In addition to being key elements in hemostasis and thrombosis, platelets amplify neutrophil function. We aimed to gain further insight into the stimuli, mediators, molecular pathways, and regulation of neutrophil extracellular trap formation mediated by human platelets. Platelets stimulated by lipopolysaccharide, a wall component of gram-negative bacteria, Pam3-cysteine-serine-lysine 4, a mimetic of lipopeptide from gram-positive bacteria, Escherichia coli, Staphylococcus aureus, or physiologic platelet agonists promoting neutrophil extracellular trap formation and myeloperoxidase-associated DNA activity under static and flow conditions. Although P-selectin or glycoprotein IIb/IIIa were not involved, platelet glycoprotein Ib, neutrophil cluster of differentiation 18, and the release of von Willebrand factor and platelet factor 4 seemed to be critical for the formation of neutrophil extracellular traps. The secretion of these molecules depended on thromboxane A(2) production triggered by lipopolysaccharide or Pam3-cysteine-serine-lysine 4 but not on high concentrations of thrombin. Accordingly, aspirin selectively inhibited platelet-mediated neutrophil extracellular trap generation. Signaling through extracellular signal-regulated kinase, phosphatidylinositol 3-kinase, and Src kinases, but not p38 or reduced nicotinamide adenine dinucleotide phosphate oxidase, was involved in platelet-triggered neutrophil extracellular trap release. Platelet-mediated neutrophil extracellular trap formation was inhibited by prostacyclin. Our results support a role for stimulated platelets in promoting neutrophil extracellular trap formation, reveal that an endothelium-derived molecule contributes to limiting neutrophil extracellular trap formation, and highlight platelet inhibition as a potential target for controlling neutrophil extracellular trap cell death.

Scar formation after brain injury is still poorly understood. To further elucidate such processes, here, we examine the interplay between astrocyte proliferation taking place predominantly at the vascular interface and monocyte invasion. Using genetic mouse models that decrease or increase reactive astrocyte proliferation, we demonstrate inverse effects on monocyte numbers in the injury site. Conversely, reducing monocyte invasion using CCR2−/− mice causes a strong increase in astrocyte proliferation, demonstrating an intriguing negative cross‐regulation between these cell types at the vascular interface. CCR2−/− mice show reduced scar formation with less extracellular matrix deposition, smaller lesion site and increased neuronal coverage. Surprisingly, the GFAP + scar area in these mice is also significantly decreased despite increased astrocyte proliferation. Proteomic analysis at the peak of increased astrocyte proliferation reveals a decrease in extracellular matrix synthesizing enzymes in the injury sites of CCR2−/− mice, highlighting how early key aspects of scar formation are initiated. Taken together, we provide novel insights into the cross‐regulation of juxtavascular proliferating astrocytes and invading monocytes as a crucial mechanism of scar formation upon brain injury.

Leptospirosis is an important global human and veterinary health problem. Humans can be infected by exposure to chronically infected animals and their environment. An important focus of the current leptospiral research is the identification of outer membrane proteins (OMPs). Due to their location, leptospiral OMPs are likely to be relevant in host-pathogen interactions, hence their potential ability to stimulate heterologous immunity. The existing whole-genome sequence of Leptospira interrogans serovar Copenhageni offers a unique opportunity to search for cell surface proteins. Predicted genes encoding potential surface proteins were amplified from genomic DNA by PCR methodology and cloned into an Escherichia coli expression system. The partially purified recombinant proteins were probed by Western blotting with sera from human patients diagnosed with leptospirosis. Sixteen proteins, out of a hundred tested, were recognized by antibodies present in human sera. Four of these proteins were conserved among eight serovars of L. interrogans and absent in the non-pathogenic Leptospira biflexa. These proteins might be useful for the diagnosis of the disease as well as potential vaccine candidates.