Cutting edge: Granzyme B proteolysis of a neuronal glutamate receptor generates an autoantigen and is modulated by glycosylation

Autoimmune processes are initiated when tolerance to self-proteins fails to be established or maintained and immune cells are stimulated by self-ags. Although intracellular autoantigens are common, the origin of extracellular autoantigens is poorly defined and possibly more dangerous. In this study, we considered a mechanism for the origin of an extracellular autoantigen from the neuronal glutamate receptor subunit 3 (GluR3) in Rasmussen's encephalitis, a severe form of pediatric epilepsy. We demonstrate that specific cleavage of GluR3 by granzyme B (GB), a serine protease released by activated immune cells, can generate the GluR3B autoantigenic peptide, but only if an internal N-linked glycosylation sequon within the GluR3-GB recognition sequence (ISND*S) is not glycosylated. However, this N-glycon sequon while glycosylated normally is inefficiently used and glycosylation can fail. These results suggest that GB/N-glycon sites may escape normal tolerance mechanisms and contribute to autoantibody-mediated immune diseases.