Tissue-infiltrating CD8⁺T cells in patients with HLH are highly activated, IFN-γ–producing T cells. (A) Representative FACS plots of CD8⁺ T cells comparing blood and CSF or bone marrow in samples obtained concurrently from 3 different patients. (B) Cumulative data for the indicated markers in samples of CSF (CS), lymph node (LN), or bone marrow (BM) from 10 patients with HLH. CD38^high/HLA-DR⁺ cells are displayed as a percentage of CD3⁺/CD8⁺ cells, the other markers are displayed as a percentage of CD8/CD38^high/HLA-DR⁺ cells. (C) Representative FACS plots and cumulative data of intracellular cytokine staining for IFN-γ, as a percentage of CD3⁺/CD8⁺ cells in either CSF or bone marrow. (D) Comparison of IFN-γ staining of CD8⁺/CD38^high T cells after stimulation of concurrent peripheral blood (BL) and bone marrow samples from 2 patients.