Genetic Screen in Chlamydia muridarum Reveals Role for an Interferon-Induced Host Cell Death Program in Antimicrobial Inclusion Rupture.

Interferon-regulated immune defenses protect mammals from pathogenically diverse obligate intracellular bacterial pathogens of the genus Interferon gamma (IFN-γ) is especially important in controlling the virulence of species and thus impacts the modeling of human chlamydial infection and disease in mice. How IFN-γ contributes to cell-autonomous defenses against species and how these pathogens evade IFN-γ-mediated immunity in their natural hosts are not well understood. We conducted a genetic screen which identified 31 -ensitive (Igs) mutants of the mouse model pathogen Genetic suppressor analysis and lateral gene transfer were used to map the phenotype of one of these mutants, Igs4, to a missense mutation in a putative chlamydial inclusion membrane protein, TC0574. We observed the lytic destruction of Igs4-occupied inclusions and accompanying host cell death in response to IFN-γ priming or various proapoptotic stimuli. However, Igs4 was insensitive to IFN-γ-regulated cell-autonomous defenses previously implicated in anti- host defense in mice. Igs4 inclusion integrity was restored by caspase inhibitors, indicating that the IFN-γ-mediated destruction of Igs4 inclusions is dependent upon the function of caspases or related prodeath cysteine proteases. We further demonstrated that the Igs4 mutant is immune restricted in an IFN-γ-dependent manner in a mouse infection model, thereby implicating IFN-γ-mediated inclusion destruction and host cell death as potent host defense mechanisms to which wild-type is resistant. Overall, our results suggest that evolved resistance mechanisms to counter IFN-γ-elicited programmed cell death and the associated destruction of intravacuolar pathogens. Multiple obligatory intracellular bacteria in the genus are important pathogens. In humans, strains of cause trachoma, chlamydia, and lymphogranuloma venereum. These diseases are all associated with extended courses of infection and reinfection that likely reflect the ability of chlamydiae to evade various aspects of host immune responses. Interferon-stimulated genes, driven in part by the cytokine interferon gamma, restrict the host range of various species, but how these pathogens evade interferon-stimulated genes in their definitive host is poorly understood. Various species can inhibit death of their host cells and may have evolved this strategy to evade prodeath signals elicited by host immune responses. We present evidence that chlamydia-induced programmed cell death resistance evolved to counter interferon- and immune-mediated killing of -infected cells.