Conformational antibodies to proteolipid protein-1 and its peripheral isoform DM20 define clinical subgroups within CNS autoimmune demyelinating diseases

In recent years, the identification of MOG and AQP4 conformational antibodies has completely revolutionized the diagnostic approach to autoimmune demyelinating diseases (ADD). These autoantibodies assist in recognizing patients with specific conditions, termed NMOSD for AQP4-IgG and MOGAD for MOG-IgG, which necessitate management distinct from that of multiple sclerosis (MS). In this context, several neuroglial targets have been investigated as potential biomarkers for ADD, with proteolipid protein 1 (PLP1) emerging as a promising candidate antigen. PLP1 is the main protein of the central nervous system myelin and is also found in the peripheral nervous system, particularly in its isoforms DM20. PLP1 has been shown to possess encephalitogenic properties in various animal models of multiple sclerosis. To date, PLP1 autoantibodies (PLP1-IgG) have been identified in MS patients, though their clinical implications remain unclear. However, most studies have relied on non-conformational immunoassays, which do not allow the detection of conformational antibodies that necessitate assays that preserve the native (tertiary) structure of the protein for their identification. In this thesis, we aim to explore the clinical significance of PLP1 conformational antibodies across the entire spectrum of autoimmune demyelinating diseases (ADD). To achieve this, we established a live cell-based assay (CBA) expressing human full-length PLP1 to investigate the presence of PLP1-IgG initially within a well-characterized retrospective cohort of patients, which included ADD patients (n=284) and controls (n=177), and subsequently in a prospective cohort of consecutive ADD patients (n=824). Overall, PLP1-IgG was identified exclusively in ADD patients (42/1104) and not in controls (0/177). PLP1-IgG positive patients mainly diagnosed as other-ADD (19/42) with frequent myelitis/encephalomyelitis (14/19), and co-existing PNS involvement (13/19). PLP1-IgG were also found in MOGAD (11/42), more frequently with PNS involvement (p=0.01), and in MS (12/42), more frequently with atypical features (p<0.001). PLP1-IgG co-reacted with DM20 in all 12 patients with PNS involvement tested compared to 17/24 (70.8%) of those with only CNS ADD. PLP1-IgG stained the myelin fibers in tissue-based assays (TBA) in 63.1% of samples and co-localized with its target in CBA-TBA, where binding was eliminated following immunoadsorption, thereby confirming the specificity of the target antigen. Furthermore, both CBA and TBA, after strong fixation, failed to detect PLP1-IgG, indicating that these autoantibodies were conformationally dependent. All patients with other-ADD accumulated disability after the first attack, with one third unable to walk at follow-up, and one fourth relapsing. Similarly, PLP1-IgG-positive MOGAD had higher EDSS score at disease peak (p<0.001) and at the end of follow-up(p=0.040). PLP1-IgG-positive MS higher severity scores (MSSS, p<0.001) compared to those PLP1-IgG-negative. This may be due to a direct pathogenic effect of PLP1-IgG, as indicated by the predominance of the IgG1 and IgG3 subclasses and their capacity to induce complement-dependent cytotoxicity (CDC) in vitro. In conclusion, we demonstrate that conformational PLP1-IgG predominantly identifies non-MS ADD patients. PLP1-IgG should primarily be tested in individuals with CNS+PNS ADD, in accordance with DM20-IgG co-reactivity, as well as in seronegative neuromyelitis optica spectrum disorder (NMOSD). Additionally, PLP1-IgG may be investigated as potential disease modifiers and prognostic markers in multiple sclerosis (MS) and MOG antibody disease (MOGAD). Preliminary evidence supports their pathogenic potential through complement activation, thereby opening new therapeutic avenues for PLP1-IgG positive patients.