Nucleated Red Blood Cells Secrete Haptoglobin to Induce Immunosuppressive Function in Monocytes

Nucleated red blood cells (NRBCs) are precursors of red blood cells (RBCs), but also possess variety of immunomodulatory effects. However, among the three types of NRBCs, the immunological effects of human CD45- NRBCs remain largely unknown. We have previously shown that cord blood-derived CD45- NRBCs and adult peripheral blood-derived monocytes cocultured in a lipopolysaccharide (LPS)-stimulated indirect coculture system that avoided cell-to-cell contact, increase IL-10 and decrease TNF-α secretion, suggesting an immunosuppressive function of CD45- NRBCs via an unknown soluble factor. The peripheral blood of fetuses and neonates has abundant NRBCs and is physiologically polycythemic, which may lead to the peripheral accumulation of toxic plasma-free hemoglobin. Plasma-free hemoglobin binds to haptoglobin, forming a haptoglobin-hemoglobin complex, which is processed within monocytes via the CD163- heme oxygenase 1 (HO-1) axis and secretes IL-10. Therefore, we hypothesized that NRBCs secrete haptoglobin and induce the immunosuppressive function of monocytes by activating the CD163-HO-1 axis. We found that immunosuppressive response decreased when the coculture medium was supplemented with an anti-CD163 blocking antibody or the HO-1 inhibitor zinc protoporphyrin IX (ZnPP-IX). Haptoglobin levels in the culture medium containing NRBCs were high and expressed the haptoglobin gene. Thus, CD45- NRBCs secreted haptoglobin and activated the immunosuppressive function of monocytes.