Heme-Regulated Differentiation of Monocytes to Macrophages Reveals Interconnections of Hemolysis, Iron Metabolism, and Macrophage Differentiation

During their development, circulation in blood, and demise from senescence, erythrocytes have key interactions with a specific subset of macrophages in bone marrow, splenic red pulp, and hepatic sinusoids (Kupffer cells). Some marrow macrophages are central macrophages of the erythroblastic islands where erythroid precursors complete differentiation into reticulocytes prior to entering circulation. Splenic red pulp macrophages form sheaths through which erythrocytes pass with the physically damaged, immunologically recognized, or senescent erythrocytes being culled by phagocytosis. The normal daily phagocytosis of the most senescent one percent of erythrocytes (about 200 billion cells) and subsequent hemoglobin degradation (with heme being catabolized to iron, carbon monoxide, biliverdin, and bilirubin) make red pulp macrophages the major recyclers of iron, which is required for sufficient hemoglobin production to supply the one percent of reticulocytes that replace the senescent erythrocytes daily. Erythroblastic island-central macrophages phagocytose extruded nuclei with associated hemoglobin (pyrenocytes) that are created as erythroblasts enucleate to become reticulocytes.¹  The reticulocytes, and to a much lesser degree mature erythrocytes, shed hemoglobin-filled vesicles, which have membrane changes similar to senescent erythrocytes that target them for macrophage-mediated phagocytosis, mostly in the liver and bone marrow.²  Free hemoglobin and heme are bound to plasma proteins, haptoglobin and hemopexin, respectively, that in turn bind to the CD163 and CD91 receptors on liver and spleen macrophages leading to endocytosis with subsequent heme degradation.³  Thus, specific macrophages in marrow, splenic red pulp, and liver sinusoids receive and catabolize large amounts of heme while recycling the iron via transferrin to developing erythroblasts, the major consumers of iron. However, even these specialized macrophages can accumulate toxic levels of heme that result in oxidative damage and apoptosis, such as when the inducible enzyme, heme oxygenase-1, that metabolizes heme, is deficient⁴  or when the amount of hemoglobin turnover is massive,⁵  as in severe hemolytic anemia.

The marrow, spleen, and liver macrophages that catabolize heme and recycle iron require the expression of Spi-C, a transcription factor related to PU.1, the major mediator of myeloid differentiation in early-stage hematopoietic progenitor cells.⁶ Using transgenic mice engineered to co-express Spic and green fluorescent protein, Dr. Malay Haldar in the lab of Dr. Kenneth Murphy at Washington University in St. Louis identified the location and characteristics of cells expressing Spic. Such cells were found in the splenic red pulp, the bone marrow, and the liver and were characterized by expression of F4/80, VCAM-1, CD169 (Siglec-1), and CD68, markers of macrophage differentiation, including central macrophages of erythroblastic islands. Subsequent in vitro studies showed that heme directly induces Spicexpression in bone marrow macrophages and monocytes. The spleen and marrow of transgenic mice lost their Spic-expressing macrophages as a consequence of heme toxicity when exogenous heme administration was combined with either genetic deficiency or chemical inhibition of heme oxygenase, or with phenylhydrazine-induced hemolysis. As theSpic-expressing macrophages were killed by heme toxicity, a population of cells with the myeloid marker CD11b and low-level expression of Spic appeared in both of these organs. This myeloid cell population was an intermediate stage of differentiation between monocytes that do not express Spic and the prominently Spic-expressing macrophages. Further investigation demonstrated that, under severe hemolytic stress, monocytes responded to heme by generating the intermediate stage cells that in turn completed differentiation into macrophages, thereby replacing those macrophages previously lost to heme toxicity. The mechanism for this hemolysis-mediated induction of Spicexpression was shown to be heme-mediated enhancement of proteasome-dependent degradation of Bach1, a transcriptional repressor of Spic.