## 2 Mechanisms of Cognitive Dysfunction
### Neutrophil Extracellular Traps
CD11b\({}^{+}\) cells are believed to be pro-inflammatory and elevated numbers of CD11b\({}^{+}\) cells correlate with disease activity in murine models of SLE [87]. Hence, LDGs appear to have a more immature and activated neutrophil phenotype compared to lupus neutrophils and control neutrophils [82]. Isolated LDGs demonstrate enhanced NET formation compared to lupus neutrophils and control neutrophils and are resistant to further NET induction (e.g., by phorbol 12-myristate 13-acetate), suggesting that LDGs may already be prestimulated _in vivo_[85, 88]. NET degradation of SLE sera has been demonstrated to be impaired in a subset of SLE patients compared to healthy controls, particularly during disease flares [89, 90]. Presence of deposited autoantibodies against NET components (e.g., anti-dsDNA and anti-histone) and complement (e.g., C1q) protect these NETs from DNase I digestion, allowing for the persistence of NETs in circulation [89, 90]. This could be due to steric hindrance of the deposited autoantibodies and complement inhibiting DNase I access [90]. Tertiary, or gelatininase, granules of neutrophils contain MMP-9 [78, 91]. Active MMP-9 is externalized on NETs at significantly higher levels in LDGs compared to lupus and control neutrophils [92]. Coculture of human umbilical vein endothelial cells (HUVECs) with human neutrophils induced to undergo NETosis leads to increased endothelial cell apoptosis [92]. This observation was completely abolished with the use of a transwell, indicating that direct contact between NETs and endothelial cells is required for cytotoxicity [92]. Furthermore, significant impairment of murine aortic endothelium-dependent vasorelaxation was observed in the presence of LDG-NETs [92]. This is mediated by MMP-9 on NETs activating endothelial matrix metalloproteinase-2 (MMP-2) to induce endothelial cell apoptosis and dysfunction [92]. Activation of endothelial MMP-2 was significantly decreased on exposure to MMP-9-depleted NETs compared to histone 2A-depleted NETs [92]. After adjusting for the concentration of NETs generated by control neutrophils and LDGs, vasorelaxation impairment induced by control NETs was similar to LDG-NETs; supporting the notion that it is the amount of MMP-9 released during NETosis that determines endothelial integrity in SLE [92]. Anti-MMP-9 antibodies are present in the sera of SLE patients and immune complexes containing MMP-9, but not irrelevant immune complexes (e.g., bovine serum albumin (BSA)/anti-BSA), enhanced NET formation via a FcyR signaling pathway [92]. Lastly, various spontaneous mouse models of SLE, in particular the MRL/MpJ-_fa_\({}^{\text{spr}}\) mice, demonstrate progressive cognitive impairment illustrated by defects in balance, learning ability and other cognitive impairments, temporally correlated with the onset of leukocyte trafficking into the cerebral microcirculation [93, 94]. Increased leukocyte-endothelial cell interactions (leukocyte rolling and adhesion) have been observed in the pial microcirculation of MRL/MpJ-_fa_\({}^{\text{spr}}\) mice at 16 weeks of age compared to control MRL\({}^{\
icefrac{{i}}{{\mu}}}\) mice using intravital microscopy [93]. Further, analysis of the infiltrating leukocytes in brains of MRL/MpJ-_fas\({}^{\rm lpr}\)_ mice at 16 weeks of age revealed mainly neutrophils as the dominant cell population present [94].