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328membrane of PMNs was also measured as a marker of PMN activation. Cell surface CD11b expression in PMNs stimulated with fMLP also increased in a dose-dependent manner (Figure 5b). Mitochondria are often referred to as the powerhouse of the cells, as they generate ATP by oxidative phosphoryla-tion. To our knowledge, changes in mitochondrial ATP levels in PMNs have not been reported. The MFI of MitoAP-1 following fMLP stimulation decreased significantly in a dose-dependent manner (Figure 5c). The MFIs of PMAP-1 and MitoAP-1 in sepsis patients were evaluated by flow cytom-etry within 24 h after diagnosis of sepsis (days 0-1). Both values were significantly higher than those of the HC group (data not shown). CD11b expression on the plasma membrane was also upregulated in sepsis patients. Activated PMNs in sepsis patients appeared to cause a burst of extra-cellular ATP release and to increase ATP synthesis by oxidative phosphorylation in the mitochondria. The same experiments were conducted at days 3-4 as the patients’ clinical conditions improved, which revealed that the MFIs of PMAP-1 and CD11b expression decreased significantly compared to those at days 0-1, whereas a high MitoAP-1 MFI was maintained. Namely, the temporal changes of Figure 4　Confocal micrographs of human polymorpho-nuclear neutrophils (PMNs) stained with PMAP-1 and MitoAP-1. a ATP on the plasma membrane was stained with PMAP-1. The bright green fluorescence observed on the plasma membrane of PMNs resulted from PMAP-1 conjugation to ATP (×100 oil objective, NA 1.4). Scale bar 10 μm.b ATP in the mitochondria was stained with MitoAP-1 (red) and MitoTracker® Green FM (green), and they co-localized well (×100 oil objective, NA 1.4). Scale bar 10 μmATP release and CD11b expression on the plasma membrane were similar, but ATP production showed distinct behavior in the mitochondria.-Complement system in multiple organ failure-Recently we are trying to elucidate the of comple-ment system in patients with multiple organ failure by the grant for Japan Society for the Promotion of Science (JSPSI, KAKENHI Grant number 19H03764). Sepsis is a life-threatening emergency that occurs when the human body reacts in an extreme way to an infection, triggering a chain reaction that exac-erbates the patient’s condition. Almost any type of infection can lead to sepsis, although the most common infections typically start in the lung, urinary tract, skin or gastrointestinal tract. One of the main problems associated with sepsis is the multiple organ failure or dysfunction that it can lead to - it is this which most often leads to compli-cation resulting in death. However, despite much research into this subject, the actual causes for multiple organ failure from sepsis (as well as severe trauma, burns and heat stroke) are not entirely clear. We have turned our attention to trying to ascertain the exact reasons why multiple organ failure occurs in sepsis patients. We have devel-