Multiple sclerosis plasma IgG aggregates induce complement-dependent neuronal apoptosis

Grey matter pathology is central to the progression of multiple sclerosis (MS). We discovered that MS plasma immunoglobulin G (IgG) antibodies, mainly IgG1, form large aggregates (>100 nm) which are retained in the flow-through after binding to Protein A. Utilizing an annexin V live-cell apoptosis detection assay, we demonstrated six times higher levels of neuronal apoptosis induced by MS plasma IgG aggregates (n = 190, from two cohorts) compared to other neurological disorders (n = 116) and healthy donors (n = 44). MS IgG aggregate-mediated, complement-dependent neuronal apoptosis was evaluated in multiple model systems including primary human neurons, primary human astrocytes, neuroblastoma SH-SY5Y cells, and newborn mouse brain slices. Immunocytochemistry revealed the co-deposition of IgG, early and late complement activation products (C1q, C3b, and membrane attack complex C5b9), as well as active caspase 3 in treated neuronal cells. Furthermore, we found that MS plasma cytotoxic antibodies are not present in Protein G flow-through, nor in the paired plasma. The neuronal apoptosis can be inhibited by IgG depletion, disruption of IgG aggregates, pan-caspase inhibitor, and is completely abolished by digestion with IgG-cleaving enzyme IdeS. Transmission electron microscopy and nanoparticle tracking analysis revealed the sizes of MS IgG aggregates are greater than 100 nm. Our data support the pathological role of MS IgG antibodies and corroborate their connection to complement activation and axonal damage, suggesting that apoptosis may be a mechanism of neurodegeneration in MS.

Automated summary

Grey matter pathology is crucial in the development of multiple sclerosis (MS). MS plasma immunoglobulin G (IgG) antibodies, particularly IgG1, form large aggregates (>100 nm) that are retained after binding to Protein A. These IgG aggregates induce significantly higher levels of neuronal apoptosis compared to other neurological disorders and healthy donors. The apoptosis is mediated by complement activation and the presence of IgG, complement activation products, and active caspase 3 in treated neuronal cells. MS plasma cytotoxic antibodies are not present in Protein G flow-through or paired plasma. The size of MS IgG aggregates is larger than 100 nm. Our findings suggest that MS IgG antibodies play a pathological role in complement activation, axonal damage, and neurodegeneration through apoptosis.