Probes for CCL2

Atypical chemokine receptor 2 (ACKR2) is a chemokine-scavenging receptor. ACKR2-/-embryos display a reduction in size of a novel, to our knowledge, embryonic skin macrophage population referred to as 'intermediate' cells. CC chemokine receptor 2 (CCR2)-/-embryos display an identical phenotype, indicating that these cells require CCR2 to enable them to populate embryonic skin. Further analysis revealed that ACKR2-/-embryos have higher circulating concentrations of the CCR2 ligand, CC ligand 2 (CCL2); thus, ACKR2 regulates intraembryonic CCL2 levels. We show that ACKR2 is strongly expressed by trophoblasts and that it blocks movement of inflammatory chemokines, such as CCL2, from the maternal decidua into the embryonic circulation. We propose that trophoblastic ACKR2 is responsible for ensuring chemokine compartmentalisation on the maternal decidua, without which chemokines enter the embryonic circulation, disrupting gradients essential for directed intraembryonic cell migration. Overall, therefore, we describe a novel, to our knowledge, molecular mechanism whereby maternal decidual chemokines can function in a compartmentalised fashion without interfering with intraembryonic leukocyte migration. These data suggest similar functions for other atypical chemokine receptors in the placenta and indicate that defects in such receptors may have unanticipated developmental consequences.

The atypical chemokine receptor 2 (ACKR2), also named D6, regulates local levels of inflammatory chemokines by internalization and degradation. To explore potential anti-inflammatory functions of ACKR2 in glomerulonephritis, we induced autologous nephrotoxic nephritis in C57/BL6 wild-type and Ackr2-deficient mice. Renal ACKR2 expression increased and localized to interstitial lymphatic endothelium during nephritis. At two weeks Ackr2–/–mice developed increased albuminuria and urea levels compared to wild-type mice. Histological analysis revealed increased structural damage in the glomerular and tubulointerstitial compartments within Ackr2−/− kidneys. This correlated with excessive renal leukocyte infiltration of CD4+ T cells and mononuclear phagocytes with increased numbers in the tubulointerstitium but not glomeruli in knockout mice. Expression of inflammatory mediators and especially markers of fibrotic tissue remodeling were increased along with higher levels of ACKR2 inflammatory chemokine ligands like CCL2 in nephritic Ackr2–/– kidneys. In vitro, Ackr2 deficiency in TNF-stimulated tubulointerstitial tissue but not glomeruli increased chemokine levels. These results are in line with ACKR2 expression in interstitial lymphatic endothelial cells, which also assures efflux of activated leukocytes into regional lymph nodes. Consistently, nephritic Ackr2–/– mice showed reduced adaptive cellular immune responses indicated by decreased regional T-cell activation. However, this did not prevent aggravated injury in the kidneys of Ackr2–/– mice with nephrotoxic nephritis due to simultaneously increased tubulointerstitial chemokine levels, leukocyte infiltration and fibrosis. Thus, ACKR2 is important in limiting renal inflammation and fibrotic remodeling in progressive nephrotoxic nephritis. Hence, ACKR2 may be a potential target for therapeutic interventions in immune complex glomerulonephritis.