The C3-C3aR axis modulates trained immunity in alveolar macrophages

Complement protein C3 is crucial for immune responses in mucosal sites such as the lung, where it aids in microbe elimination, and enhances inflammation. While trained immunity - enhanced secondary responses of innate immune cells after prior exposure - is well-studied, the role of the complement system in trained immune responses remains unclear. We investigated the role of C3 in trained immunity and found that alveolar macrophage (AM) C3 and C3aR1 expression increased in humans after an intranasal exposure to a training stimulus. In vivo, trained wild-type mice showed significantly elevated proinflammatory cytokines and increased C3a levels upon a second stimulus. Ex vivo, trained C3-deficient AMs displayed reduced chemokine and cytokine output as well as impaired phagocytosis and reactive oxygen species production compared to wild-type AMs. Real-time confocal microscopy of live, intact mouse alveoli revealed that AMs internalize C3 rapidly after alveolar microinstillation, as compared to C3a. Correspondingly, the blunted cytokine output was restored by exogenous C3 but not by C3a. Inhibiting C3aR, both pharmacologically and with a genetic C3aR knockout, prevented this restoration, indicating the necessity of C3aR engagement. Mechanistically, trained WT AMs demonstrated enhanced glycolytic activity compared to C3-deficient AMs - a defect corrected by exogenous C3 in a C3aR-dependent manner. These findings reveal that C3 modulates trained immunity in AMs through C3aR signaling and highlight a novel role for C3 in trained immunity.