Activated αβ T and reduced mucosa-associated invariant T cells in LGI1- and CASPR2-encephalitis.

Anti-leucine-rich glioma inactivated-1 (LGI1) and anti-contactin-associated-protein-2 (CASPR2) autoimmune encephalitis (AIE) are common and characterized by pathogenic antibodies targeting neuronal autoantigens. However, the drivers of the antibody-secreting cells and involvement of T cells remain unresolved. We performed single-cell RNA sequencing of fresh CSF and parallel blood samples of 15 patients with LGI1-AIE (n = 9) and CASPR2-AIE (n = 6) compared with control patients [multiple sclerosis (n = 15) and idiopathic intracranial hypertension (n = 18)]. We validated our observations in independent cohorts using flow cytometry of CSF and blood. We confirmed autoantibody specificity using recombinant human monoclonal antibodies. In comparison to idiopathic intracranial hypertension and multiple sclerosis controls, we observed clonal CSF-specific antibody-secreting cell expansion in LGI1/CASPR2-AIE despite mostly normal CSF findings. Antibody-secreting cells were dominantly plasmablasts and transcribed IgG4 and IgG1/2 heavy chains. Expanded clones showed signs of affinity maturation and bound the respective neuronal autoantigen. Within CD4 and CD8 T-cell clusters, CD4 and CD8 central memory T cells were activated, clonally restricted and expanded. T-cell clones were often shared between CSF and blood. We also observed a shift of natural killer cells and loss of mucosa-associated invariant T (MAIT) cells in the CSF of LGI1-AIE and the blood of LGI1- and CASPR2-AIE compared with idiopathic intracranial hypertension and multiple sclerosis controls. MAIT-like T cells were detected in autopsied brains of LGI1- and CASPR2-AIE patients, and mice lacking MAIT cells displayed an increased antibody seroconversion and higher titres following active LGI1/CASPR2 immunization. Our data: (i) confirm the intrathecal antigen-specific plasma cell expansion in LGI1- and CASPR2-AIE in a large cohort of untreated AIE patients; (ii) suggest that activated and expanded central memory CD4 and CD8 T cells in the CSF participate in disease pathogenesis; and (iii) implicate invariant T-cell receptor-expressing lymphocytes in the brain, CSF and blood in disease pathogenesis.