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The human-bacterial pathogen interaction is a complex process that results from
a prolonged evolutionary arms race in the struggle for survival. The pathogen employs
virulence strategies to achieve host colonization, and the latter counteracts using defense
programs. The encounter of both organisms results in drastic physiological changes
leading to stress, which is an ancient response accompanying infection. Recent evidence
suggests that the stress response in the host converges with the innate immune pathways
and influences the outcome of infection. However, the contribution of stress and the exact
mechanism(s) of its involvement in host defense remain to be elucidated. Using the model
bacterial pathogen Shigella flexneri, and comparing it with the closely related pathogen
Salmonella Typhimurium, this study investigated the role of host stress in the outcome of
infection.
Shigella infection is characterized by a pronounced pro-inflammatory response
that causes intense stress in host tissues, particularly the intestinal epithelium, which
constitutes the first barrier against Shigella colonization. In this study, inflammatory
stress was simulated in epithelial cells by inducing oxidative stress, hypoxia, and cytokine
stimulation. Shigella infection of epithelial cells exposed to such stresses was strongly
inhibited at the adhesion/binding stage. This resulted from the depletion of sphingolipidrafts
in the plasma membrane by the stress-activated sphingomyelinases. Interestingly,
Salmonella adhesion was not affected, by virtue of its flagellar motility, which allowed the
gathering of bacteria at remaining membrane rafts. Moreover, the intracellular replication
of Shigella lead to a similar sphingolipid-raft depletion in the membrane across adjacent
cells inhibiting extracellular bacterial invasion.
Additionally, this study shows that Shigella infection interferes with the host stress
granule-formation in response to stress. Interestingly, infected cells exhibited a nuclear
depletion of the global RNA-binding stress-granule associated proteins TIAR and TIA-1
and their accumulation in the cytoplasm.
Overall, this work investigated different aspects of the host stress-response in the
defense against bacterial infection. The findings shed light on the importance of the host
stress-pathways during infection, and improve the understanding of different strategies
in host-pathogen interaction.
In neoplastic diseases the tumor stroma and especially tumor-associated macrophages (TAMs) play an important role in tumor growth and progression. TAMs exhibit an intensive cross-talk with tumor cells resulting in the promotion of angiogenesis and the inhibition of local protective immune responses in certain tumor entities. Therefore, TAMs are a potential target for tumor therapy. Here it was shown that intravenously applied intracellular bacteria like Salmonella and Shigella primarily target TAMs. To exploit this feature a growth attenuated Shigella strain with the capacity to induce apoptosis in macrophages was designed. Shigella are invasive bacteria that penetrate the colonic tissue and initiate an acute inflammation. In macrophages, Shigella rapidly induces caspase-1 processing and apoptosis via the virulence factor IpaB. By genomic deletion of the aroA-locus a metabolically attenuated strain defective in intracellular growth but with retained capacity of infection, cell-to-cell spread, caspase-1 processing and apoptosis induction in macrophages was designed. It was shown that this strain primarily targets TAMs in 4T1 cell induced and transgenic MMTV-HER2/new breast cancer models. Shigella were almost exclusively found intracellularly, whereas growth attenuated Salmonella were also found extracellularly at late time points. The metabollically attenuated Shigella strain with retained virulence, but not avirulent Shigella strains, was able to activate caspase-1 and induce apoptosis in TAMs at all time points (4 h, 6 h and 7 d p.i.) in both breast cancer models. This unrestricted apoptosis induction translated into a substantial, long-lasting and highly significant reduction of TAMs number (up to 70 %) in both models. In contrast, Salmonella could only induce apoptosis in TAMs at early time points (6 h p.i.) and failed to reduce TAMs in both models. In the 4T1 model, the effect on tumor size was monitored and treatment of the mice with the attenuated Shigella strain resulted in a complete block of tumor growth. Finally, Shigella primarily infected the macrophage fraction, activated caspase-1 and induced apoptosis in cells derived from a human ovarian carcinoma ex vivo. Taken together, this data suggests that growth attenuated intracellular bacteria capable of inducing apoptosis in TAMs are a promising therapeutic option for certain cancer diseases where TAMs have a proven role for tumor growth or progression.