1) To characterize terminal complement component split products in the cerebrospinal fluid (CSF) and plasma from amyotrophic lateral sclerosis (ALS) patients compared with healthy controls, 2) To investigate the relationship between systemic and CSF activation of complement, and 3) To correlate the presence of complement activation with markers of neurodegeneration and neuroinflammation/remodeling.

ALS is a progressive, fatal, neurodegenerative disease characterized by degeneration of motor neurons leading to progressive loss of voluntary muscle action. Hyperactivation of the complement system has been shown to be a characteristic feature of several neurodegenerative diseases.

We obtained commercially sourced CSF and plasma samples from both ALS patients and age matched healthy controls. We used both multiplexed and singleplex ELISAs to monitor analytes in CSF and plasma.

We demonstrate a significant elevation of terminal complement pathway activation, C5a and soluble C5b-9 (sC5b-9) in CSF and sC5b-9 in plasma of ALS patients. Our data suggest that distinct pathways lead to the activation of complement in CSF (alternative pathway) versus plasma (classical or lectin pathways). Further, we note that C5a and sC5b-9 levels produced in CSF did not correlate with those observed in plasma. Taken together with the albumin-quotient (CSF/plasma), a marker of blood-brain barrier permeability, these data support the possibility that local complement activation in the CNS is independent from that observed in the periphery. Finally, we detect elevation of markers of axonal neurodegeneration (NFL and pNFH) and neuroinflammation (CHIT1) in CSF from ALS patients.

These studies deepen our understanding of the activation of the terminal complement pathway in ALS patients, enabling the leverage of these analytes as potential pharmacodynamic and patient selection or stratification biomarkers, and supporting the potential benefit of terminal complement inhibition in ALS patients.