The CD34⁺CD38⁻ HSC fraction but not the progenitor population in HLH patients is sensitive to engulfment by macrophages. (A) Relationship between the level of CD47 expression and the phagocytosis index tested in vitro. CD34⁺CD38⁻ cells and CD34⁺CD38⁺ cells from 3 HLH patients (unique patient number [UPN] 4, 16, and 22) and healthy controls were cultured with activated macrophages. CD47 expression index represents the relative surface CD47 level calculated as the median CD47 levels of analyzed cells/those in normal blood mononuclear cells. The bar shows the regression line; the coefficient of correlation value was −0.91. (B) The number of CD34⁺CD38⁻ HSCs and CD34⁺CD38⁻ progenitor cells in the BM of HLH patients (n = 14) and healthy controls (n = 6) are shown in the box and whisker plot. HLH patients had significantly decreased numbers of HSC and progenitor populations, but the magnitude of suppression of HSCs was more profound. The bottom and top of the boxes are the 25th and 75th percentile values. The bands in the middle of the boxes represent the 50th percentile (median). Error bars show 1 SD differences above and below the mean of the data. *P < .01 by conventional t test.