Mixed chimera status in patients with Wiskott-Aldrich syndrome (WAS) after hematopoietic stem cell transplantation ; evaluation by flow cytometric analysis of intracellular WAS protein expression.

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The Wiskott-Aldrich syndrome (WAS) is caused by defects in the WAS protein (WASP) gene on the X-chromosome. Previously we reported that flow cytometric analysis of intracellular WASP expression (FCM-WASP) was useful for the diagnosis of WAS patients and carriers. In this study, we applied FCM-WASP to evaluate the mixed chimera (MC) status of 12 WAS patients who underwent hematopoietic stem cell transplantation (HST). After HST, donor and recipient-derived peripheral blood mononuclear cells (PBMC) could be easily distinguished by this method, as the donor cells are observed to be WASP^<bright>, while the defective recipient cells are WASP^<dim>. Furthermore, by two-color FCM-WASP, the MC status could be characterized by cell lineage. Six of 12 WAS patients were revealed to have the MC status after HST, while others had the complete chimera status. The MC was observed in every cell lineage examined. However, among PBMC, recipient cells were most commonly observed in the monocyte population. Finally, to investigate the naive/memory status of donor and recipient T cells in these patients, three-color FCM-WASP using anti-CD45RA or CD45RO was performed. It was demonstrated that, in contrast to WASP^<bright> T cells, most WASP^<dim> T cells remained naive (CD45RA^+/RO^~) more than one year after HST. No imbalance in the ratio of naive/memory T cells was observed in WAS patients before HST. We conclude that FCM-WASP is a potentially useful method for clinical follow-up of WAS patients who underwent HST. This study may also have significant implications regarding the role of WASP during hematopoietic development.