Do neural autoantibodies always prove brain disease?

Neural autoantibodies have heterogeneous clinical meaning: some directly disrupt neuronal or glial function, whereas others chiefly identify an underlying immune process. Their interpretation requires matching the antibody, specimen, assay, and clinical syndrome.

Functional and injury-related evidence

• Antibodies to accessible neuronal-surface targets can directly alter signaling. Anti-NMDAR antibodies cross-link GluN1-containing receptors, drive receptor internalization, reduce synaptic receptor abundance, and produce behavioral abnormalities in transfer models. Anti-LGI1 antibodies disrupt LGI1–ADAM22/23 interactions and reduce AMPAR- and Kv1.1-related signaling; anti-GABA(_B) receptor antibodies can inhibit receptor function.
• Neural antibodies may also serve principally as markers of immune-mediated injury. Intracellular/onconeural antibodies generally indicate an associated cytotoxic T-cell response rather than direct antibody-mediated injury to intact neurons.

Mechanisms of inflammatory CNS injury

• MOG-IgG has experimental support for complement- and Fc-receptor-mediated demyelination, with impaired neural conduction arising secondarily; passive-transfer effects often require CNS access or inflammatory priming.
• AQP4-IgG targets astrocytic AQP4 and causes predominantly complement-mediated astrocyte injury, with Fc-mediated cellular injury and inflammation also implicated. Demyelination is generally downstream of astrocyte damage.

Interpreting serum results

• Serum positivity alone is not proof of pathogenic antibody activity in the brain. Its relevance depends on phenotype concordance, target, titer, assay, pretest probability, and the tested compartment.
• In real-world serum cell-based testing, positive predictive values were 69.7% for NMDAR, 91.8% for LGI1, and 78.8% for GABA(_B) receptor antibodies; tissue-based confirmation raised these to 97.1%, 96.4%, and 100%. For NMDAR antibodies, CSF sensitivity was 99.1% versus 86.8% in serum, and CSF detection is required for anti-NMDAR encephalitis diagnosis.

Bottom line

• Neural-directed antibodies can be functionally pathogenic or informative markers of immune injury, but a serum result is a diagnostic clue—not standalone evidence that the antibody is causing brain disease.