Circulating FABP5 released by dying macrophages function as a DAMP to exacerbate septic inflammation

Damage-associated molecular patterns (DAMPs) often cause an exaggerated immune response. Fatty acid-binding protein 5 (FABP5) is traditionally considered a cytosolic protein responsible for the transport of fatty acids. However, little is known about the role of FABP5 in sepsis. Herein, we found that circulating FABP5 levels were higher in patients with sepsis and were associated with adverse outcomes. The circulating FABP5 originated mainly from macrophage pyroptosis in the later stages of sepsis despite a decrease in cytoplasm-resident FABP5 expression. Functionally, circulating FABP5 penetrated the living macrophage membrane and bound to the intracellular domain of TLR4, ultimately inducing secondary inflammation through the NF-κB and MAPK pathways. Unlike circulating oxidized FABP5, cytoplasm-resident FABP5 was present in its reduced form and suppressed macrophage pyroptosis. Clearance of circulating DAMP FABP5 by preventing the passive release from dying macrophages or using a specific blocking antibody can improve survival in mice with sepsis.