White blood cells help control levels of red blood cells

Macrophages - white blood cells that play a key role in immune response - also help to both produce and eliminate the body's red blood cells (RBCs), according to a new study.
The findings from researchers at Albert Einstein College of Medicine of Yeshiva University in US and the Icahn School of Medicine at Mount Sinai could lead to novel therapies for diseases or conditions in which the red blood cell production is thrown out of balance.
"Our findings offer intriguing new insights into how the body maintains a healthy balance of red blood cells," said study leader Paul Frenette, professor of medicine and of cell biology and director of the Ruth L and David S Gottesman Institute for Stem Cell and Regenerative Medicine Research at Einstein.

"We've shown that macrophages in the bone marrow and the spleen nurture the production of new red blood cells at the same time that they clear ageing red blood cells from the circulation," Frenette said in a statement.
"This understanding may ultimately help us to devise new therapies for conditions that lead to abnormal RBC counts, such as hemolytic anemia, polycythemia vera, and acute blood loss, plus aid recovery from chemotherapy and bone marrow transplantation," he said.
Previous research has shown that bone marrow macrophages express a cell surface molecule called sialoadhesin, or CD169 - a target that could be used for selectively eliminating macrophages from bone marrow.

Researchers in the current study found that selectively eliminating CD169-positive macrophages in mice reduces the number of bone marrow erythroblasts - evidence that these macrophages are indeed vital for the survival of erythroblasts, which develop into RBCs.

"After we depleted the macrophages in the bone marrow, we discovered that we had also depleted CD169-positive macrophages present in the spleen and liver," Frenette said.
"It turns out that the macrophages in these two organs are quite important in removing old red blood cells from the peripheral circulation. Taken together, the findings show that these macrophages have a dual role, both producing and clearing red blood cells," he said.

The researchers also examined the role of macrophages in polycythemia vera, a genetic disease in which the bone marrow produces too many RBCs, typically leading to breathing difficulties, dizziness, excessive blood clotting and other symptoms.
Using a mouse model of polycythemia vera, they found that depleting CD169-positive macrophages in bone marrow normalises the RBC count.
The study was published in the journal Nature Medicine.