To report 4 novel autoantibodies (resembling amphiphysin-IgG on murine brain tissue immunofluorescence assay [TIFA]) targeting antigens critical to neural synaptic vesicular trafficking.

Amphiphysin regulates synaptic vesicle endocytic recycling and is a target of paraneoplastic neurological autoimmunity (stiff-person syndrome or encephalomyeloneuropathies associated with breast adenocarcinoma and small cell carcinoma). Synaptic autoantibodies resembling amphiphysin-IgG are routinely encountered by TIFA in the Mayo Clinic Neuroimmunology Laboratory but have remained uncharacterized. 

October 2022-September 2023, 258 patient specimens (133 serums; 125 CSFs) meeting criteria for unclassified neural-restricted synaptic antibodies by TIFA were screened using protein microarray and phage-display immunoprecipitation sequencing (PhIP-Seq). Top-ranking candidate neural antigens were validated by dual-staining confocal microscopy, and ≥1 protein-specific assay (recombinant-protein western blot [all antigens] and cell-based assay [antigens #1, SNAP91; and #4, SYT3]). Clinical information was reviewed.

Four novel autoantibodies resembling amphiphysin-IgG by TIFA (but amphiphysin blot negative) that target other proteins critical for neuronal synaptic vesicle release or endocytosis were identified and validated in 9 patients: antigen #1 (SNAP91), 3 patients; antigen #2 (SNAP25), 3 patients; antigen #3 (SYNJ1), 1 patient; antigen #4 (SYT3), 2 patients. Five patients were female; median age, 56 years (range, 11-68). Clinical syndromes were: encephalitis: 2; brainstem encephalitis, 2; movement disorder, 2 (chorea, 1; PERM, 1); encephalomyelitis, myelitis, and myeloneuropathy, 1 each. Inflammatory CSF findings were present in all 3 tested (pleocytosis [1], elevated IgG index or synthesis rate [2], CSF-exclusive oligoclonal bands [1]). MRI revealed inflammatory-appearing T2 hyperintensities in 3/8 patients (cerebrum [2]; brainstem [1]; spinal cord [1, longitudinally extensive [MOG/AQP4 antibody negative]) and cerebellar atrophy, 1. One patient had known paraneoplastic cause (lung adenocarcinoma). Outcome data were available for 6 patients (clinical improvement/stability with immunotherapy [3]; clinical progression [3] including death [2], all untreated).

Antigens of the neural synaptic vesicular trafficking pathway, beyond amphiphysin, are targets in autoimmune neurological disease.