Defining innate immunity to Clostridium difficile

Clostridium difficile is a spore-forming anaerobic bacillus and a leading cause of nosocomial diarrhoea. C. difficile infection occurs often following antibiotic treatment leading to alteration of the gut microbiota enabling C. difficile to thrive and cause a range of symptoms from asymptomatic carriage to severe diarrhoea, PMC and death. C. difficile virulence is associated with production of toxins (A, B and CDT) nonetheless this bacteria harbours an array of other virulence factors. In the present study, C. difficile-mediated innate immunity response was investigated utilising four different strains including: R20291 a hypervirulent strain (A+B+, CDT+), 630 a fully sequenced strain (A+B+) and variant strains M68 and CF5 (A-B+). Initial studies showed that all strains shared comparable survival and sporulation capacity whereas strains R20291, M68, and CF5 achieved similar growth kinetics. R20291 showed significant adherence to IEC and was the most potent strain. 630 and M68 were found to be as cytotoxic leading to significant cell apoptosis, IEC TJ disruption and increased paracellular permeability. In contrast, CF5 had the least effect on cell death and IEC barrier integrity. Although all four strains induced antimicrobial immunity and pro-inflammatory cytokines, CF5 mediated the least pro-inflammatory responses. Similar findings were also found in an ex vivo model of infection utilising human colonic explants. C. difficile strains up-regulated murine DC maturation markers leading to a predominant anti-inflammatory IL-10 secretion, which is known to suppress IL-12 and IL-23 induction although C. difficile may generate a cytokine milieu that favours dual Th1/Th17 immunity. C. difficile toxin mutant strains showed DC activation and cytokine production in a toxin-dependent and independent manner and triggered an ASC-containing inflammasome causing the activation of caspase-1 and release of mature IL-1β. These findings indicated that multiple bacterial factors may play a role in initiating host innate immune responses to C. difficile infection