Kinetics of local C3 production orchestrates neutrophil recruitment in lung injury

Complement component 3 (C3) is crucial for host defense against bacteria. While the liver is the primary source of circulating C3, local C3 production at barrier surfaces such as the lung is key in early responses. Yet, how local complement-mediated responses are initiated at mucosal barriers is unknown. This study investigates the kinetics and necessity of lung-derived C3 during the initial hours of an infection. Using models of bacterial pneumonia in ex vivo-perfused human lungs and mice deficient in liver-derived C3, we demonstrate that intrapulmonary C3 production and activation precedes the accumulation of circulating C3 into the bronchoalveolar space. Utilizing mice deficient in lung-derived C3, we demonstrate that epithelial cell-derived C3 is required for early neutrophil recruitment in pneumonia. Transcriptomic and proteomic analyses reveal that neutrophil chemotactic pathways such as C5a and CXCL2 depend on lung epithelial cell-derived C3. These findings demonstrate how lung epithelial-derived C3 influences early mucosal responses to infection via both canonical (direct) and non-canonical (indirect) pathways. SUMMARYAlburquerque et al show an initial, entirely local phase of complement-mediated mucosal protection before a subsequent, systemic response occurs in the setting of barrier disruption. Their work suggests that complement component C3 derived locally at a barrier from the epithelium influences early responses to infection by recruiting neutrophils via multiple pathways independent of circulating C3.