Absolute Counts of Circulating T Follicular Helper Cell Subsets in Acute‐Phase AQP4 ‐ IgG –Positive Neuromyelitis Optica Spectrum Disorder

ABSTRACT Objectives Neuromyelitis optica spectrum disorder (NMOSD) is an autoimmune inflammatory disease characterized by pathogenic anti‐aquaporin‐4 (AQP4) antibodies. Previous studies have shown increased circulating T follicular helper (cTfh) cells and altered Tfh subset frequencies in NMOSD, but the absolute counts of Tfh subsets remain unclear. We evaluated relative frequencies and absolute counts of cTfh subsets in acute‐phase AQP4‐IgG–positive NMOSD. Methods This retrospective single‐center study included 12 patients with AQP4‐IgG–positive NMOSD and seven disease controls. Flow cytometry assessed the frequency of cTfh cells among CD4 + T cells, the relative frequencies of Tfh1, Tfh2, and Tfh17 subsets, and their absolute counts. Acute‐phase NMOSD was compared with chronic‐phase NMOSD and disease controls. Results The frequency of cTfh cells among CD4 + T cells was significantly higher in acute‐phase NMOSD than in chronic‐phase NMOSD (15.7% vs. 12.7%, p = 0.026) and disease controls (15.7% vs. 12.2%, p = 0.012). Neither the relative frequencies of Tfh1, Tfh2, and Tfh17 subsets nor the (Tfh2 + Tfh17)/Tfh1 ratio differed significantly between groups. However, the absolute Tfh1 count was significantly higher in acute‐phase NMOSD than in chronic‐phase NMOSD (72.5 vs. 30.7 cells/μL, p = 0.034), whereas the absolute Tfh2 count was significantly higher than that in disease controls (50.5 vs. 32.4 cells/μL, p = 0.038). Conclusions Acute‐phase AQP4‐IgG–positive NMOSD is characterized by a higher absolute Tfh1 count than chronic‐phase NMOSD and a higher absolute Tfh2 count than disease controls, together with an increased cTfh frequency, despite unchanged relative Tfh‐subset frequencies and an unchanged (Tfh2 + Tfh17)/Tfh1 ratio. Analysis of absolute cTfh‐subset counts may complement conventional assessments based on relative Tfh‐subset frequencies and the (Tfh2 + Tfh17)/Tfh1 ratio, providing additional insight into the immunopathogenesis of NMOSD.