Rapid Flow Cytometric Diagnosis of XIAP Deficiency

Introduction X-linked inhibitor of apoptosis protein (XIAP) deficiency is an inborn error of immunity caused by pathogenic variants in XIAP. It presents with a wide range of symptoms, including recurrent hemophagocytic lymphohistiocytosis and inflammatory bowel disease. Previous reports investigated the intracellular tumor necrosis factor-alpha (TNF-α) production capacity following stimulation with muramyl dipeptide (MDP)—a specific agonist of nucleotide-binding and oligomerization domains 2 (NOD2), a specific agonist of XIAP. However, obtaining the analysis results required two days, making it a time-consuming test. Method We established a method to measure the downregulation of L-selectin (CD62L) on the cell surface following MDP stimulation in monocytes and neutrophils from patients with XIAP deficiency using flow cytometry, with the entire process taking a total of 4 hours. Additionally, we measured TNF-α and CD62L levels following MDP stimulation in patients with XIAP deficiency after allogeneic hematopoietic cell transplantation (HCT) to evaluate the utility of this method for functional analysis. Results We analyzed 6 patients with XIAP deficiency. We evaluated the percentage of inhibition (%Inhibition) of the mean CD62L expression levels in monocytes and neutrophils, respectively. The inhibition rates of CD62L expression in patients’ monocytes and neutrophils were 8.5% and 11.3%, respectively, on average, and were significantly lower compared to healthy controls (monocytes: 85.4%, neutrophils: 85.4%). Furthermore, in 3 patients with XIAP deficiency following HCT, we measured TNF-α and CD62L levels after MDP stimulation post-HCT and confirmed that these parameters improved in accordance with donor chimerism. Conclusion In XIAP deficiency, measuring cell surface CD62L following MDP stimulation enabled more rapid functional analysis. Additionally, these analyses proved useful for functional assessment following HCT.