Description: Objective— In contrast to other antibodies involved in transfusion-related acute lung injury, anti–HNA-3a antibodies are incapable of inducing direct neutrophil activation and seem to interact with endothelial cells (ECs) primarily. In animal studies, anti–HNA-3a-mediated transfusion-related acute lung injury could be precipitated in the absence of neutrophils, but was stronger when neutrophils were present. In a different context the target protein of these antibodies, choline transporter-like protein-2 (CTL-2), was reported to interact with a protein of the inner ear carrying 2 von Willebrand factor (VWF) A-domains. These observations prompted us to investigate whether VWF might be involved in anti–HNA-3a-mediated neutrophil activation, and whether signaling via CD11b/CD18 is involved, as in various other experimental settings. Approach and Results— Cell adhesion demonstrated specific binding of CTL-2 to VWF. Immunoprecipitation analysis of CTL-2/CD11b/CD18 coexpressing cells indicated that anti–HNA-3a colocalizes CTL-2 and CD11b/CD18 when VWF is present. Functional studies revealed that anti–HNA-3a-mediated neutrophil agglutination is an active, protein kinase C-dependent and partially Fc-dependent process. Agglutination and the production of reactive oxygen species seem to require the formation of a trimolecular complex between the target antigen (CTL-2), CD11b/CD18 and VWF. In line with these observations, anti–HNA-3a induced less severe transfusion-related acute lung injury and less neutrophil recruitment to the alveolar space in VWF knockout mice. Conclusions— We introduce CTL-2 as a new binding partner for VWF. Interaction of neutrophils with VWF via CTL-2 allows anti–HNA-3a to induce signal transduction via CD11b/CD18, which leads to neutrophil activation and agglutination. In transfusion-related acute lung injury, this mechanism may further aggravate endothelial leakage.