Immune reactivity to neurofilaments and dipeptide repeats in ALS progression

ObjectiveTo test antibody response and formation of immune-complexes to neurofilaments and dipeptide-repeats, the translational products of the mutated C9orf72 gene, as potential biomarkers for clinical stratification of amyotrophic lateral sclerosis (ALS). MethodsUsing neurofilament protein isoforms plasma expression as reference, antibodies and immune-complexes against neurofilament-light, medium and heavy chain and poly-(GP)-GR dipeptide-repeats were tested in blood from 105 fast and slow progressing ALS patients, 26 C9orf72 mutation carriers (C9+ve) ALS patients and 77 healthy controls (HC) using single-molecule and immune-capture assays. Longitudinal antibody/immune-complex responses were measured in serial blood samples from 37 (including 11 C9+ve) patients. ResultsAntibodies and immune-complex reactivity was higher in ALS patients than HC, particularly in C9+ve ALS patients, and modestly correlated with total neurofilament protein isoforms (r:0.24 p=0.002; r:0.18 p=0.02 respectively). Neurofilament-light immune-complexes and neurofilament-heavy antibodies had the best diagnostic performances distinguishing ALS subtypes from HC (AUC=0.68 p<0.01; AUC=0.68 p<0.001 respectively). Neurofilament-light immune-complexes (AUC=0.69 p<0.01) and poly-(GP) dipeptide-repeats antibodies (AUC=0.71 p<0.001) separated C9+ve from C9-ve patients. Multivariate mortality hazard ratio and Kaplan-Meier analyses showed low neurofilament-heavy antibody levels associated with increased survival. Longitudinal analysis identified raising levels of antibodies against neurofilaments in fast progressing ALS and of neurofilament-light immune-complexes in C9+ve patients. InterpretationC9+ve and fast progressing ALS patients have a distinct neurofilament and dipeptide-repeat immuno-phenotype, with increasing levels of blood neurofilament-light immune-complexes and neurofilament antibodies with disease progression. The study of the expression of these biomarkers in the natural history of ALS may shed light on disease initiation and progression and provide novel pharmacodynamic biomarkers in emerging C9orf72 gene silencing therapies.