Type III secretion system effectors form robust and flexible intracellular virulence networks

Area of Science:

• Microbiology
• Molecular Biology
• Immunology

Background:

• Gram-negative bacterial infections often utilize type III secretion system (T3SS) effectors to manipulate host cells.
• These effectors are crucial for pathogen virulence and host immune evasion.

Purpose of the Study:

• To investigate the robustness and adaptability of the T3SS effector network in Gram-negative pathogens.
• To determine the impact of effector network contractions on pathogenicity and host immune responses.

Main Methods:

• Utilized *Citrobacter rodentium* as a model organism, performing sequential gene deletions of its 31 T3SS effectors.
• Employed *in vivo* infection models in mice to assess pathogenicity and immune responses.
• Developed and trained a machine learning model using data from over 100 mutant combinations to predict colonization outcomes.

Main Results:

• The T3SS effector network demonstrated significant robustness, tolerating up to 60% contraction while maintaining pathogenicity.
• Different effector networks induced distinct host cytokine profiles (e.g., IL-22, IL-17, IFN-γ, GM-CSF), yet all elicited protective immunity.
• Machine learning models accurately predicted colonization outcomes, which were experimentally validated.
• Transferring a human-restricted effector repertoire did not confer efficient colonization in *C. rodentium*, suggesting host adaptation roles for effector networks.

Conclusions:

• T3SS effector networks exhibit remarkable resilience to genetic variation, highlighting their adaptability during host infection.
• Host immune responses are robust and adaptable to diverse T3SS effector network compositions.
• Effector network composition plays a critical role in pathogen host adaptation and host range.