Neutrophil Diversity in the Pathogenesis of Ischemic Acute Kidney Injury

Abstract

Acute kidney injury (AKI) affects millions of patients worldwide yet has few treatment options. There is a critical need to identify novel interventions for AKI, especially approaches targeting cell types that are central to the disease, such as neutrophils. Neutrophils are professional phagocytic cells that respond early to tissue injury. In rodent models of severe ischemic-reperfusion-injury AKI, neutrophils transiently infiltrate the injured kidney, appearing within 6 hours, and are gone by 72 hours. These infiltrating neutrophils are considered proinflammatory and harmful to tissue repair and recovery of kidney function. However, neutrophils can exhibit atypical activity such as antigen presentation and have a central role in recovery from myocardial ischemic injury. Furthermore, little is known of neutrophil polarization, atypical activity, or neutrophil diversity in AKI. Lastly, the kidney generated and renal-protective immunomodulatory protein uromodulin (Tamm-Horsfall Protein, THP) regulates granulopoiesis. In the absence of uromodulin, there is a systemic increase in neutrophils and mouse kidneys are sensitive to injury in AKI. To elucidate neutrophil diversity in AKI and their sensitivity to uromodulin, I performed a series of single-cell sequencing experiments to generate transcriptional profiles of neutrophils from the blood and kidneys of wild-type and THPknockout mice after renal ischemic-reperfusion-injury (IRI). Neutrophil diversity was detected following IRI of the mouse kidney in the blood and kidney. The distribution of subpopulations was sensitive to the kidney milieu. Within the kidney, this diversity and the transcriptional programs of neutrophil subpopulations was sensitive to the severity of ischemic injury. Lastly, Cxcl3 was uniquely upregulated in specific neutrophils after severe ischemic injury. Using single-cell sequencing of uromodulin knock-out mice, I detected the upregulation of toll-like receptor pathways and complement cascades across neutrophil subpopulations in a THP sensitive manner. Furthermore, CXCR2 ligand expression was a combination of moderate and severe injury in wild-type mice. This confirmed previously reported cytokine dysregulation in the uromodulin knock-out mouse after IRI and uncovers a novel role for Cxcl3. Thus, upon revisiting the well-studied neutrophil, I have uncovered novel neutrophil diversity that correlates with recovery of kidney function in AKI and suggests new roles for an old player.