To ascertain the clinical significance of two important costimulatory molecules required for T-cell activation in the peripheral blood of patients with NMOSD, we examined the expression levels of a membrane- and soluble-type inducible costimulatory molecule (ICOS), its ligand (ICOSL), programmed death-1 (PD-1), and its ligand (PD-L1) in the peripheral blood of 30 patients with NMOSD and compared these levels with those in patients with longitudinally extensive transverse myelitis (LETM), those with optic neuritis (ON), and healthy controls (HCs). Here, PDCD1 is linked to optic neuritis.