Complement activation drives Transfusion-related acute lung injury via macrophage trafficking and formation of NETs.

Transfusion-related acute lung injury (TRALI) is one of the leading causes of transfusion-related fatalities and to date, without available therapies. Here we investigated the role of the complement system in TRALI. Murine anti-major histocompatibility complex (MHC) class I antibodies were used in TRALI mouse-models, in combination with analyses of TRALI patient plasma samples. We found that in vitro complement activation was related to in vivo antibody-mediated TRALI induction, which was correlated with increased macrophage trafficking from the lungs to the blood in a Fc-dependent manner and that this was dependent on C5. hIgG1 variants of the murine TRALI-inducing antibody 34-1-2S, either unable to activate complement and/or bind to Fcγ-receptors (FcγRs), revealed an essential role for the complement system, but not for FcγRs, in the onset of 34-1-2S-mediated TRALI in mice. In addition, we found high levels of complement activation in plasma of human TRALI patients (n=53) which correlated with elevated neutrophil extracellular trap (NET) markers. In vitro we found that NETs could be formed in a murine, 2-hit model, mimicking TRALI with lipopolysaccharide (LPS) and C5a stimulation. Collectively, this reveals a critical role for Fc-mediated complement activation in TRALI, with a direct relation to macrophage trafficking from the lungs to the blood and an association with NET formation, suggesting that targeting the complement system may be an attractive therapeutic approach for combatting TRALI.