Long-term calorie restriction of two laboratory mouse strains caused differential changes in infection susceptibility, reactive oxygen species/antioxidant production and parasite life history traits

Research has demonstrated many benefits of calorie restriction (CR) across many species. Such benefits include increased median and maximal lifespans, delayed onset of cancer and kidney disease, and enhanced immune functions. However, previous research found that CR animals had increased susceptibility to pathogens despite a similar or stronger immune response as ad libitum (AL) fed animals. This conflict between strength of immune response and pathogen susceptibility is unresolved. Much research has demonstrated a relationship between reactive oxygen species (ROS) and antioxidant concentrations with CR and with immune function. Moreover, some strains of laboratory mice (Mus musculus) use ROS to different degrees to counter pathogen infection. My experiment used two strains of laboratory mice to examine if changes in ROS/antioxidant production after CR would affect susceptibility to H. bakeri infection and subsequent physiology of the parasites themselves. Mice were given calorie restriction for six months then infected with H. bakeri for two, four, or six weeks after which I measured mouse antioxidants and ROS production, and also H. bakeri antioxidants and oxidative damage. Additionally, I measured parasite motility, length, and in vitro reproduction. For the C57 strain, CR fed mice had a greater worm burden than AL fed mice that decreased over time while the SJL strain had similar worm burden throughout the experiment with no difference between AL and CR individuals. CR resulted in higher hydrogen peroxide (H2O2) levels in the small intestine for both mouse strains, and uninfected SJL mice had greater H2O2 production than infected mice. AL uninfected mice for both strains had higher levels of superoxide dismutase (SOD) than CR fed mice. Mouse catalase (CAT) levels were higher in uninfected AL fed C57 mice; while SJL mice had similar values for CR and AL fed animals, SJL uninfected mice had greater CAT production overall. Worm CAT was similar for both AL and CR derived worms for both mouse strains. Worms from C57 mice had similar glutathione-S-transferase (GST) levels when taken from AL and CR individuals but worms from CR fed SJL mice had greater GST levels than AL fed mice. Oxidative damage as indicated by 4-hydroxynonenal (HNE) was higher in worms taken from C57 mice when worms were exposed to H2O2 media in vitro; but, oxidative damage of worms taken from SJL mice did not increase with in vitro exposure to H2O2. Regarding worm life history, for worms taken from either strain, motility was greater in control media than H2O2 media. And male worms were the same length regardless of strain or diet, while females taken from both strains grew longer over time but did not vary with mouse food intake. In vitro reproduction for female worms coming from C57 mice had greater egg output for CR fed mice than AL fed mice and when females were exposed to H2O2 media, egg output decreased by 47%. For female worms taken from SJL mice in vitro reproduction did not vary with host CR or with experiment duration. Additionally, there was a three way interaction of media, diet, and experiment duration. Female worms taken from AL fed mice and infected for 2 weeks and placed in control media laid more eggs than females in H2O2 media taken from CR fed mice. Overall, effects of CR on host susceptibility to infection and ROS/antioxidant production depended on mouse strain and parasite antioxidant defense and resulting changes in parasite life history varied with the host environment where the worms lived. Future studies to examine the relationship between host food intake and parasite susceptibility should account for possible strain effects when using laboratory mice.