Background: Clostridium difficile is an anaerobic, gram positive bacterium that causes ~500,000 gastrointestinal infections each year in the United States, with clinical manifestations ranging from diarrhea to life-threatening colitis. There are over 15,000 deaths attributable to CDI in the U.S., 80% of which occur in patients 65 years or older. CDI is difficult to treat as the pathogenesis and range in outcomes reflect the interplay between infecting strain virulence, host physiology, and intestinal microbial composition. The higher incidence of CDI in more elderly patients suggests that the impact of aging, whether it be on the immune system, epithelial integrity or microbial community stability and composition, is impacting CDI pathogenesis and leading to worse outcomes. At the same time, research across multiple labs including the Young lab have found that alterations in infecting strain characteristics also impact the range of disease severity and the disease course in humans and mice. As some facets of CDI pathogenesis are difficult to study in isolation, we are attempting to study CDI using a systems biology approach by studying disease outcomes in our mouse model over two experimental variables: two levels of toxin production (low vs high toxin production) and two mouse physiologies (young vs aged). We hypothesize that aged mice will have a more severe clinical course because of immunosenescence, altered microbiota, and functional reserve, similar to humans.
Methods: Mice were treated with 10 days of cefoperazone to render them susceptible to C. difficile infection and then infected with either a high toxin producing (VPI 10463) or low toxin producing (630g) strain of C. difficile. Mice were split into two groups: young (2 – 3 months old) or aged (18 – 24 months old). Systemic disease was measured by clinical scoring (weight loss, posture, coat, activity, and signs of diarrhea) and local damage was measured by histopathology scoring of the colon and cecum (edema, epithelial damage, and immune cell infiltration). The immune response was measured systemically by serum cytokines values and locally through flow cytometry of cecal and colon tissue. Intestinal microbial communities were sampled by 16S rRNA sequencing of cecal content and feces.
Results: In our experiments, we found that certain aged mice had either no or low colonization when infected as compared to young mice, which are almost always colonized once infected. Even with mice with low/no colonization removed, aged mice infected with 630g still experienced lower levels of colonization compared to young mice. In the mice that were colonized, we found that aged mice infected with VPI 10463 had higher clinical scores, similar weight loss and cecum scores, but lower colon scores. On the host response side, VPI 10463 infected aged mice had lower levels of systemic KC, and higher levels of IL-17, IL-13 and VEGF. VPI 10463 infected aged mice also had lower levels of CD45+ cells and neutrophils in their cecum and colon compared to young mice infected with VPI 10463.
Future Directions: Our future analysis will include looking at the microbial compositions and local transcription levels of host response genes. We are interested in understanding the mechanism and impact of the reduced neutrophil accumulation in the tissue, altered systemic cytokine profile, and lower levels of colonization and colon damage seen in infected aged mice.