Innate immune responses yield tissue-specific bottlenecks that scale with pathogen dose.

Area of Science:

• Immunology
• Microbiology
• Infectious Disease

Background:

• Pathogens face host immune system barriers, known as infection bottlenecks, which limit pathogen entry and determine disease outcome.
• Understanding how pathogen dose influences these bottlenecks is crucial for predicting infection dynamics.

Purpose of the Study:

• To investigate the relationship between pathogen inoculum size and the effectiveness of innate immune responses.
• To introduce and define the concept of "dose scaling" in the context of infection.

Main Methods:

• Utilized a model of systemic *Escherichia coli* infection in hosts.
• Analyzed how infection bottlenecks change with varying inoculum sizes.
• Investigated the role of Toll-like receptor 4 (TLR4) and lipopolysaccharide (LPS).
• Experimentally mimicked high pathogen doses using heat-killed bacteria.

Main Results:

• Identified "dose scaling," where innate immune response efficacy varies with pathogen dose.
• Demonstrated that dose scaling during *E. coli* infection is tissue-specific.
• Showed that dose scaling depends on the lipopolysaccharide (LPS) receptor TLR4.
• Confirmed that sensing pathogen molecules, not live bacteria, drives dose scaling.

Conclusions:

• Dose scaling quantitatively links innate immunity with infection bottlenecks.
• This framework provides a new understanding of how pathogen inoculum size influences infection outcomes.
• Highlights the importance of pathogen sensing mechanisms in modulating immune barrier effectiveness.