C5a alters blood–brain barrier integrity in a human<i>in vitro</i>model of systemic lupus erythematosus

<jats:title>Summary</jats:title><jats:p>The blood–brain barrier (<jats:styled-content style="fixed-case">BBB</jats:styled-content>) plays a crucial role in brain homeostasis, thereby maintaining the brain environment precise for optimal neuronal function. Its dysfunction is an intriguing complication of systemic lupus erythematosus (<jats:styled-content style="fixed-case">SLE</jats:styled-content>).<jats:styled-content style="fixed-case">SLE</jats:styled-content>is a systemic autoimmune disorder where neurological complications occur in 5–50% of cases and is associated with impaired<jats:styled-content style="fixed-case">BBB</jats:styled-content>integrity. Complement activation occurs in<jats:styled-content style="fixed-case">SLE</jats:styled-content>and is an important part of the clinical profile. Our earlier studies demonstrated that C5a generated by complement activation caused the loss of brain endothelial layer integrity in rodents. The goal of the current study was to determine the translational potential of these studies to a human system. To assess this, we used a two dimensional<jats:italic>in vitro</jats:italic><jats:styled-content style="fixed-case">BBB</jats:styled-content>model constructed using primary human brain microvascular endothelial cells and astroglial cells, which closely emulates the<jats:italic>in vivo</jats:italic><jats:styled-content style="fixed-case">BBB</jats:styled-content>allowing the assessment of<jats:styled-content style="fixed-case">BBB</jats:styled-content>integrity. Increased permeability monitored by changes in transendothelial electrical resistance and cytoskeletal remodelling caused by actin fiber rearrangement were observed when the cells were exposed to lupus serum and C5a, similar to the observations in mice. In addition, our data show that C5a/C5aR1 signalling alters nuclear factor‐<jats:italic>κ</jats:italic>B translocation into nucleus and regulates the expression of the tight junction proteins, claudin‐5 and zonula occludens 1 in this setting. Our results demonstrate for the first time that C5a regulates<jats:styled-content style="fixed-case">BBB</jats:styled-content>integrity in a neuroinflammatory setting where it affects both endothelial and astroglial cells. In addition, we also demonstrate that our previous findings in a mouse model, were emulated in human cells<jats:italic>in vitro</jats:italic>, bringing the studies one step closer to understanding the translational potential of C5a/C5aR1 blockade as a promising therapeutic strategy in<jats:styled-content style="fixed-case">SLE</jats:styled-content>and other neurodegenerative diseases.</jats:p>