Eosinophils promote monocyte to macrophage differentiation and anti-bacterial immunity

Chemokine receptors control cell migration within the body. Here we reveal a novel interaction between eosinophils and monocytes in the bone marrow, indirectly controlled by the atypical chemokine receptor ACKR2. We demonstrate that ACKR2 maintains eosinophil levels within the bone marrow by scavenging CCL11. In the absence of ACKR2, elevated CCL11 leads to increased egress of eosinophils from the bone marrow into the bloodstream. As a result, eosinophil and monocyte interactions are reduced within the bone marrow niche, leading to changes in monocyte gene expression. Monocytes from ACKR2-/- mice are recruited to the tissues but are fundamentally altered in their ability to differentiate into macrophages, in the lung, peritoneal cavity and cavity wall. Bacterial elimination is impaired in ACKR2-/- mice during peritoneal infection. ACKR2 is therefore a key regulator of eosinophil-driven monocyte education in the bone marrow, required for full monocyte differentiation and macrophage function within the tissues.