Hemophagocytic Lymphohistiocytosis Is Caused by Engulfment of Hematopoietic Stem Cells by Macrophages Through Stem Cell-Specific Suppression of CD47 Under Influence of Cytokine Deregulation

Abstract 522

Hemophagocytic lymphohistiocytosis (HLH) is a syndrome characterized by hemophagocytosis in the bone marrow with systemic inflammatory reactions. We have reported that the inflammatory cytokinemia including interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) might play a key role in activating macrophages to induce HLH (Akashi et al. Br J Haematol, 1994). Recent studies have shown that the engulfment of blood cells by macrophages is regulated in part by the interaction of CD47 and its ligand, signal regulatory protein alpha (SIRPA). Blocking the CD47 and SIRPA interaction by using anti-CD47 monoclonal antibodies can induce engulfment of blood cells, suggesting that the SIRPA signaling provide “don't eat me signals” to prevent macrophages from digesting self blood cells. These data led us to hypothesize that HLH is caused by the disruption of self-recognition by macrophages through impairment of the CD47-SIRPA system. To test this hypothesis, we evaluated the expression level of CD47 in bone marrow cells in 24 patients with HLH. Interestingly, the expression of CD47 was significantly downregulated (by ∼2-fold reduction in average at the protein level) in the CD34+CD38− hematopoietic stem cell (HSC) fraction in HLH patients, whereas that of the CD34+CD38+ progenitor and other mature blood cell fractions was unchanged. We then purified the CD34+CD38− HSCs and CD34+CD38+ progenitor cells from HLH patients and normal controls, co-cultured them with human macrophages in the presence of IFN-gamma and lipopolysaccharide, and evaluated the percentage of phagocyting macrophages. The numbers of phagocyting macrophages were significantly higher in cultures of CD34+CD38− HSCs from HLH patients, as compared to those in cultures of normal HSCs and of progenitor populations, suggesting that the expression level of CD47 inversely correlated with the efficiency of hemophagocytosis. Furthermore, normal HSCs but not CD34+CD38+ progenitors or other mature blood cells down-regulated CD47 in vitro in response to IFN-gamma, TNF-alpha, and IL-6, suggesting that these cytokines plays a critical role in down-regulation of CD47 especially in HSCs. In contrast, the expression level of SIRPA did not differ in myeloid cells between HLH patients and normal controls, and mutations of SIRPA were not found in HLH patients. Thus, in HLH, inflammatory cytokines down-regulate CD47 selectively in HSCs, resulting in the engulfment of HSCs by macrophages. Our data strongly suggest that the cytokine deregulation can lead to HLH in human, through disruption of self-recognition guided by the CD47-SIRPA system at the HSC stage.