Transfusion-related acute lung injury (TRALI) is the leading cause of transfusion-related fatalities and is characterized by acute respiratory distress following blood transfusion. Donor antibodies are frequently involved; however, the pathogenesis and protective mechanisms in the recipient are poorly understood, and specific therapies are lacking. Using newly developed murine TRALI models based on injection of anti-major histocompatibility complex class I antibodies, we found CD4CD25FoxP3 T regulatory cells (Tregs) and CD11c dendritic cells (DCs) to be critical effectors that protect against TRALI. Treg or DC depletion in vivo resulted in aggravated antibody-mediated acute lung injury within 90 minutes with 60% mortality upon DC depletion. In addition, resistance to antibody-mediated TRALI was associated with increased interleukin-10 (IL-10) levels, and IL-10 levels were found to be decreased in mice suffering from TRALI. Importantly, IL-10 injection completely prevented and rescued the development of TRALI in mice and may prove to be a promising new therapeutic approach for alleviating lung injury in this serious complication of transfusion.

Key Points• CRP enhances antibodymediated lung damage when infused into TRALI-resistant mice.• CRP and TRALI-inducing antibodies generate a synergistic increase in MIP-2 production and pulmonary neutrophil accumulation in vivo.Transfusion-related acute lung injury (TRALI) is a syndrome of respiratory distress triggered by blood transfusions and is the leading cause of transfusion-related mortality. TRALI has primarily been attributed to passive infusion of HLA and/or human neutrophil antigen antibodies present in transfused blood products, and predisposing factors such as inflammation are known to be important for TRALI initiation. Because the acute-phase protein C-reactive protein (CRP) is highly upregulated during infections and inflammation and can also enhance antibody-mediated responses such as in vitro phagocytosis, respiratory burst, and in vivo thrombocytopenia, we investigated whether CRP affects murine antibody-mediated TRALI induced by the anti-major histocompatibility complex antibody 34-1-2s. We found that BALB/c mice injected with 34-1-2s or CRP alone were resistant to TRALI, however mice injected with 34-1-2s together with CRP had significantly enhanced lung damage and pulmonary edema. Mechanistically, 34-1-2s injection with CRP resulted in a significant synergistic increase in plasma levels of the neutrophil chemoattractant macrophage inflammatory protein-2 (MIP-2) and pulmonary neutrophil accumulation. Importantly, murine MIP-2 is the functional homolog of human interleukin-8, a known risk factor for human TRALI. These results suggest that elevated in vivo CRP levels, like those observed during infections, may significantly predispose recipients to antibodymediated TRALI reactions and support the notion that modulating CRP levels is an effective therapeutic strategy to reduce TRALI severity. (Blood. 2015;126(25):2747-2751 Introduction Transfusion-related acute lung injury (TRALI) is characterized by respiratory distress following blood transfusions and is the leading cause of transfusion-related mortality.1,2 Approximately 80% of TRALI cases are associated with HLA-or human neutrophil antigen-specific antibodies present in donor blood.3,4 A 2-hit model was hypothesized to underlie antibody-mediated TRALI where the first hit comprises patient predisposing factors, such as inflammation, and the second hit is due to antibodies in the transfused blood.1 The pathogenesis of antibody-mediated TRALI, however, is still poorly understood, and controversy has arisen using animal models. For example, several cell types have been implicated in inducing lung damage by the TRALI-inducing anti-major histocompatibility complex (MHC) class I antibody 34-1-2s.5 Neutrophils were originally thought to be stimulated by 34-1-2s to produce reactive oxygen species that damaged the pulmonary endothelium in an Fc-dependent manner.1,5-13 Strait et al, 14 however, showed that neutrophil involvement was limited and that activated endothelial cells and complement interactions were the primary events in TRALI induction. Furth...

Key Points• Gastrointestinal flora contributes to development of antibody-mediated murine TRALI.• Depletion of gastrointestinal flora prevents TRALI by inhibiting MIP-2 secretion and pulmonary neutrophil accumulation.Transfusion-related acute lung injury (TRALI) is a syndrome of respiratory distress upon blood transfusion and is the leading cause of transfusion-related fatalities. Whether the gut microbiota plays any role in the development of TRALI is currently unknown. We observed that untreated barrier-free (BF) mice suffered from severe antibody-mediated acute lung injury, whereas the more sterile housed specific pathogen-free (SPF) mice and gut flora-depleted BF mice were both protected from lung injury. The prevention of TRALI in the SPF mice and gut flora-depleted BF mice was associated with decreased plasma macrophage inflammatory protein-2 levels as well as decreased pulmonary neutrophil accumulation. DNA sequencing of amplicons of the 16S ribosomal RNA gene revealed a varying gastrointestinal bacterial composition between BF and SPF mice. BF fecal matter transferred into SPF mice significantly restored TRALI susceptibility in SPF mice. These data reveal a link between the gut flora composition and the development of antibody-mediated TRALI in mice. Assessment of gut microbial composition may help in TRALI risk assessment before transfusion.