A CD57 super(+) CTL Degranulation Assay Effectively Identifies Familial Hemophagocytic Lymphohistiocytosis Type 3 Patients

Familial hemophagocytic lymphohistiocytosis type 3 (FHL3) is a genetic disorder that results in immune dysregulation. It requires prompt and accurate diagnosis. A natural killer (NK) cell degranulation assay is often used to screen for FHL3 patients. However, we recently encountered two cases of late-onset FHL3 carrying novel UNC13D missense mutations: in these cases, the degranulation assays using freshly isolated and interleukin (IL)-2-activated NK cells yielded contradictory results. Since the defective degranulation of CD57 super(+) cytotoxic T lymphocytes (CTLs) in these cases was helpful for making the diagnosis, we assessed whether the CD57 super(+) CTL degranulation assay more effectively identified FHL3 patients than the NK cell assays. Forty additional patients with hemophagocytic lymphohistiocytosis were prospectively screened for FHL3 by measuring the perforin expression in NK cells and the expression of Munc13-4, syntaxin-11, and Munc18-2 in platelets and by performing NK cell and CTL degranulation assays. The results were confirmed by genetic analysis. The freshly isolated NK cell degranulation assay detected FHL3 patients with high sensitivity (100%) but low specificity (71%). The IL-2-stimulated NK cell assay had improved specificity, but 3 out of the 31 non-FHL3 patients still showed degranulation below the threshold level. The CD57 super(+) CTL degranulation assay identified FHL3 patients with high sensitivity and specificity (both 100%). The CD57 super(+) CTL degranulation assay more effectively identified FHL3 patients than the NK cell-based assays.