Complement Activation Induces Neutrophil Adhesion and Neutrophil-Platelet Aggregate Formation on Vascular Endothelial Cells

INTRODUCTION: Atypical hemolytic uremic syndrome is a thrombotic microangiopathy, which is linked to hereditary or autoimmune defects in complement activators or regulators present in blood and on vascular endothelial cells. Acute thrombotic microangiopathy episodes are typically preceded by infections, which by themselves would not be expected to manifest HUS. Thus, it is possible that the host immune response contributes to the precipitation of aHUS. However, the mechanisms involved are not fully understood. We hypothesized that neutrophils trigger aHUS via initiating platelet aggregate formation on complement-activated endothelial cells. METHODS: We investigated neutrophil adhesion to complement-activated endothelial cells under static and flow conditions in vitro and ex vivo. RESULTS: Our results show that complement activation on endothelial cells promotes neutrophil adhesion, which is significantly reduced when the complement terminal pathway is blocked. When neutrophils and platelets are perfused simultaneously, neutrophils adhering to endothelial cells also induce the formation of platelet-neutrophil aggregates on these cells. Sera from patients with aHUS recapitulated these results. DISCUSSION: Therefore, our findings of (i) neutrophils adhering to complement-activated endothelial cells, (ii) the formation of neutrophil-platelet aggregates on endothelial cells, and (iii) the ability of aHUS serum to induce similar effects identify a possible role for neutrophils in aHUS manifestation.