Dimeric Cxcl12 (Sdf-1) Binds To Atypical Chemokine Receptor 1 (Ackr1/Darc)

The chemokine CXCL12/SDF-1 has diverse capability to influence leukocyte migration, tumor metastasis, and hematopoiesis, with signaling pathways previously described through a G-protein coupled receptor (CXCR4) and an atypical chemokine receptor (ACKR3). Here we show that CXCL12 as a dimer is also able to bind to the atypical chemokine receptor 1 (ACKR1), previously known as Duffy antigen/receptor for chemokines (DARC). Using nuclear magnetic resonance spectroscopy and isothermal titration calorimetry, we demonstrate that dimeric CXCL12 binds to the flexible, extracellular N-terminal domain of ACKR1 with low nanomolar affinity, while the binding affinity of monomeric CXCL12 is orders of magnitude lower. Furthermore, ACKR1-transfected cells and primary human Duffy-positive erythrocytes bound and internalized a dimeric CXCL12 construct (CXCL12-LD) but not a constitutively monomeric one (CXCL12-LM) in competition with a professional ligand. Together, these reveal a new interaction between CXCL12 and ACKR1, and provide another layer of regulation for the multiple biological functions of CXCL12, relevant especially in microenvironments with high CXCL12 concentration like the hematopoietic stem cell niche. The capability of ACKR1 to bind CXCL12 and potentially also other chemokines with altered affinity for the dimeric form offers new insights into mechanisms behind known functions of ACKR1 in the in the bone marrow and on venular endothelial cells.