Thesis (Ph.D.) - Indiana University, Biology, 2015The host response towards chlamydial infection mobilizes elements of both innate and adaptive immunity, culminating in the production of the Th1 cytokine interferon-gamma (IFN-γ). IFN-γ induces a variety of anti-chlamydial programs, many of which are unique to either humans or mice. Consequently, the human pathogen Chlamydia trachomatis and its near-identical murine relative C. muridarum have evolved to survive and cause disease in their respective hosts. Understanding the mechanisms that contribute to chlamydial niche specificity will facilitate the creation of improved mouse models for the study of human chlamydial disease. To this end, we examined a small chlamydial genomic region of extreme divergence called the Plasticity Zone (PZ) in C. muridarum for its alleged roles in host specificity and virulence. Using a newly adapted reverse genetic technique, we determined that much of the PZ is in fact dispensable in the murine genital tract. We concomitantly embarked on a screen for C. muridarum mutants that were no longer resistant to IFN-γ and uncovered a gain-of-function mutation in an orf outside the PZ that led to dramatic attenuation in mice. IFN-γ is secreted by multiple cell types including macrophages, which participate in innate as well as adaptive immune responses. IFNγ-dependent and -independent defense mechanisms mounted by macrophages contribute significantly to bacterial clearance. Specifically, the amount of nitric oxide (NO) produced by macrophages in response to different infectious doses determines infection outcome in mice. We discovered that the in vivo regulation of infection by NO is also replicated in cell culture. Murine macrophage cell lines infected at high multiplicities of infection cause chlamydial death because of a massive NO response that is triggered by elevated levels of reactive oxygen species (ROS) and lysosomal cathepsin B activity. Taken together, our work elucidates the mechanisms behind important host and chlamydial defense strategies, offering insight into the evolutionary adaptations that are a consequence of the relentless host-pathogen arms race.