Complement Activation Linked to Type II Interferon Signaling in Still Disease

Still disease (SD) is an autoinflammatory syndrome characterized by innate immune dysregulation. Although complement can drive inflammation, its involvement in SD remains to be defined. Thus, we aimed to assess complement activation in SD.

Complement was assessed using transcriptomic, proteomic, and in vitro approaches. RNA sequencing of monocytes was performed in healthy donors (n = 15), those with nonsystemic juvenile idiopathic arthritis (JIA; n = 8), patients with SD at onset (n = 19) and remission (n = 18), and those with macrophage activation syndrome (n = 2). Whole-blood NanoString analysis of complement and interferon (IFN)-related gene expression was conducted in patients with SD (active n = 41, inactive n = 33) and JIA (n > 600). Complement products and inflammatory mediators were measured by Luminex and enzyme-linked immunosorbent assay. Functional complement activity was evaluated in SD (active n = 30, inactive n = 67) and JIA sera (n = 12). In vitro assays examined monocytic C1q induction and complement-mediated CD8⁺ T cell activation.

Transcriptomic analysis of monocytes from patients with SD at onset revealed enrichment of the complement cascade compared with patients in remission (adjusted P = 3.7 × 10^-36), ranking among the top 10 up-regulated pathways. Classical complement genes (C1QB/C1QC) were markedly up-regulated in onset SD compared with patients with remission SD and JIA. Patients with active SD showed increased C1q, C3a, C5a, and terminal complement complex protein levels, with enhanced functional classical complement activity. Whole-blood C1QB/C1QC expression correlated with IFN-related markers, including interleukin-18, CXCL9, and CXCL10. Recombinant IFN-γ induced monocytic C1q, whereas C1q enhanced IFN-γ production by CD8⁺ T cells, supporting a feed-forward loop.

SD is characterized by complement activation with marked up-regulation of C1q, which is closely linked to IFN-γ/type II signaling.