Macrophages employ quorum licensing to regulate collective activation

Abstract

Macrophage-initiated inflammation is tightly regulated to eliminate threats such as infections while suppressing harmful immune activation. However, individual cells signaling responses to pro-inflammatory cues are heterogeneous; subpopulations emerge with high or low activation states, though why this occurs is unknown. To address this question, we used single-cell tracking and dynamical modeling to develop and validate a revised model for macrophage activation by lipopolysaccharide (LPS) that invokes a mechanism we term quorum licensing. Bimodal phenotypic partitioning of macrophages is primed during the resting state, depends on cumulative history of cell density, is predicted by extrinsic noise in transcription factor expression, and is independent of canonical LPS-induced intercellular feedback in the tumor necrosis factor (TNF) response. Our analysis shows how this density-dependent coupling produces a nonlinear effect on collective TNF production. We speculate that by linking macrophage density to activation, this mechanism could amplify local responses to threats and prevent false alarms.