Introduction
SLE is a chronic, multisystem autoimmune connective tissue disorder of unknown aetiology.1 2 Nervous system involvement presenting as neurologic, psychiatric and cognitive disorders (CDs) occurs in approximately 50% of patients.3 This spectrum of disorders is referred to as neuropsychiatric lupus (NPSLE) and includes 19 standardised central or peripheral nervous system conditions.3 4 CD is an NPSLE syndrome stemming from diffuse central nervous system (CNS) pathology and has a variable prevalence of 15%–79% due to multiple challenges associated with obtaining an accurate diagnosis.1 5–8 Diffuse, central NPSLE manifestations are more common than focal manifestations and have a significant effect on the quality of life.9 10 Interestingly, CD can also occur despite the quiescence of other SLE manifestations.11
The pathophysiology of NPSLE, including CD, remains poorly understood although there are several proposed theories. Prior research has shown that inflammatory molecules12 gain access to the CNS via a perturbed blood–brain barrier (BBB)13, brain–CSF barrier (choroid plexus), meningeal barrier and glymphatic system and result in direct stimulation of neurons and microglia.11 14 Microglia are antigen-presenting cells in the CNS that are also important for fine-tuning neuronal connections.3 Microglia are thought to play a role in the loss of neuronal dendrites and are activated by a variety of inflammatory molecules.5 Injection of SLE serum, and specifically SLE IgG, into mouse CSF has been shown to result in microglial activation and production of proinflammatory cytokines, suggesting that peripheral immune mediators play a role in inducing CNS inflammation via microglia.15–17 Microglia have been shown to remain activated for many months beyond an initial CNS insult, which is a speculative aetiology for the disconnection between SLE disease activity and CD.18
The renin–angiotensin system (RAS) is important in haemodynamic and mineralocorticoid homeostasis and also includes multiple neuroactive peptides that activate microglia and contribute to neuroinflammation in CD.5 Inactive angiotensin I is converted into active angiotensin II by ACE which is expressed throughout the body including in neurons.5 Angiotensin II receptor 1 blockers (ARBs) directly block angiotensin II at its site of action, while ACE inhibitors block the conversion of angiotensin I to angiotensin II. Some of these agents, such as captopril, lisinopril, ramipril and perindopril, cross the BBB and are termed ‘centrally acting’.19 ACE inhibitors and ARBs are cornerstones of the management of hypertensive, cardiovascular and proteinuric renal disorders. In patients with SLE, they are indicated for the treatment of hypertension and proteinuria in the case of lupus nephritis.20
Angiotensin II has been shown to cause microglial activation and direct neuronal injury/cell death at high levels.5 RAS suppression has also been shown to result in lower levels of bradykinin which suppresses microglial activation in mice.5 Furthermore, direct activation of microglia by renin via the prerenin receptor and stimulation of the production of proinflammatory cytokines has been demonstrated in rodent microglia.21 Pretreatment of microglia with angiotensin II resulted in enhanced proinflammatory cytokine secretion induced by renin.21 These findings suggest that ACE inhibitors may be a promising emerging therapy for CD partially via inhibitory effects on microglial activation. Supporting this theory, centrally acting ACE inhibitors (cACEi) have been shown to decrease microglial activation and improve cognitive deficits in mice.19 In a mouse model of N-methyl-D-aspartate receptor antibody (DNRAb)22 -mediated CD, treatment with captopril was shown to lead to less microglial activation compared with mice treated with enalapril, an ACE inhibitor that does not cross the BBB.5 Furthermore, mice treated with captopril had preserved neuronal dendrite complexity compared with enalapril-treated mice.5 The degree of complexity was similar to mice lacking DNRAb, and these mice had a normal number of dendritic spines suggesting a reversal of neuronal pathology.5 Cognitive function, as assessed by the object–place memory task, was preserved in mice treated with captopril and perindopril supporting an ACE-targeted class effect.5 These findings support the notion that blockade of the RAS, particularly with cACEi, may lead to decreased microglia activation. This may then lead to improved dendritic cell and synapse morphology and a lower incidence of CD.
Investigating cACEi/ARBs as a potential treatment for SLE-associated CD is an important next step in the study of SLE-CD. Clinical trials are now underway although results can be expected to take several years.5 In the interim, this study aimed to determine whether RAS suppression (use of cACEi and/or ARBs) was associated with lower odds of CD in a ‘real-world’, prospective cohort of patients with SLE.