Hematopoietic cell transplantation for hemophagocytic lymphohistiocytosis: recent advances and controversies

Hemophagocytic lymphohistiocytosis (HLH) is a life-threatening hyperinflammatory clinical syndrome of uncontrolled immune response which results in hypercytokinemia due to underlying primary or secondary immune defect. A number of genetic defects in transport, processing and function of cytotoxic granules which result in defective granule exocytosis and cytotoxicity of cytotoxic T lymphocytes (CTL) and natural killer (NK) cells have been well identified at the cellular and molecular level. Important advances have been made during the last 20 years in the diagnosis and treatment of HLH. The Histiocyte Society has proposed diagnostic guideline using both clinical and laboratory findings in HLH-2004 protocol, and this has been modified partly in 2009. HLH used to be a fatal disease, but the survival of HLH patients has improved to more than 60% with the use of chemoimmunotherapy combined with hematopoietic cell transplantation (HCT) over the past 2 decades. However, HCT is still the only curative option of treatment for primary HLH and refractory/relapsed HLH after proper chemoimmunotherapy. The outcome of HCT for HLH patients was also improved steadily during last decades, but HCT for HLH still carries significant mortality and morbidity. Moreover, there remain ongoing controversies in various aspects of HCT including indication of HCT, donor selection, timing of HCT, conditioning regimen, and mixed chimerism after HCT. This review summarized the important practical issues which were proven by previous studies on HCT for HLH, and tried to delineate the controversies among them.