Patient with SMBA and CAEBV disease progressing to lethal HLH shows expansion of hyperinflammatory memory NK cells absent in her MZ twin. (A) Titers of reactive immunoglobulin G (IgG) antibodies against EBV viral capsid antigen and early antibody (Ea) antigens in sera from patient and MZ twin 6 months after referral. (B) CD3⁺, CD4⁺, CD8⁺, and CD56⁺CD16 lymphocyte counts from the patient and her twin at 6 months after referral. (C) Procedure for in vitro expansion of NK cells from PBMCs. (D) t-distributed stochastic neighbor embedding (t-SNE) clustering of expanded NK cells from the patient and her MZ twin based on the expression of 24 NK cell activating and inhibitory receptors. Clusters labeled by donor (red, patient and orange, MZ twin) (left), and phenograph of further subpopulations (right), are shown. (E) Surface expression of NK cell receptors in each phenograph subpopulation as a heat map of z scores of the mean fluorescence intensity of each receptor. (F) Frequency of NK cells from the MZ twin and patient in each phenograph subpopulation. (G) Concentration of IFN-γ produced by PBMCs after stimulation with IL-2 (80 IU/mL), IL-12 (100 μg/mL), IL-18 (100 μg/mL), or IL-12 with IL-18. (H) Percentage of CD56^dimCD16^dim NK cells among the total IFN-γ⁺ PBMCs from the patient and her MZ twin when unstimulated (H₂O), or when stimulated with IFN-α (1000 IU/mL) + IL-2 or PMA (phorbol 12-myristate 13-acetate; 5 ng/mL)/ionomycin (iono; 1 μg/mL). (I) Histogram of intracellular IFN-γ staining among CD56^dimCD16^dim NK cells from the patient and her MZ twin when unstimulated (H2O) or stimulated with IFNa + IL-2 or PMA/ionomycin, normalized to the mode of cell counts (left) and percentage of cells gated IFN-γ⁺ among CD56^dimCD16^dim NK cells (right) are shown. Data are shown from single replicates due to the limited amount of primary sample.