Efficacy of T-cell assays for the diagnosis of primary defects in cytotoxic lymphocyte exocytosis

•TCR-triggered T-cell and Fc receptor–triggered NK-cell assays are most accurate for diagnosing defective cytotoxic lymphocyte exocytosis.•The standard K562 cell assay has high interindividual variability and is affected by expanded NK-cell subsets and transportation stress. [Display omitted] Primary hemophagocytic lymphohistiocytosis (HLH) is a life-threatening disorder associated with autosomal recessive variants in genes required for perforin-mediated lymphocyte cytotoxicity. A rapid diagnosis is crucial for successful treatment. Although defective cytotoxic T lymphocyte (CTL) function causes pathogenesis, quantification of natural killer (NK)–cell exocytosis triggered by K562 target cells currently represents a standard diagnostic procedure for primary HLH. We have prospectively evaluated different lymphocyte exocytosis assays in 213 patients referred for evaluation for suspected HLH and related hyperinflammatory syndromes. A total of 138 patients received a molecular diagnosis consistent with primary HLH. Assessment of Fc receptor–triggered NK-cell and T-cell receptor (TCR)–triggered CTL exocytosis displayed higher sensitivity and improved specificity for the diagnosis of primary HLH than routine K562 cell–based assays, with these assays combined providing a sensitivity of 100% and specificity of 98.3%. By comparison, NK-cell exocytosis after K562 target cell stimulation displayed a higher interindividual variability, in part explained by differences in NK-cell differentiation or large functional reductions after shipment. We thus recommend combined analysis of TCR-triggered CTL and Fc receptor–triggered NK-cell exocytosis for the diagnosis of patients with suspected familial HLH or atypical manifestations of congenital defects in lymphocyte exocytosis. Primary hemophagocytic lymphohistiocytosis is a hyperinflammatory disorder caused by biallelic loss-of-function mutations in genes required for CD8 T-cell and natural killer (NK)–cell cytotoxicity. Diagnosis requires a constellation of clinical and laboratory findings, including functional impairment in NK-cell cytotoxicity, which can be challenging. Chiang et al report on an encouraging novel in vitro assay that quantifies cytotoxic lymphocyte exocytosis more accurately than standard K562 cell line–based assays, warranting further assessment for routine incorporation into diagnostic workups.