CD163 is the macrophage Hb scavenger receptor: the crucial role of Hp at low ligand concentrations. (A) CD163 is involved in Hb uptake in human macrophages. Human monocytes purified by plastic adherence and cultured for 1 day show low expression levels of CD163, and, as expected, these cells have a low Hbfl-uptake capacity. After 8-day culture in the presence of human serum, an increase occurs in CD163 expression (as measured by staining with FITC anti-CD163 [clone 5C6-FAT] and subsequent FACS analysis) and in Hbfl uptake. Polyclonal rabbit IgG directed against CD163 (which is known to inhibit CD163/Hb-Hp interaction) almost completely inhibits macrophage uptake of Hbfl. Data represent mean ± SD of 3 experiments performed with monocytes obtained from 3 different donors. (B) Hp increases Hb uptake of macrophages at low (1 μg/mL), but not at high (100 μg/mL), ligand concentrations. Human macrophages were incubated with increasing concentrations of either free Hbfl or Hbfl-Hp for 30 minutes. Cell-associated fluorescence was determined by FACS. Data represent the mean ± SD of triplicate values obtained from a representative experiment.