We recently described a severe, potentially lethal, but treatment-responsive
encephalitis that associates with
autoantibodies to the
NMDA receptor (NMDAR) and results in behavioral symptoms similar to those obtained with models of genetic or pharmacologic attenuation of NMDAR function. Here, we demonstrate that patients' NMDAR
antibodies cause a selective and reversible decrease in NMDAR surface density and synaptic localization that correlates with patients' antibody titers. The mechanism of this decrease is selective antibody-mediated capping and internalization of surface NMDARs, as
Fab fragments prepared from patients'
antibodies did not decrease surface receptor density, but subsequent cross-linking with anti-Fab
antibodies recapitulated the decrease caused by intact patient NMDAR
antibodies. Moreover, whole-cell patch-clamp recordings of miniature EPSCs in cultured rat hippocampal neurons showed that patients'
antibodies specifically decreased synaptic NMDAR-mediated currents, without affecting
AMPA receptor-mediated currents. In contrast to these profound effects on NMDARs, patients'
antibodies did not alter the localization or expression of other
glutamate receptors or synaptic
proteins, number of synapses, dendritic spines, dendritic complexity, or cell survival. In addition, NMDAR density was dramatically reduced in the hippocampus of female Lewis rats infused with patients'
antibodies, similar to the decrease observed in the hippocampus of autopsied patients. These studies establish the cellular mechanisms through which
antibodies of patients with
anti-NMDAR encephalitis cause a specific, titer-dependent, and reversible loss of NMDARs. The loss of this subtype of
glutamate receptors eliminates NMDAR-mediated synaptic function, resulting in the learning, memory, and other behavioral deficits observed in patients with
anti-NMDAR encephalitis.