targeted toxin inhibition.

• DFG Programme: Research Grants