Identification of Determinants that Mediate Chlamydia Muridarum Tissue Tropism

Abstract

Bacteria within the genus Chlamydia are obligate intracellular pathogens that infect a wide variety of vertebrates. Despite having highly similar genomes, closely related Chlamydia species can have different tissue tropisms. The human pathogen Chlamydia trachomatis exemplifies this; strains in three genetically similar biovars (trachoma, chlamydia, and lymphogranuloma venereum) target different human tissues (ocular epithelia, urogenital epithelia, and the lymphatic system, respectively). Studies using the mouse pathogen Chlamydia muridarum have suggested that tropism is heavily influenced by evasion of host immune responses, particularly, responses mediated by the cytokine interferon-gamma (IFNγ). My goal was to use an unbiased approach to investigate how C. muridarum circumvents immunity. I conducted a tropism screen using a mutagenized C. muridarum library and murine-derived cells from distinct tissues. This screen identified mutants that have reduced ability to proliferate in specific cell types in IFNγ-dependent and independent manners. Some of the mutants had complex phenotypes that could not be linked to single mutations. However, I also identified two isolates whose growth was restricted in murine rectal and oviduct epithelia in an IFNγ- independent manner. Both of these isolates had the same missense mutation in the gene tc0237. I determined that complementation of the mutants with wild type tc0237 was sufficient to reverse their tropism phenotype. Blocking the ability of host cells to respond to infection by treating them with a protein synthesis inhibitor revealed that the factors responsible for restricting tc0237 mutants were cell-line specific rather than a shared characteristic of epithelial cells. Several attempts to localize TC0237 using different approaches were unsuccessful, but bioinformatic analyses revealed that TC0237 likely remains associated with chlamydial cells and could oligomerize. Overall, my findings suggest that TC0237 plays a critical role in C. muridarum immune evasion and show that chlamydial tropism is not solely defined by IFNγ.