Neutrophils, the most abundant human immune cells, are rapidly recruited to sites of infection, where they fulfill their life-saving antimicrobial functions. While traditionally regarded as short-lived phagocytes, recent findings on long-term survival, neutrophil extracellular trap (NET) formation, heterogeneity and plasticity, suppressive functions, and tissue injury have expanded our understanding of their diverse role in infection and inflammation. This review summarises our current understanding of neutrophils in host-pathogen interactions and disease involvement, illustrating the versatility and plasticity of the neutrophil, moving between host defence, immune modulation, and tissue damage.

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The role of platelets in hemostasis is to produce a plug to arrest bleeding. During thrombocytopenia, spontaneous bleeding is seen in some patients but not in others; the reason for this is unknown. Here, we subjected thrombocytopenic mice to models of dermatitis, stroke, and lung inflammation. The mice showed massive hemorrhage that was limited to the area of inflammation and was not observed in uninflamed thrombocytopenic mice. Endotoxin-induced lung inflammation during thrombocytopenia triggered substantial intraalveolar hemorrhage leading to profound anemia and respiratory distress.By imaging the cutaneous Arthus reaction through a skin window, we observed in real time the loss of vascular integrity and the kinetics of skin hemorrhage in thrombocytopenic mice. Bleeding-observed mostly from venulesoccurred as early as 20 minutes after challenge, pointing to a continuous need for platelets to maintain vascular integrity in inflamed microcirculation. IntroductionInflammation and hemostasis are tightly intertwined. In particular, this is becoming very evident in platelet biology. While the classical role of platelets is to mediate hemostatic plug formation, it has been demonstrated that platelets also play an important role in inflammation. For example, previous studies show that platelets promote inflammatory responses in atherosclerosis, in hepatitis, and after cerebral ischemia. 1-4 Furthermore, early in inflammation prothrombotic functions of platelets are reduced, 5 and activated platelets are capable of up-regulating inflammatory molecules on the endothelium. 6,7 Recently, our group showed that in angiogenesis-which is strongly linked to inflammation 8 -platelets play an important role in preventing hemorrhage of sprouting vessels. 9 That platelets support vascular integrity during injury is well established. Early studies also demonstrated a supportive role for platelets during organ perfusion with buffers 10,11 and growthpromoting effects on endothelial cultures. 12 Whether platelets have a supportive role in inflamed microcirculation still remains experimentally unexplored.In humans, profound thrombocytopenia is found, for example, in patients suffering from idiopathic thrombocytopenic purpura. Interestingly, in the absence of injury, some patients bleed while others do not show spontaneous bleeding despite equally low platelet counts. 13 Thus, thrombocytopenia alone cannot explain this phenomenon and other, yet to be defined contributing factors are required to induce bleeding in thrombocytopenic patients, as suggested in a recent review. 14 As thrombocytopenia may lead to life-threatening bleedings, it is important to further understand the cofactors leading to hemorrhage.In the present study, we investigate the effects of inflammation on vascular integrity during thrombocytopenia. We challenged mice in 4 different inflammatory models and observed the affected blood vessels over time in the presence or absence of platelets. We show that thrombocytopenia rapidly induces massive bleeding in inflamed sk...

Optic atrophy 1 (OPA1) is a mitochondrial inner membrane protein that has an important role in mitochondrial fusion and structural integrity. Dysfunctional OPA1 mutations cause atrophy of the optic nerve leading to blindness. Here, we show that OPA1 has an important role in the innate immune system. Using conditional knockout mice lacking Opa1 in neutrophils (Opa1N∆), we report that lack of OPA1 reduces the activity of mitochondrial electron transport complex I in neutrophils. This then causes a decline in adenosine-triphosphate (ATP) production through glycolysis due to lowered NAD+ availability. Additionally, we show that OPA1-dependent ATP production in these cells is required for microtubule network assembly and for the formation of neutrophil extracellular traps. Finally, we show that Opa1N∆ mice exhibit a reduced antibacterial defense capability against Pseudomonas aeruginosa.

Neutrophil granule serine proteases contribute to immune responses through cleavage of microbial toxins and structural proteins. They induce tissue damage and modulate inflammation if levels exceed their inhibitors. Here, we show that the intracellular protease inhibitors Serpinb1a and Serpinb6a contribute to monocyte and neutrophil survival in steady-state and inflammatory settings by inhibiting cathepsin G (CatG). Importantly, we found that CatG efficiently cleaved gasdermin D (GSDMD) to generate the signature N-terminal domain GSDMD-p30 known to induce pyroptosis. Yet GSDMD deletion did not rescue neutrophil survival in Sb1a.Sb6a À/À mice. Furthermore, Sb1a.Sb6a À/À mice released high levels of pro-inflammatory cytokines upon endotoxin challenge in vivo in a CatG-dependent manner. Canonical inflammasome activation in Sb1a.Sb6a À/À macrophages showed increased IL-1b release that was dependent on CatG and GSDMD. Together, our findings demonstrate that cytosolic serpins expressed in myeloid cells prevent cell death and regulate inflammatory responses by inhibiting CatG and alternative activation of GSDMD.