AbstractObjectiveTo report clinical and immunological investigations of contactin‐associated protein‐like 2 (Caspr2), an autoantigen of encephalitis and peripheral nerve hyperexcitability (PNH) previously attributed to voltage‐gated potassium channels (VGKC).MethodsClinical analysis was performed on patients with encephalitis, PNH, or both. Immunoprecipitation and mass spectrometry were used to identify the antigen and to develop an assay with Caspr2‐expressing cells. Immunoabsorption with Caspr2 and comparative immunostaining of brain and peripheral nerve of wild‐type and Caspr2‐null mice were used to assess antibody specificity.ResultsUsing Caspr2‐expressing cells, antibodies were identified in 8 patients but not in 140 patients with several types of autoimmune or viral encephalitis, PNH, or mutations of the Caspr2‐encoding gene. Patients' antibodies reacted with brain and peripheral nerve in a pattern that colocalized with Caspr2. This reactivity was abrogated after immunoabsorption with Caspr2 and was absent in tissues from Caspr2‐null mice. Of the 8 patients with Caspr2 antibodies, 7 had encephalopathy or seizures, 5 neuropathy or PNH, and 1 isolated PNH. Three patients also had myasthenia gravis, bulbar weakness, or symptoms that initially suggested motor neuron disease. None of the patients had active cancer; 7 responded to immunotherapy and were healthy or only mildly disabled at last follow‐up (median, 8 months; range, 6–84 months).InterpretationCaspr2 is an autoantigen of encephalitis and PNH previously attributed to VGKC antibodies. The occurrence of other autoantibodies may result in a complex syndrome that at presentation could be mistaken for a motor neuron disorder. Recognition of this disorder is important, because it responds to immunotherapy. Ann Neurol 2011