Human cerebrospinal fluid monoclonal N-methyl-D-aspartate receptor autoantibodies are sufficient for encephalitis pathogenesis

Antibodies against the NR1 subunit of the NMDA receptor are suspected to underlie anti-NMDA receptor encephalitis. Kreye et al. provide direct evidence by showing that monoclonal human NR1 antibodies are sufficient to downregulate synaptic NMDA receptors. They show too that patients harbour a much broader auto-antibody repertoire than previously thought.See Zekeridou and Lennon (doi:related-article ext-link-type=doi id=RA3 related-article-type=companion xlink:href=10.1093/brain/aww21310.1093/aww213related-article) for a scientific commentary on this article. Antibodies against the NR1 subunit of the NMDA receptor are suspected to underlie anti-NMDA receptor encephalitis. Kreye et al. provide direct evidence by showing that monoclonal human NR1 antibodies are sufficient to downregulate synaptic NMDA receptors. They show too that patients harbour a much broader auto-antibody repertoire than previously thought.Anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis is a recently discovered autoimmune syndrome associated with psychosis, dyskinesias, and seizures. Little is known about the cerebrospinal fluid autoantibody repertoire. Antibodies against the NR1 subunit of the NMDAR are thought to be pathogenic; however, direct proof is lacking as previous experiments could not distinguish the contribution of further anti-neuronal antibodies. Using single cell cloning of full-length immunoglobulin heavy and light chain genes, we generated a panel of recombinant monoclonal NR1 antibodies from cerebrospinal fluid memory B cells and antibody secreting cells of NMDAR encephalitis patients. Cells typically carried somatically mutated immunoglobulin genes and had undergone class-switching to immunoglobulin G, clonally expanded cells carried identical somatic hypermutation patterns. A fraction of NR1 antibodies were non-mutated, thus resembling 'naturally occurring antibodies' and indicating that tolerance induction against NMDAR was incomplete and somatic hypermutation not essential for functional antibodies. However, only a small percentage of cerebrospinal fluid-derived antibodies reacted against NR1. Instead, nearly all further antibodies bound specifically to diverse brain-expressed epitopes including neuronal surfaces, suggesting that a broad repertoire of antibody-secreting cells enrich in the central nervous system during encephalitis. Our functional data using primary hippocampal neurons indicate that human cerebrospinal fluid-derived monoclonal NR1 antibodies alone are sufficient to cause neuronal surface receptor downregulation and subsequent impairment of NMDAR-mediated currents, thus providing ultimate proof of antibody pathogenicity. The observed formation of immunological memory might be relevant for clinical relapses.