# Rapid flow cytometric diagnosis of XIAP deficiency

## TL;DR

This paper introduces a faster flow cytometry method to diagnose XIAP deficiency by measuring CD62L downregulation within 4 hours, compared to previous 2-day methods.

## Contribution

A novel rapid functional assay for XIAP deficiency using CD62L downregulation after MDP stimulation is developed and validated.

## Key findings

- MDP-flow CD62L analysis showed significantly lower CD62L inhibition in XIAP-deficient patients compared to healthy controls.
- The method was effective in assessing post-HCT recovery in XIAP-deficient patients.
- MDP-flow CD62L provides faster results than the previous MDP-flow TNF-α method.

## Abstract

X‐linked inhibitor of apoptosis protein (XIAP) deficiency is an inborn error of immunity caused by pathogenic variants of XIAP. It presents diverse symptoms, including recurrent hemophagocytic lymphohistiocytosis and inflammatory bowel disease. Previous reports established a functional analysis method that quantitatively evaluates intracellular tumor necrosis factor‐alpha (TNF‐α) production capacity following muramyl dipeptide (MDP) stimulation using flow cytometry (MDP‐flow TNF‐α) for assessing XIAP deficiency. However, this method required 2 days to obtain results, which is a limitation.

We established a method to measure the downregulation of L‐selectin (CD62L) on the cell surface after MDP stimulation of monocytes and neutrophils from patients with XIAP deficiency using flow cytometry (MDP‐flow CD62L) within 4 h. Moreover, we also evaluated MDP‐flow CD62L in patients with XIAP deficiency after allogeneic hematopoietic cell transplantation (HCT) to evaluate its usefulness in functional analysis.

Six patients with XIAP variants, two with interleukin‐1 receptor‐associated kinase 4 deficiency, and healthy controls were analyzed. The mean percent inhibition of CD62L expression (%inhibition) was evaluated in monocytes and neutrophils. The mean inhibition rates of CD62L expression in monocytes and neutrophils from patients with XIAP deficiency were 5.96% and 6.20%, respectively, significantly lower than those from healthy controls (monocytes, 85.4%; neutrophils, 85.4%). Furthermore, in three patients with XIAP deficiency after HCT, the MDP‐flow CD62L was evaluated post‐HCT, confirming improvement in accordance with donor chimerism.

In XIAP deficiency, MDP‐flow CD62L enabled faster functional analysis than MDP‐flow TNF‐α. These analyses are also useful for post‐HCT functional assessment.