Serum cystatin C and anti‐N‐methyl‐D‐aspartate receptor encephalitis

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Cystatin C (CysC) is a non‐glycosylated 13 kDa basic protein, consisting of 120 amino acids. It is a secreted cysteine inhibitor encoded by the CST3 (Cst3) gene, which is located on the short arm of chromosome 20.1 CysC belongs to the cystatin superfamily of cysteine proteinase inhibitors that modulate protein degradation and cell‐matrix interactions.2 CysC is produced by nearly all cells and is secreted into the extracellular space, blood and cerebrospinal fluid (CSF). It is commonly used as a biomarker of renal function3 and is a risk factor and strong predictor of cardiovascular disease.4 Moreover, CysC is thought to play a major role in modulating immune cell activation and inflammation‐driven cell death and counteracts the action of cathepsins, which are a family of lysosomal proteins released by activated microglia/macrophages.6 In addition, recent studies have shown that CysC can protect neuronal cells from cell death through activating the autophagy pathway.7
Therefore, CysC may exert multiple functions in the pathogenesis and progression of immunological and neurological diseases. Indeed, a significant association of CysC with autoimmune diseases, such as systemic lupus erythematosus (SLE)9 and chronic arthritis,10 has been reported. Furthermore, CysC is associated with a variety of neurological diseases, including inflammatory neurological diseases, such as Guillain‐Barre syndrome (GBS), chronic inflammatory demyelinating polyneuropathy (CIDP) and multiple sclerosis (MS)11 and neurodegenerative disorders, such as Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD).7
Anti‐N‐methyl‐D‐aspartate receptor (NMDAR) encephalitis is a rapid‐onset, immune‐mediated disorder that is often associated with psychosis, seizures, various movement disorders and autonomic system disturbances.17 It develops through the action of immunoglobulin G (NMDAR antibodies) against the GluN1 subunit of the NMDAR.18 Anti‐NMDAR encephalitis is a severe and rare disorder that can affect patients of all ages with or without tumours in various regions, such as ovarian teratomas19 Multiple studies have shown that immune cells, including B cells and T cells, are involved in the pathogenesis of anti‐NMDAR encephalitis.21
However, the importance of CysC in anti‐NMDAR encephalitis is unknown. Here, we analysed serum CysC levels in anti‐NMDAR encephalitis patients and investigated the associations between the serum CysC levels and clinical parameters in these patients.

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