CSF reactivity in GABA_A receptor antibody encephalitis - Immunocytochemical distribution in the murine brain

The y-aminobutyric acid A receptor (GABA_AR) participates in most neurophysiological processes. Mutations cause epilepsy and neuropsychiatric pathologies. Recently a severe encephalitis with refractory seizures and antibodies against GABA_ARs has been described. Considering the complex subunit distribution of GABA_ARs, binding patterns of human GABA_AR antibodies will help to understand the pathophysiology underlying diverse clinical pictures. We therefore investigated the cerebrospinal fluid (CSF) reactivity of a patient with GABA_AR encephalitis using immunocytochemistry on murine brain sections and compared its specificity with commercial GABA_AR antibodies. The immunoreactivity of the patient's CSF showed excellent agreement with previously reported GABA_AR mRNA expression. Colocalization with neuronal and glial markers verified the neuronal specificity of GABA_AR. Patient antibodies strongly bound to neuropil in the external layers of the olfactory bulb, CA1 and CA2 of the hippocampus, neocortex, pallidum and granular cells of the cerebellum. Distribution patterns suggest the presence of polyclonal CSF GABA_AR antibodies targeting multiple receptor subunits. The comparison with commercial antibodies revealed large overlap, but also specific differences. For example, commercial antibodies accumulated on dendrites, while CSF created a homogeneous neuropil signal. The number of GABAergic synapses stained with CSF exceeded those labeled with the commercial antibodies. In some areas, commercial antibodies and CSF even stained complementary populations of GABAergic neurons. The data indicate the presence of additional anti-neuronal autoantibodies in the CSF, which could be assessed in future studies with individual recombinant monoclonal antibodies from CSF B cells. This strategy would confirm antibody pathogenicity and likely explain variable clinical pictures in autoimmune encephalitis patients.