Atypical Chemokine Receptor 4 is Expressed in Glomerular Parietal Epithelial Cells and Mitigates the Severity of Experimental Glomerulonephritis

Abstract

Background and Aims Cell migration contributes to the innumerable physiological and pathological processes driven and directed by chemotactic cytokines (chemokines), small structurally homologous proteins signaling via their cognate GPCRs on motile cells. In addition to GPCRs, chemokines importantly interact with atypical chemokine receptors (ACKRs) that are characterized by expression in various stromal cells and distinctive ligand specificities. ACKR4 binds CCL19, CCL20, CCL21, CCL22 and CCL25, chemokines involved primarily in adaptive immune responses. Accordingly, ACKR4 is prominently expressed in the primary and secondary lymphoid organs, thymus, bone marrow, lymph nodes and spleen where it regulates distinct cell migratory steps driven by its cognate ligands. Furthermore, ACKR4 is expressed by dissimilar cell types in multiple parenchymal organs, the heart, the liver and the gut. Currently it is not entirely clear how chemokine scavenging by ACKR4 in these organs might affect their pathophysiology. Here we investigated the expression of ACKR4 in murine kidney and assessed its contribution to nephrotoxic serum nephritis (NTSN), an experimental murine model of immune complex glomerulonephritis. The pathomechanisms in NTSN are known to rely on the contribution of multiple chemokines and their receptors and functions of Th1 cells, Th17 cells, neutrophils and macrophages exerting pathogenic effects in both lymphoid organs and the kidney. Method The expression of renal ACKR4 was evaluated in healthy ACKR4-eGFP reporter mice as well as after the induction of NTSN. To investigate the contribution of ACKR4 to NTSN, ACKR4-deficient mice and WT controls were subjected to an anti-basal membrane immunization protocol and the parameters of immunopathogenesis and the ensuing kidney disease were evaluated at 7 and 14 days after the immunization. Results Multicolour immunofluorescence confocal microscopy revealed that ACKR4 is expressed in the kidney exclusively in the glomeruli by a discrete subset of parietal epithelial cells localizing adjacently to the vascular glomerular pole. In mice with NTSN, the overall proportion of ACKR4 expressing cells in glomeruli at the vascular pole did not change as compared to healthy mice, however ACKR4+ cells were not detected in glomeruli corresponding with the increased abundance of alpha-SMA, a marker of renal fibrosis. The ACKR4-deficient mice showed delayed antibody response following immunization and reduced readouts of cellular immunity. However, despite this, disease parameters of NTSN, including albuminuria, PAS-score and crescent formation were significantly increased in ACKR4-deficient mice as compared to the WT controls. Renin, a hormone secreted by the cells adjacent to ACKR4+ cells in the kidney was significantly increased in serum of nephritic ACKR4-/- mice. Conclusion We report for the first time that ACKR4 is expressed in the kidney by a small subpopulation of parietal epithelial cells. Reduced parameters of humoral and cellular immunity in ACKR4-deficient mice contrast with a more severe manifestations of NTSN seen in these mice, suggesting a tissue-specific functional bias of ACKR4 in the kidney that limits the development of NTSN. Furthermore, the immediate proximity of ACKR4 expressing cells to the juxtaglomerular apparatus and the observed increased renin secretion in ACKR4-/- mice suggest a potential contribution of ACKR4 to the regulation of renin-driven pathways, by a yet unclear mechanism.