Pathological Mechanisms of Immune-Mediated Cerebellar Ataxia with Associated Sez6L2 Autoantibodies

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Project Summary/Abstract Sez6L2 autoantibodies have been reported in patients with subacute cerebellar ataxia. Patients present with gait ataxia, slurred speech, and ocular motor symptoms. Some patients also develop limb ataxia, cognitive deficits, bradykinesia, and/or Parkinsonism. These case studies have led to the hypothesis that autoimmunity against Sez6L2 can directly cause cerebellar damage leading to ataxia. Sez6L2 is a transmembrane protein expressed by most neurons in the brain including Purkinje cells and granule cells of the cerebellum. Currently, no animal studies have been performed to help understand if Sez6L2 autoantibodies are directly pathologic, whether they are a biomarker of cell-based autoimmunity to Sez6L2, or whether are simply superfluous to the cerebellar pathology. We plan to test two mouse models of Sez6L2 autoimmunity in order to understand the mechanisms underlying cerebellar ataxia with associated Sez6L2 autoantibodies. Aim 1 of this project will determine whether mice immunized with Sez6L2 protein develop cerebellar ataxia. We will also seek to understand the disease mechanisms using behavioral assessments, immunohistochemistry, and flow cytometry analysis of infiltrating immune cell populations. Aim 2 will determine whether Sez6L2 antibodies alone can cause cerebellar ataxia in mice, or block Sez6L2’s complement inhibitory functions in vitro. We anticipate that these studies will provide strong mechanistic evidence that the antibody and/or full immune response to Sez6L2 is a pathological cause of ataxia and related symptoms. These studies, coupled with the presently reported human case studies, should encourage prompt and routine screening for Sez6L2 autoantibodies in suspected immune-mediated presentations of cerebellar ataxia. The results on disease mechanisms could also help guide clinicians to the immunotherapies that may be most effective for patients and help justify the risks of prolonged immunosuppression.