NINJ1 Defines a Hyperinflammatory Monocyte State Associated with Severe COVID-19

Background/Objectives: Severe coronavirus disease 2019 (COVID-19) is characterized by dysregulated immune activation and excessive inflammation; however, the specific immune cell states that drive disease progression remain incompletely understood. Methods: Here, we integrated single-cell RNA sequencing analysis of peripheral blood mononuclear cells with clinical validation to investigate the role of Ninjurin-1 (NINJ1) in COVID-19-associated immune dysregulation. Results: Single-cell profiling revealed that NINJ1 expression was largely confined to monocytes and defined as a distinct NINJ1⁺ inflammatory monocyte state in patients with COVID-19. Compared with NINJ1^- monocytes, NINJ1⁺ monocytes exhibited enhanced inflammatory transcriptional programs, altered regulon activity, and increased cell-cell communication capacity, suggesting their potential role as a highly activated immune state involved in inflammatory amplification. Further ligand-receptor interaction analysis demonstrated that NINJ1⁺ monocytes functioned as key communication hubs within the COVID-19 immune landscape, coordinating extensive interactions with other immune populations. In an independent cohort of 99 COVID-19 patients and 51 healthy controls, serum NINJ1 levels were significantly elevated in infected individuals and were further increased in non-survivors. Circulating NINJ1 levels correlated with inflammatory and immune dysregulation indicators, including C-reactive protein, procalcitonin, prothrombin time, neutrophil-to-lymphocyte ratio, and lymphocyte depletion. Receiver operating characteristic analysis demonstrated that serum NINJ1 exhibited significant predictive value for in-hospital mortality. Conclusions: Collectively, our findings identify NINJ1⁺ inflammatory monocytes as an activated immune state associated with the transition from protective innate defense to pathological inflammation, providing new insights into COVID-19 immunopathogenesis and highlighting NINJ1 as a promising biomarker candidate for risk stratification and therapeutic exploration.