## 2 Mechanisms of Cognitive Dysfunction
### Neutrophil Extracellular Traps  
Neutrophils act as the sentinels of the innate immune system, migrating from the circulating blood to sites of infection where they combat pathogens by phagocytosis, degranulation and the release of neutrophil extracellular traps (NETs) [70; 71; 72]. The discovery of NETs by [PERSON] in 2004 led to an entire new field of granulocyte investigation [71]. Activated neutrophils undergo \"NETosis\" to release NETs, a unique form of cell death that is distinct to necrosis and apoptosis [73; 74]. NETs are fibrous structures consisting of chromatin backbones with diameters of \(\sim\)17 nm with attached globular domains with diameters of \(\sim\)50 nm [75; 76]. The globular domains comprise of histones (H1, H2A, H2B, H3, and H4) and a granule-derived enzyme (neutrophil elastase, a neutrophil-specific serine protease) [71; 75]. Other granule-derived enzymes present in NETs include myeloperoxidase and MMP-9 [71]. In total, less than 30 proteins are present in NETs and histones account for 70% of the protein mass [75; 76]. The antimicrobial activity of histones is well established, killing bacteria at nanomolar concentrations more effectively than other antimicrobials [71; 76]. NET formation has been documented in other conditions, including experimental models of neuroinflammatory diseases [77; 78]. Transmigration across an activated endothelium _in vivo_ is known to induce an increase in reactive oxygen species in neutrophils [78]. _In vitro_, transmigrated murine neutrophils (CD11b/Ly6G\({}^{+}\)) across IL-1\(\beta\) stimulated brain endothelium to undergo NETosis and caused cell death when applied to primary murine neuronal cultures [78]. Similar to the bacteria-killing experiments, addition of a monoclonal antibody against H2A-H2B-DNA complex improved neuronal viability [78]. A mixture of neutrophil proteases inhibitors (including inhibitor against neutrophil elastase) rescued neuronal viability when transmigrated neutrophils were applied to neurons in the presence of the protease inhibitors [78]. Interestingly, the protease inhibitors were ineffective when using conditioned medium from transmigrated neutrophils, suggesting that the cascade of events after the release of NETs might be neurotoxic [78]. _In vivo_, NETosis was observed in the cerebral cortex after sterotaxic intracerebral injection of LPS [78]. Taken together, the release of NETs containing histones and proteases that takes place normally as part of an antimicrobial defense potentially contributes to neurotoxicity [78]. This is in concordance with the observation that stroke patients with systemic infections have poorer outcomes [77; 78]. Neutrophils from SLE patients (referred subsequently in text as lupus neutrophils) have altered functional properties, including diminished phagocytic capabilities, increased intravascular activation, increased platelet-neutrophil aggregation and increased production of reactive oxygen species compared to neutrophils from healthy controls (referred subsequently in text as control neutrophils) [79; 80; 81; 82]. In addition, PBMC transcriptional signatures reveal overexpression of granulopoiesis-related genesin SLE patients but not in healthy controls [83]. This \"granulocyte signature\" is due to the presence of low-density granulocytes (LDGs) that co-purify with mononuclear cells during density centrifugation [83, 84]. LDGs appear to be a distinct subset of immature neutrophils that includes a spectrum of pro-, myelo-, meta-myelocytes, early and late bands as well as some segmented neutrophils [82, 83]. In addition, gene expression profiling of LDGs revealed an upregulation of mRNA of serine proteases (e.g., neutrophil elastase) present in primary, or azurophilic, granules [85]. Levels of mRNA expression that encode neutrophil serine proteases are greatest at the promylocytic stage of neutrophil differentiation and are downregulated as the neutrophil matures [85]. The profile of cell surface markers of LDGs shows an increased expression of CD11b [86].