Coverslips with age-matched hippocampal cells exposed to human control serum or anti-NMDAR-IgA serum of the index patient (dilution 1:200, 3 days) were transferred to a submerged recording chamber reperfused with artificial CSF (ACSF) (see e-Methods for details). correlated with the titer of IgA NMDAR antibodies and were reversed after removing patients’ serum from the culture media. When available, comprehensive clinical assessment and brain metabolic imaging showed neurologic improvement after immunotherapy. Conclusions: A subset of patients with slowly progressive cognitive impairment has an underlying synaptic autoimmunity that decreases the density of NMDAR and other synaptic proteins, and alters synaptic currents. This autoimmunity can be exhibited examining patients’ serum and CSF for NMDAR IgA antibodies, identifying possible candidates for immunotherapy. The forming of new memories, attention, and planning require proper neuronal activity in frontal and temporal regions that largely depends on normal fine-tuned ion channel activation and distribution in the brain. For example, dysregulation of one type of excitatory glutamatergic receptors, the synaptic NMDA receptor (NMDAR), can contribute to neuropsychiatric disorders such as dementia or schizophrenia.1C3 Recent studies show that in patients with anti-NMDAR CXXC9 encephalitis the immunoglobulin G (IgG) antibodies cause internalization of NMDAR resulting in a significant decrease of these receptors at synapses.4 These patients develop a characteristic clinical picture including psychiatric symptoms, decreased levels of consciousness, hypoventilation, epileptic seizures, and dyskinesias. After the acute stage of the disease patients are often left with chronic frontal and temporal lobe dysfunction (impulsivity, behavioral disinhibition, poor memory, attention, and planning) that may progressively improve until full or partial recovery.4C6 This and other recently identified autoimmune disorders of memory and cognition associated with antibodies against synaptic proteins (AMPA, GABA[B] receptors, mGluR5, LGI1, and Caspr2 proteins) strengthen the concept that autoimmunity can result in progressive cognitive dysfunction.7C10 Demonstration of novel antibodies to synaptic proteins can be used as a diagnostic tool to uncover autoimmune, potentially treatable, disorders that ITIC-4F otherwise would be considered primary degenerative diseases with limited therapeutic options.11 Here, we report a subset of patients with slowly progressive cognitive decline associated with a humoral immune response that alters the density of several synaptic proteins and synaptic currents. We also show that determination of immunoglobulin A (IgA)-NMDAR antibodies could be used as a serologic test to uncover this autoimmune mechanism. METHODS Patients. In 2009 2009, the identification of a patient with cognitive dysfunction and IgA-NMDAR antibodies who substantially improved with immunotherapy (see index patient) led to investigate these antibodies in serum and CSF of 23 additional patients with progressive cognitive decline of unclear etiology (15 women). Of these 23 patients, 6 were found to have IgA-NMDAR antibodies. This obtaining further extended our studies to examine the serum or CSF of 238 individuals, including 75 healthy ITIC-4F individuals and 163 patients with several disorders: 70 had neuropsychiatric diseases (15 schizophrenia, 20 bipolar disorder, 20 multiple sclerosis, 15 amyotrophic lateral sclerosis), 29 well-defined dementia (10 Alzheimer disease [AD], 10 Lewy body disease [LBD], 9 frontotemporal lobe degeneration [FTLD]), and 64 anti-NMDAR encephalitis (IgG antibodies to NMDAR-NR1 subunit). Clinical information was obtained by the authors or referring physicians. Standard protocol approvals, registrations, and patient consents. Studies were approved by the Charit University Hospital Institutional Review Board and written informed consent was obtained from patients or representatives. Detection of NMDAR antibodies. Testing for NMDAR antibodies was performed with frozen sections of rat hippocampus and cerebellum and recombinant immunofluorescence with HEK293 cells transfected with NR1 or NR1/NR2b subunits of the NMDAR4,12 (physique 1). Other antibodies were examined with HEK cells transfected with GAD65, LGI1, CASPR2, AMPAR, and GABA(B1) receptor. Classic paraneoplastic antibodies (i.e., anti-Hu, -Yo, -Ri, -Ma, -CV2, -amphiphysin) were determined by indirect immunofluorescence and line immunoblot (Euroimmun, Lbeck, Germany). NMDAR antibody index (NMDAR-AI) was calculated as the ratio between the CSF/serum quotient for NMDAR-IgA antibodies, ITIC-4F and the CSF/serum quotient for total IgA. Values >4 were considered as evidence of intrathecal NMDAR-specific IgA antibody synthesis.13,14 Open in a separate window Determine 1 Intense downregulation of NMDA receptors (NMDAR) by immunoglobulin A (IgA) NMDAR antibodies(A) Immunostaining of HEK cells transfected with the NR1 subunit of NMDAR and probed with patient serum and anti-IgA secondary antibodies. (B) No staining was observed using an antiCimmunoglobulin G (IgG) secondary antibody. (C) Nontransfected HEK293 cells served as unfavorable ITIC-4F control. Strong immunopositive staining of primate cerebellum (D) and rat hippocampus (E) with IgA-positive serum (n = 3), but not with control serum (F). (G) Primary mouse hippocampal neurons were incubated for 3 days with patient serum (1:100 dilution). Following removal of incubation medium cells were cultured for further 4 days with growth serum alone (rescue). A membrane fraction was obtained from harvested cells and processed for Western blotting. Staining against NR1 subunits revealed a.