Coexistence of CN1A autoantibodies in GAD65 encephalitis exacerbates neurodegeneration : Novel autoantibodies in GAD65 encephalitis

Abstract

Background: Autoantibodies targeting the intracellular 65-kDa isoform of glutamic acid decarboxylase (anti-GAD65) have been associated with a variety of autoimmune-related syndromes involving a spectrum of difficult-to-treat neurological disorders. However, the pathophysiological role of anti-GAD65 in neuroinflammation remains vague. Its understanding may be complicated by the possible pathogenic interaction between anti-GAD65 and potentially coexisting autoantibodies.

Methods: We combined a broad spectrum of approaches ranging from antibody-antigen identification, immunoblotting, immunoprecipitation, mass-spectrometry, cell-based assays, subcellular binding pattern analysis in primary neuronal cultures, and immunohistochemistry to in vitro assays of neuronal uptake, viability, and multi-electrode arrays.

Results: In anti-GAD65-positive neurological patients, mass-spectrometric analysis revealed cytosolic 5'-nucleotidase 1 A (CN1A syn. NT5C1A) as the most abundant antigen. Subsequent screening of 118 anti-GAD65-positive patients revealed that 32 of them had additional autoantibodies targeting CN1A, which were also present in all available corresponding CSF samples. Limbic encephalitis was more often diagnosed in anti-CN1A/anti-GAD65-positive compared to the anti-GAD65-positive patients. Functionally, incubation of primary hippocampal neurons with anti-GAD65, but not with anti-CN1A, resulted in uptake into GABAergic neurons, neuronal cell death, and increased neuronal network activity. Moreover, simultaneous incubation with both antibodies (anti-CN1A/anti-GAD65) resulted in concomitant intraneuronal uptake in a concentration-dependent manner, which correlated with enhanced autophagy followed by massive neuronal death.

Conclusion: GAD65 antibodies directly affect neuronal viability and network activity. Co-existing autoantibodies against CN1A, present in anti-GAD65-positive patients, enhance autophagy and subsequent neuronal death in vitro. Clinically, anti-GAD65-positive patients should be screened for anti-CN1A-associated diseases, and evaluation of anti-CN1A in anti-GAD65-related autoimmune conditions may clarify links between systemic autoimmunity and epilepsy.

Keywords: Autoimmune encephalitis; Cytosolic 5'-nucleotidase 1A (CN1A); Glutamate decarboxylase 65 (GAD65); Limbic encephalitis; Temporal lobe epilepsy.