Naturally Occurring Alpha-Synuclein Autoantibodies in Parkinson’s Disease: Sources of (Error) Variance in Biomarker Assays
Alpha-synuclein ($α$-Syn) plays a pivotal role in the pathophysiology of Parkinson's disease (PD), which can partly be modulated by innate and adaptive immune functions, and vice versa. Here, naturally occurring $α$-Syn autoantibodies ($α$-Syn-nAbs) may be effective against $α$-Syn pathoetiology and may serve as a PD biomarker. However, serum and cerebrospinal fluid $α$-Syn-nAbs levels still lack consistent evidence as required for a reliable PD biomarker. Serum and cerebrospinal fluid $α$-Syn-nAbs levels of 66 PD patients and 69 healthy controls were assessed using a validated ELISA assay. Moreover, potential sources of error variance including unspecific ELISA background signals, free serum hemoglobin concentrations, $α$-Syn plate coating procedures, and differences in $α$-Syn-nAbs standards, were investigated. PD patients and controls did not differ in serum (p = .49) nor cerebrospinal fluid (p = .29) $α$-Syn-nAbs levels. Interestingly, free serum hemoglobin concentrations were negatively correlated with $α$-Syn-nAbs levels in controls (Spearman $ρ$ = -.41, p<.001), but not in PD patients ($ρ$ = .16, p = .21). ELISA $α$-Syn plate coating procedures impacted inter-assay variability (same day coating: 8-16%; coating on different days: 16-58%). $α$-Syn-nAbs standards from different purification batches differed regarding optical density measured in ELISAs suggesting differences in $α$-Syn affinity. While $α$-Syn-nAbs levels may represent a potential PD biomarker, several methodological issues have to be considered to increase reproducibility of $α$-Syn-nAbs findings. Further studies using standardized protocols minimizing sources of error variance may be necessary to establish a reliable PD $α$-Syn-nAbs biomarker.