This study aims to develop Fc-fusion proteins (“Baitbodies”) to neutralize pathogenic anti-LGI1 autoantibodies as a treatment approach for autoimmune encephalitis.

LGI1 is the second-most-common neuronal target involved in autoimmune encephalitis. Anti-LGI1 autoantibodies cause increases in neuronal excitability, manifesting as seizures and cognitive dysfunction. Since the effects of immunosuppressive therapies (e.g., Rituximab) can be delayed due to pathogenic antibodies remaining in circulation, strategies for rapid, specific neutralization of these autoantibodies are needed to improve treatment timelines.

Soluble, bivalent constructs were engineered by fusing the LRR domain of LGI1 to the fragment crystallizable (Fc) domain of immunoglobulin G. The LRR-Fc-fusion constructs were validated in ELISA with anti-LRR antibodies derived from anti-LGI1 encephalitis patients. Neutralization capabilities of the LRR-Fc-fusion constructs were evaluated in immunofluorescence and electrophysiological assays with cultured rat hippocampal neurons.

While patient-derived anti-LRR antibodies bound the native LRR-Fc-fusion in ELISA, the effects of the same antibodies were not neutralized by the construct in neuron-based immunofluorescence inhibition tests. The immunofluorescence assays also revealed binding of the LRR-Fc-fusion to the neurons, both independent of and colocalized with anti-LRR autoantibodies. As a strategy to prevent this interaction, the amino acids of the LRR domain involved in LGI1-LGI1 dimerization were mutated, resulting in new LRR-Fc-fusion variants. Although most amino acid substitutions decreased autoantibody binding, two LRR-Fc-fusion variants showed increased binding signals in ELISA for different patient-derived autoantibodies. These two variants showed neutralization capabilities in neuron-based immunofluorescence assays but were still localized at the neurons. However, this neuronal binding showed no pathological effects in electrophysiological analysis. The two engineered LRR-Fc-fusion variants effectively neutralized the increased neuronal excitability caused by patient-derived antibodies.

The engineered LRR-Fc constructs showed increased autoantibody binding. The effective neutralization of anti-LRR autoantibodies pathology showcases the potential of designed Fc-fusions as a treatment strategy for autoimmune disorders.