Comprehensive flow cytometry-based diagnosis of XIAP deficiency

Deficiency of X-linked inhibitor of apoptosis protein (XIAP) is an X-linked recessive inborn error of immunity characterized by abnormal immune responses leading to inflammatory bowel disease and hemophagocytic lymphohistiocytosis. Although XIAP protein expression analysis by flow cytometry (XIAP flow) is commonly used to diagnose XIAP deficiency, certain variants may not affect the protein expression, thereby complicating the diagnostic process. XIAP is crucial for the nucleotide-binding and oligomerization domain 2 (NOD2) signaling pathway. In this study, we aimed to perform a comprehensive analysis of nine patients diagnosed with XIAP deficiency through genetic testing. In addition to XIAP flow, we employed a previously reported method utilizing muramyl dipeptide (MDP) stimulation, a specific agonist of NOD2, to quantitatively evaluate the downstream tumor necrosis factor-alpha (TNFα) production by flow cytometry in patient monocytes (MDP flow). The median mean fluorescence intensity in healthy controls with XIAP flow was 711 (95% confidence interval [CI], 653-815) compared to 195 (95% CI, 161-386) in patients with XIAP deficiency (P < 0.0001). The median percentage of TNFα-producing monocytes in controls with MDP flow was 29.1% (95% CI, 19.6-53.7), while in patients it was 0.34% (95% CI, 0.18-0.82) (P = 0.0008). The receiver operating characteristic curves demonstrated that both XIAP flow and MDP flow exhibited 100% sensitivity and specificity. Taken together, combining XIAP flow and MDP flow analyses allows for a highly accurate diagnosis.