CD169⁺ Macrophages Regulate Erythropoiesis Under Homeostasis, Recovery From Erythron Injury and in JAK2^V617F-Induced Polycythemia Vera

Abstract 80

The role of macrophages (MΦ) in erythropoiesis was suggested several decades ago with the description of “erythroblastic islands” in the bone marrow (BM) composed of a central MΦ surrounded by developing erythroblasts. This hypothesis was strengthened by in vitro observations using cell culture systems showing that MΦ promote erythroblast proliferation and survival. However, the in vivo role of MΦ in erythropoiesis under homeostasis or disease remains unclear. Central MΦ reportedly express CD169 (or Sialoadhesin), an antigen that specifically marks tissue resident MΦ among mononuclear phagocytes of the bone marrow and spleen. Specific depletion of CD169⁺ MΦ markedly reduced erythroblasts in the BM (40.4+1.8%) but did not result in overt anemia under homeostasis, likely due to concomitant compensatory splenic erythropoiesis and alterations in RBC clearance. However, MΦ depletion significantly impaired erythroid recovery from PHZ-induced hemolytic anemia (reticulocytes: 8.2-fold lower, p<0.01 and hematocrit: 2-fold lower, p<0.01 on day 6 post-PHZ challenge) and acute blood loss (reticulocytes: 3.2-fold lower, p<0.001 and hematocrit: 1.6-fold lower, p<0.001 on day 4 post-phlebotomy). Furthermore, depletion of CD169⁺ MΦ in the BM and spleen impaired erythroblast recovery seven days after bone marrow transplantation (BM: 8.2-fold lower, p<0.01 and spleen: 120-fold lower, p<0.05 on day 7 post-BMT) and delayed recovery of reticulocyte numbers (4-fold lower, p<0.001 on day 10 post-BMT) and hematocrit (1.1-fold lower, p<0.05 on day 14 post-BMT). Mechanistically, we observed a rapid drop in reticulocyte hemoglobin content (CHr) in CD169⁺ MΦ-depleted animals starting four days post-BMT, but iron supplementation was unable to correct the impaired expansion of erythroblasts, suggesting other mechanisms. We determined that VCAM-1 expressed by BM CD169⁺ MΦ and BMP4 derived from splenic red pulp macrophages were critical for the efficient recovery of the erythron after BMT. Moreover, depletion of host-derived, radioresistant macrophages shortly after transplantation was sufficient to delay erythroblast recovery, implicating a critical role for this population until donor-derived macrophages can repopulate post-BMT. In addition, we characterized a CD169⁺ VCAM1⁺ MΦ population in human BM aspirates that represents the first step in clinically targeting the analogous BM resident macrophage population in humans. Since CD169⁺ MΦ support recovery after erythropoietic injury, we hypothesized that MΦ depletion could potentially normalize the erythron in a JAK2^V617F-driven murine model of polycythemia vera (PV). Indeed, we observed that MΦ depletion in PV mice reduced erythroblasts in the BM (1.6-fold lower, p<0.05 after 4 weeks of depletion) and spleen (14-fold lower, p<0.01 after 4 weeks of depletion). This reduction of the expanded PV erythron was associated with an efficient (within 20 days of MΦ depletion) and durable (up to 40 days after last depletion) normalization of the hematocrit. A rapid and durable reduction in CHr was observed after MΦ depletion in PV mice, but systemic iron chelation did not produce the same effect as MΦ depletion, further confirming the contribution of additional mechanisms. MΦ depletion abrogated the induction of BMP4 (3.4-fold lower, p<0.001) and stress erythropoiesis (stress BFU-E: 790-fold reduction, p<0.05) in the spleen. Importantly, MΦ depletion reduced the number of erythropoietin-independent colonies in the spleen of PV mice (endogenous BFU-E: 29-fold lower, p<0.05 and endogenous CFU-E: 1400-fold lower, p<0.05), indicating that erythropoiesis in PV, unexpectedly, remains under the control of MΦ in the BM and splenic microenvironments. Altogether, these studies strongly support the notion that CD169⁺ MΦ promote erythrocyte development and that modulation of the MΦ compartment represents a novel strategy to treat erythropoietic disorders.