CD163 is the macrophage scavenger receptor for native and chemically modified hemoglobins in the absence of haptoglobin

Abstract

CD163 mediates the internalization of hemoglobin-haptoglobin (Hb-Hp) complexes by macrophages. Because Hp binding capacity is exhausted during severe hemolysis, an Hp-independent Hb-clearance pathway is presumed to exist. We demonstrate that Hb interacts efficiently with CD163 in the absence of Hp. Not only is Hb internalized into an endosomal compartment by CD163 as a result of active receptordependent endocytosis; it also inhibits the uptake of Hb-Hp complexes, suggesting a common receptor-binding site. Free Hb further induces heme oxygenase mRNA expression in CD163 ؉ HEK293 cells, but not in CD163 ؊ cells. Additional evidence for Hp-independent Hb-CD163 interaction is provided by the demonstration that CD163 mediates the uptake of ␣␣-DBBF crosslinked Hb, a chemically modified Hb that forms minimal Hp complexes. Moreover, certain modifications to Hb, such as polymerization or the attachment of specific functional groups (3 lysyl residues) to the ␤-Cys93 can reduce or enhance this pathway of uptake. In human macrophages, Hp-complex formation critically enhances Hb uptake at low (1 g/mL), but not at high (greater than 100 g/mL), ligand concentrations, lending support for a concentrationdependent biphasic model of macrophage Hb-clearance. These results identify CD163 as a scavenger receptor for native Hb and small-molecular-weight Hb-based blood substitutes after Hp depletion. IntroductionHeme, which is mainly derived from hemoglobin (Hb), is a strong oxidant and has potent pro-inflammatory properties. These properties become apparent if the intricate intra-erythrocytic compartmentalization of heme is compromised after the destruction of erythrocytes. [1][2][3][4] Large quantities of free hemoglobin can be found in the circulation of patients who have undergone transfusion with cell-free hemoglobin products as a blood substitute. 5 Macrophages are the primary scavengers of Hb after systemic hemolysis and during wound healing. These cells also play a key role in the clearance of exogenously administered blood substitutes. 6 CD163 is a member of the cysteine-rich scavenger receptor family and is exclusively expressed by cells of monocyte/ macrophage lineage. 7 Resident tissue macrophages contain the highest levels of CD163, most notably Kupffer cells in the liver and macrophages within the bone marrow and spleen red pulp. [8][9][10] To date, the Hb-haptoglobin (Hp) complex is the only known ligand of CD163, 11,12 and neither Hp alone nor free Hb has been found to display high-affinity binding to the receptor. Because the Hb-Hp complex binds to CD163 with high affinity and the receptor system has a high endocytotic capacity, CD163 is thought to mediate the clearance of Hb-Hp complexes from the blood. 13 Several lines of evidence indicate that CD163 plays a key role in the anti-inflammatory and wound-healing process. First, there is a high level of CD163 expression by macrophages during the down-regulatory phase of inflammatory reactions. 8,14 Second, CD163 expression is strongly induced by glucocorticoids 15,16 and...

Iron transport proteins: Gateways of cellular and systemic iron homeostasis

“…Hemoglobin in the plasma binds to the acutephase protein haptoglobin to form a hemoglobin-haptoglobin complex that is recognized by the scavenger receptor CD163, which is exclusively expressed on macrophages (61,62). Hemoglobin that is not bound to haptoglobin can also be taken up by CD163 (63). Interestingly, hemoglobin clearance from the plasma is largely unaffected in mice lacking CD163 (64), indicating that alternative hemoglobin and hemoglobin-haptoglobin clearance pathways exist (such as in hepatocytes as noted below).…”

The Mononuclear Phagocyte System in Homeostasis and Disease: A Role for Heme Oxygenase-1

“…On the other hand, severe hemolysis rapidly depletes Hp levels (140,169). On the surface of circulating monocytes or tissue macrophages, CD163 binds to Hb complexed with Hp at high affinity or, when Hp is depleted, to free Hb with low affinity (151). Ligation of the CD163 receptor by Hp:Hb leads to receptor-mediated endocytosis and clearance of Hb.…”

An alternative view of the proposed alternative activities of hemopexin

“…For example, Hb exhibits low affinity binding in vitro to the Hb-Hp receptor CD163 on macrophages. 116 Although any number of mechanisms could be proposed as to how hemopexin associates with the apoA-1 containing HDL (e.g., through recognition of the carbohydrate portion of hemopexin or through specific or nonspecific electrostatic interactions), establishment of an anti-inflammatory role for hemopexin in this aspect of atherosclerosis requires additional evidence.…”