Porcine Monocyte DNA Traps Formed during Infection with Pathogenic Clostridioides difficile Strains

Clostridioides (Clostridium) difficile is an enteric pathogen of several mammalian species including man, frequently involving nosocomial resurgence, following oral administration of broad-spectrum antibiotics, but also with human-to-human infection occurring, and neonatal pigs with zoonotic transmission. To date, the immune response to C. difficile has mostly focused on neutrophils and cytokine/chemokines, particularly in human infection. The neonatal pig is now recognized as a valuable model for human infection. We show that porcine monocytes respond to C. difficile differently compared with many other bacterial infections. Infection of porcine monocytes with human C. difficile strains CD630 (Ribotype 078) or R20291 (Ribotype 027) for 3 or 24 h post-infection (pi) resulted in a lack of oxidative burst or nitrite ion production when compared to uninfected controls (p > 0.05). The survival dynamics of both CD630 and R20291 in monocytes were similar with intracellular bacterial numbers being similar at 3 h pi and 24 h pi (p > 0.05). However, we show that porcine monocytes entrap C. difficile via extracellular DNA traps. This process began as early as 3 h pi, and at 24 h pi the nuclei appeared to be depleted of DNA, although extracellular DNA was associated with the cell membrane. Our preliminary study also suggests that entrapment of C. difficile by extracellular DNA may occur via a process of monocyte etosis.