The macrophage CD163 surface glycoprotein is an erythroblast adhesion receptor

Abstract

Erythropoiesis occurs in erythroblastic islands, where developing erythroblasts closely interact with macrophages. The adhesion molecules that govern macrophage-erythroblast contact have only been partially defined. Our previous work has implicated the rat ED2 antigen, which is highly expressed on the surface of macrophages in erythroblastic islands, in erythroblast binding. In particular, the monoclonal antibody ED2 was found to inhibit erythroblast binding to bone marrow macrophages. Here, we identify the ED2 antigen as the rat CD163 surface glycoprotein, a member of the group B scavenger receptor cysteine-rich (SRCR) family that has previously been shown to function as a receptor for hemoglobinhaptoglobin (Hb-Hp) complexes and is believed to contribute to the clearance of free hemoglobin. CD163 transfectants and recombinant protein containing the extracellular domain of CD163 supported the adhesion of erythroblastic cells. Furthermore, we identified a 13-amino acid motif (CD163p2) corresponding to a putative interaction site within the second scavenger receptor domain of CD163 that could mediate erythroblast binding. Finally, CD163p2 promoted erythroid expansion in vitro, suggesting that it enhanced erythroid proliferation and/or survival, but did not affect differentiation. These findings identify CD163 on macrophages as an adhesion receptor for erythroblasts in erythroblastic islands, and suggest a regulatory role for CD163 during erythropoiesis. IntroductionThe functional unit for definitive erythropoiesis in the bone marrow is the erythroblastic island, a multicellular structure composed of a central macrophage surrounded by erythroblasts at various stages of differentiation. [1][2][3] The contact between erythroblasts and macrophages supports the growth, survival, and differentiation of erythroblasts, and allows for phagocytosis of the extruded erythroid nucleus. Thus far, the molecular interaction(s) that mediate the formation of erythroblastic islands have only been partly defined. 2,4 First, interactions between vascular cell adhesion molecule-1 (VCAM-1) on macrophages and ␣ 4 integrins on erythroblasts have been implicated in the contact between these cells. 5 Furthermore, a molecule called erythroblast macrophage protein (Emp) has been identified, which is expressed on macrophages, erythroblasts, and other cells. [6][7][8] Emp is believed to mediate erythroblast adhesion to macrophages, probably by homophilic interaction. Of relevance, Emp-deficient fetuses, which die perinatally, have significantly reduced numbers of erythroblastic islands and defective erythropoiesis. This phenotype appears to result from a deficiency in both macrophage development as well as disturbed erythroblast nuclear extrusion. Finally, the erythroid intercellular adhesion molecule-4 (ICAM-4) has been demonstrated to bind to the ␣ v integrin expressed by macrophages in erythroblastic islands, and this has also been shown to contribute to erythroblastic island formation in vivo. 9,10 We have previously identified the rat mac...

Extrinsic and intrinsic control by EKLF (KLF1) within a specialized erythroid niche

“…Extensive macrophage-erythroblast and erythroblast-erythroblast adhesive interactions are necessary for a thriving definitive erythropoietic community. As a result, structural proteins that mediate these interactions (Fabriek et al, 2007;Lee et al, 2006;Liu et al, 2007;Mankelow et al, 2004;Sadahira et al, 1995;Soni et al, 2006) as well as transcriptional factors (Gutiérrez et al, 2004;Iavarone et al, 2004;Kusakabe et al, 2011) are both crucial for promoting an effective differentiative environment. Erythroblasts can proliferate, mature and enucleate in vitro in the absence of other cell types; however, this process is typically very inefficient at all stages (Hanspal et al, 1998;Rhodes et al, 2008).…”

Wild-type macrophages reverse disease in heme oxygenase 1-deficient mice

“…20 A subset of tissue macrophages known as alternatively activated (M2) macrophages 21 are the only cells in the mouse that express significant amounts of the Hp-Hb complex scavenging receptor, CD163 (also known as ED2). [22][23][24] CD163 mRNA expression was very low in the BM, spleens, and livers of Hmox1 2/2 animals, and the clearance of released Hb from the bloodstream of these animals was minimal. We found significantly increased CD163 expression in the KO BMT group in comparison with KO Ctr ( Figure 6B).…”

“…Thus, the restoration of CD163-expressing macrophages may ameliorate the previously unexplained anemia in Hmox1 2/2 animals by providing a supportive environment for effective erythropoiesis. 23,25 To explore whether our conclusions about the loss of CD163 1 macrophages were relevant to the pathophysiology of Hmox1…”

CD163-L1 Is an Endocytic Macrophage Protein Strongly Regulated by Mediators in the Inflammatory Response

“…CD163 has been identified and characterized as an endocytic receptor for the haptoglobin-hemoglobin (Hp-Hb) complex and is believed to be essential for the removal of free hemoglobin by macrophages during hemolysis (8)(9)(10). In addition, CD163 has also been identified as a potential erythroblast adhesion receptor and as a novel pattern recognition receptor for bacteria (11,12). Indeed, the ability to bind microbial microorganisms has been shown for several members of the SRCR superfamily including SR-AI, gp-340, MARCO, CD5, CD6, and Spa, suggesting a possible unifying function as pattern recognition receptors in host defense (13)(14)(15)(16)(17)(18).…”