Incomplete T cell tolerance and epitope-specific responses to the NR1 subunit in NMDA receptor antibody-associated encephalitis

Abstract

NMDA receptor antibody associated (NMDAR-Ab) encephalitis primarily affects young women and children, and often requires prolonged intensive care admission. Currently available first and second-line therapies are effective, but patients can be refractory and residual deficits are not uncommon. Although B cell lineages and specific NMDAR (NR1 subunit)-antibodies have been examined in detail, the subtypes and roles of antigen-specific T cells providing T-help to drive the IgG antibody response, cytotoxic effects, or other roles, have not been reported. Peripheral blood mononuclear cells from eight patients and eight healthy volunteers underwent three rounds of stimulation with pooled 15mer peptides spanning the length of the NR1 sequence. At each round of stimulation, functional CD4+ and CD8+ T cell responses to NR1-derived peptide pools were assessed using flow cytometry. CD4+ and CD8+ T responses to NR1-derived peptide pools were identified in both patients and healthy volunteers. Patient CD4+ T cell responses were more frequent compared to healthy volunteers after the first round of stimulation, with fewer responses after the third round. Healthy volunteer CD4+ T cells showed the opposite trend (P=0.004, Chi square test for trend). Patient CD8+ T cell responses were also stronger than healthy volunteers after just one round of stimulation, but not thereafter. Surprisingly, both patient and healthy volunteer CD8+ T cells responded more strongly to NR1-peptide pools than CD4+ T cells. Epitope mapping after round 3 identified two NR1 peptide epitopes as targets for CD4+ T cells and eight peptide epitopes as targets for CD8+ T cells. These epitopes were distributed throughout the NR1 sequence and were distant from the well-established antibody epitope. Although the kinetics of responses to NR1 peptides in patients may be compatible with in vivo priming of autoreactive T cells or their reduced regulation, overall, the difference between patient and healthy volunteer responses was modest. Further investigation is needed to better characterise these responses and determine their implications for disease pathogenesis, prognosis and therapy. The existence of natural responses to NR1 as a potential autoantigen, particularly in the CD8+ T cell lineage, should also be explored.