Innate and Adaptive Immune Responses Shape the Outcome of Clostridium difficile Infection

Clostridium difficile infection (CDI) is the number one hospital-acquired gastrointestinal infection in the United States. C. difficile is listed as one of three urgent threats by the Centers for Disease Control and Prevention and is the cause of 29,000 deaths annually in the United States alone. Several risk factors have been associated with this infection, including the use of broad-spectrum antibiotics, long-term stays in healthcare settings and older age. In addition, several clinical studies have identified inflammatory bowel disease (IBD) in its two major forms (ulcerative colitis and Crohn’s disease) as an independent risk factor for CDI. The incidence of CDI is higher in IBD patients compared to those without IBD, and this infection tends to be more severe as evidenced by higher mortality, endoscopy and bowel surgery rates. Although this increased susceptibility of IBD patients to the infection is well documented, the underlying mechanism was poorly understood. In order to understand the mechanism of increased severity of CDI in IBD patients, we utilized a Dextran Sulfate Sodium (DSS) murine model of inflammatory colitis. In support of clinical observations, our data showed that mice treated with DSS and then infected with C. difficile developed a more severe C. difficile disease compared to untreated mice. Increased severity of disease was measured by increased mortality and clinical scores. Importantly, disease severity did not correlate with C. difficile burden, toxin A/B levels or major differences in the microbiota, suggesting that increased severity of disease might be due to the host immune response to infection. Immunophenotyping of immune cells recruited to the colon at the peak of CDI revealed increased levels of CD4+ T cells at the site of infection in mice with prior DSS colitis. Furthermore, depletion of CD4+ T cells using a monoclonal antibody protected mice with prior DSS colitis from severe C. difficile disease and adoptive transfer of these cells caused severe CDI in naïve mice. Later we identified that DSS caused a skew towards type 3 immunity. Type 3 immunity is mediated by RORγt+ lymphocytes that express IL-17A alone or co-express IL-22. These lymphocytes include CD4+ Th17 cells, CD8+ Tc17 cells and type 3 innate lymphoid cells (ILC3s). In this study, we found that DSS treatment led to increased Th17 cells that persisted beyond the resolution of inflammation. Adoptive transfer of these cells was sufficient to worsen the outcome of CDI and this was dependent on IL-17A production by Th17 cells. Previous work in our lab and others studied the roles of certain type 3 cytokines during CDI, including IL-6, IL-17A and IL-23, all of which have been associated with increased CDI severity. However, this study was one of few that assessed the role of adaptive immunity in general and CD4 T cells in particular during CDI. Furthermore, this work was the first to identify a role for colitis-induced Th17 cells in the pathogenesis of C. difficile. This is extremely important because an increase in Th17 cells has been documented in IBD patients and aberrant Th17 responses have been associated with the pathogenesis of IBD. Our work suggests that inhibiting Th17 cells or one or more of their effector cytokines may provide protection against CDI in IBD patients who are highly susceptible to the infection.