“The liver, not the spleen, is the major on-demand site of red blood cell elimination and iron recycling,” according to Filip Swirski, PhD, of the Massachusetts General Hospital Center for Systems Biology, and his colleagues.

The liver relies on a buffer system consisting of bone marrow–derived monocytes that consume damaged red blood cells (RBCs) in the blood and settle in the liver, where they become the transient macrophages capable of iron recycling, the researchers concluded in a study published in Nature Medicine .

The study was designed to examine how the body clears old and damaged RBCs without releasing toxic levels of free iron. Damaged RBCs can release unbound forms of iron-carrying hemoglobin, which can cause kidney injury and can lead to anemia.

The researchers used several different models of RBC damage to examine RBC clearance and iron recycling in a mouse model. Damaged RBCs in the bloodstream prompted an increase in monocytes that took up the damaged cells and traveled to both the liver and the spleen. Within hours, almost all of the damaged RBCs were located within specialized macrophages seen only in the liver. Chemokines drew the monocytes to the liver, resulting in the accumulation of the iron-recycling macrophages.

Monocytes that express high levels of lymphocyte antigen 6 complex, locus C1 ingest stressed and senescent erythrocytes, accumulate in the liver via coordinated chemotactic cues, and differentiate into ferroportin 1–expressing macrophages that can deliver iron to hepatocytes. Monocyte-derived FPN1+Tim-4neg macrophages are transient, reside alongside embryonically derived T-cell immunoglobulin and mucin domain containing 4high Kupffer cells, and depend on the growth factor Csf1 and the transcription factor Nrf2, the researchers wrote.

Blocking that process resulted in impaired RBC clearance, toxic levels of free iron and hemoglobin, and signs of liver and kidney damage.

“If overactive, (the mechanism) could remove too many RBCs, but if (the mechanism is) sluggish or otherwise impaired, it could lead to iron toxicity. Further study could provide us with details of how this mechanism occurs in the first place and help us understand how to harness or suppress it in various conditions,” Dr. Swirski said in a press release.

The researchers had no financial conflicts of interest. The study was funded by the National Institutes of Health.

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