Whole-genome expression profiling of embryonic and monocyte-derived Kupffer cells

Iron is an essential component of the erythrocyte protein hemoglobin and is crucial to oxygen transport in vertebrates. In the steady state, erythrocyte production is in equilibrium with erythrocyte removal1. In various pathophysiological conditions, erythrocyte life span is severely compromised, which threatens the organism with anemia and iron toxicity 2,3. Here we identify anon-demand mechanism specific to the liver that clears erythrocytes and recycles iron. We showthat Ly-6Chigh monocytes ingest stressed and senescent erythrocytes, accumulate in the liver, and differentiate to ferroportin 1 (FPN1)-expressing macrophages that can deliver iron to hepatocytes. Monocyte-derived FPN1+ Tim-4neg macrophages are transient, reside alongside embryonically-derived Tim-4high Kuppfer cells, and depend on Csf1 and Nrf2. The spleenlikewise recruits iron-loaded Ly-6Chigh monocytes, but they do not differentiate into ironrecycling macrophages due to the suppressive action of Csf2, and are instead shuttled to the livervia coordinated chemotactic cues. Inhibiting this mechanism by preventing monocyte recruitment to the liver leads to kidney failure and liver damage. These observations identify the liver as the primary organ supporting emergency erythrocyte removal and iron recycling, and uncover a mechanism by which the body adapts to fluctuations in erythrocyte integrity. Pregnant CX3CR1-CreERT+/+ Stopfl/fl tdTomato mice were injected subcutaneously with tamoxifen (8 mg/animal in 400 µl corn oil s.c.) on days 13.5and 15.5 p.c to induce tdTomato expression in embryonic monocytes/macrophages. At the age of 3 months, the progeny was transplanted with GFP bone marrow after busulfan pre-conditioning. After establishment of stable chimerism among blood leukocytes (3 weeks), mice were transfused with stressed red blood cells (sRBC). Embryonic (eKCs, CD45+ CD11b- F4/80+ GFP- tdTomato+) and monocyte-derived Kupffer cells (mKCs, CD45+ CD11b- F4/80+ GFP+ tdTomato-) of the liver were FACS-sorted 7 days after sRBC treatment and subjected to whole-genome expression profiling