Natural killer cell reduction and dysfunction define a pathogenic and diagnostic axis in neuromyelitis optica spectrum disorder.

Neuromyelitis optica spectrum disorder (NMOSD) is a severe relapsing autoimmune disease of the central nervous system, where early diagnosis and monitoring are essential to prevent long-term disability. The cell-based assay recommended for aquaporin-4 immunoglobulin G (AQP4-IgG) serology offers well-established diagnostic accuracy, however, it is costly, time-consuming and not universally accessible in some hospitals. Here, we identify peripheral natural killer (NK) cell reduction as a readily accessible and quantifiable complementary diagnostic biomarker for NMOSD. Across two independent cohorts, NK cell was significantly reduced in NMOSD patients compared to healthy controls and other central nervous system (CNS) demyelinating disorders including multiple sclerosis (MS), myelin oligodendrocyte glycoprotein antibody-associated disease (MOGAD) and autoimmune encephalitis (AE). Using single-cell RNA sequencing, we demonstrate that NK cells exhibit distinct transcriptional dysfunctions involving cytokine production, immune regulation, and cellular migration. Further, cell-cell interaction analysis revealed impaired interferon-gamma (IFN-γ) signaling between NK and B cells, likely contributing to pathological B cell activation. Longitudinal analyses showed persistent NK cell reduction during active and post-treatment phases, with partial recovery in remission, supporting their use in disease monitoring. Finally, weighted gene co-expression network analysis identified NK gene modules strongly associated with disease severity and enriched in proinflammatory and B cell-activating pathways. Together, our findings position NK cell reduction and dysfunction as central features of NMOSD immunopathology and establish NK cell proportion and gene activity as potential biomarkers for diagnosis, differential diagnosis, and disease progression.