Identification of the haemoglobin scavenger receptor

Intravascular haemolysis is a physiological phenomenon as well as a severe pathological complication when accelerated in various autoimmune, infectiou s (such as malaria) and inherited (such as sickle cell disease) disorders(1 ). Haemoglobin released into plasma is captured by the acute phase protein haptoglobin, which is depleted from plasma during elevated haemolysis(1). H ere we report the identification of the acute phase-regulated and signal-in ducing macrophage protein, CD163, as a receptor that scavenges haemoglobin by mediating endocytosis of haptoglobin-haemoglobin complexes. CD163 binds only haptoglobin and haemoglobin in complex, which indicates the exposure o f a receptor-binding neoepitope. The receptor-ligand interaction is Ca2+-de pendent and of high affinity. Complexes of haemoglobin and multimeric hapto globin (the 2-2 phenotype) exhibit higher functional affinity for CD163 tha n do complexes of haemoglobin and dimeric haptoglobin (the 1-1 phenotype). Specific CD163-mediated endocytosis of haptoglobin-haemoglobin complexes is measurable in cells transfected with CD163 complementary DNA and in CD163- expressing myelo-monocytic lymphoma cells.