Percentage of circulating CD38^high/HLA-DR⁺ CD8⁺ T cells is significantly increased in patients with all forms of secondary HLH and reliably distinguishes patients with MAS from those with active sJIA. (A) Representative fluorescence-activated cell sorter (FACS) profile showing the percentage of peripheral blood CD38^high/HLA-DR⁺ cells, gated on CD3⁺CD8⁺ T cells, in each group of patients. Results obtained in mononuclear cells isolated from bone marrow of 3 patients with primary HLH (BM pHLH) are also reported. (B) Percentage of CD38^high/HLA-DR⁺ cells, gated on CD8⁺ T cells, in the different patient groups. Patients who were receiving glucocorticoids longer than 2 weeks are reported as filled squares. *P ≤ .05; ***P ≤ .001. (C) ROC curve analysis of CD38^high/HLA-DR⁺ CD8⁺ T cells was performed comparing all patients with MAS/sHLH (n = 30) and patients with active sJIA (n = 27).