The immunopathogenesis of Clostridium difficile and its toxins

Clostridium difficile is a Gram-positive, spore forming bacillus and the most common cause of antibiotic-associated diarrhea in the United States. Infection with C. difficile is associated with disruption of the microbiota, which commonly occurs after administration of antibiotics. Clinical outcomes of C. difficile infection (CDI) range from asymptomatic colonization to pseudomembranous colitis, sepsis and death. Recurrent disease is a major problem in treatment of CDI, as approximately 25% of patients who undergo treatment for CDI experience at least one additional infection. Pathogenesis of C. difficile is primarily mediated by the action of the Rho-glucosylating Toxins A and B, which cause dramatic host cell death and induce potent pro-inflammatory signaling. Certain strains of C. difficile also express a third toxin, known as C. difficile transferase or CDT. CDT expression is associated with more severe disease outcome and increased rates of recurrence. In addition to the toxins, multiple innate immune signaling pathways have been implicated in establishing an inflammatory response during infection. The pattern recognition receptors (PRRs), including Toll-like Receptors 2 and 4 (TLR2 and TLR4), are particularly essential in recognition of C. difficile and help shape the subsequent inflammatory response. Although multiple studies demonstrate protective immune responses during murine infection, recent clinical data suggests inflammatory markers correlate closely with disease severity. The combination of these findings emphasizes the importance of immune balance during infection and the protective capacity of an immune response which eradicates bacterial threats while preserving tissue integrity and promoting tissue repair. Based on this knowledge, we hypothesized that C. difficile toxins A, B, and CDT play an important role in shifting the immune response towards a pathogenic state which enhances disease severity. We show that all three toxins act synergistically to promote activation of the immune response via a signaling complex known as the inflammasome, and demonstrate that toxin enzymatic activity is required for this process to occur. We also demonstrate that activation of this complex promotes production of the pathogenic cytokine Interleukin-23 in response to C. difficile, and clarify the role of Toll-like Receptors in this process. Finally, we demonstrate that intoxication dramatically influences the composition of immune cells during infection by specifically depleting eosinophils. This work suggests that intoxication by C. difficile toxins A, B and CDT has profound and synergistic inflammatory consequences which lead to pathogenic signaling and shape the course of disease. Thus, the host immune response is a truly double-edged sword which must be carefully wielded to achieve protection.