Macrophages employ quorum licensing to regulate collective activation

Area of Science:

• Immunology
• Cell Biology
• Systems Biology

Background:

• Macrophage-initiated inflammation is crucial for host defense but requires tight regulation to prevent autoimmunity.
• Heterogeneity in single-cell responses to pro-inflammatory stimuli, like lipopolysaccharide (LPS), leads to distinct macrophage activation states.
• Existing models often overlook the impact of cellular density on macrophage activation dynamics.

Purpose of the Study:

• To develop and validate a revised model of LPS-induced macrophage activation incorporating cell density effects.
• To investigate the mechanism of "quorum licensing" in regulating macrophage phenotypic heterogeneity.
• To understand how cell density influences collective inflammatory responses, specifically tumor necrosis factor (TNF) production.

Main Methods:

• Single-cell tracking of macrophage behavior and activation dynamics.
• Development and validation of a dynamical mathematical model for macrophage signaling.
• Analysis of transcription factor expression noise and its relation to phenotypic partitioning.
• Modeling the nonlinear effects of density-dependent coupling on collective TNF production.

Main Results:

• Macrophage bimodal phenotypic partitioning (high/low activation) is primed in the resting state.
• This partitioning depends on cumulative cell density history and extrinsic noise in transcription factor expression.
• The observed heterogeneity is independent of canonical LPS-induced intercellular feedback in TNF response.
• Density-dependent coupling results in a nonlinear effect on collective TNF production.

Conclusions:

• A novel mechanism, "quorum licensing," links macrophage density to activation states.
• This density-dependent mechanism primes macrophages for differential responses based on their past environment.
• Quorum licensing may amplify local inflammatory responses to pathogens while preventing systemic overactivation or false alarms.