Characterization of Homotypic Fusion in Chlamydia muridarum

ABSTRACT. Chlamydia trachomatis (Ct) is the most common cause of bacterial sexually transmitted infections worldwide. Since Chlamydia infections do not generate protective immunity, frequent reinfections are common and can result in substantial reproductive consequences such as pelvic inflammatory disease and infertility. Without a preventative vaccine, mitigating Ct-induced tissue damage is paramount. Thus, my long-term goal is to identify how interactions between Ct and its host contribute to virulence and result in pathological outcomes. Ct is an obligate intracellular bacterium that replicates in a parasitic niche called the inclusion. At a multiplicity of infection (MOI) >1, each bacterium enters the cell and forms its own inclusion. High MOIs would be encountered in vivo when adjacent infected cells rupture and release infectious Ct at the end of their life cycle. The inclusions in highly infected cells ultimately fuse together into a single compartment through the process of homotypic fusion, in which the protein CtIncA is involved. Non-fusogenic Ct causes reduced clinical symptoms and reduced cervical burdens in infected women, highlighting the clinical importance of homotypic fusion. However, the mechanism by which homotypic fusion contributes to these processes is unknown. Identifying how homotypic fusion supports Ct virulence has been hampered by the lack of a suitable in vivo model for Ct pathogenesis. Here, I propose to use the surrogate species Chlamydia muridarum (Cm) in a murine infection model to determine the effect of homotypic fusion on Chlamydia pathogenesis. Using a combination of in vitro and in vivo approaches, I will use a newly generated non-fusogenic IncAKO Cm strain to (1) determine the impact of homotypic fusion on Cm fitness in a cell culture and animal model and (2) compare the in vivo host response to infection between WT and IncAKO Cm. For the first time, this study will establish how homotypic fusion contributes to Cm virulence. Ultimately, this work will allow us to apply our knowledge of homotypic fusion in Ct to a powerful animal model to identify the molecular basis by which homotypic fusion impacts Chlamydia pathogenesis.