The Tubular Cells Produce Fms-like Tyrosine Kinase Receptor 3 Ligand to Recover Kidney Function after Acute Kidney Injury Disease

INTRODUCTION: Dendritic cells (DC) are involved in the recovery process of acute kidney injury disease (AKD). Fms-like tyrosine kinase receptor 3 ligand (Flt3L) is critical for instructing DC generation and maintenance in organs. In healthy kidney, nonhematopoietic cells produce most Flt3L, rather than immune cells. However, it remains unknown which cells produce Flt3L, and which cellular source drives DC development during AKD. Further, We hypothesized that tubular cells probably produce Flt3L, which improves kidney function after AKD by fostering the accumulation of DC.

METHODS: Blood specimens were collected from patients of prerenal or renal AKD and healthy controls for analysis of serum Flt3L and DC subsets. Wild type mice and IRF8^KO mice were treated with rFlt3L or Flt3 inhibitor gilteritinib. They were subjected to unilateral ischemia-reperfusion (IR) to induce post-ischemic AKD or were injected with cisplatin to induce nephrotoxic AKD. Serum and kidneys were collected and analyzed. Kidney function, tubular injury, primary/proximal tubular cell numbers and Flt3L expression were quantified.

RESULTS: Initially, we observed an increased level of serum Flt3L in patients with AKD. Further analysis revealed that this increase was specific for trauma- rather than heart failure- or sepsis-induced AKD. A similar increase in serum Flt3L was found in mice with IR-AKD but not cisplatin-induced AKD. Flow cytometric analysis revealed a reduced expression level of Flt3L among intrarenal CD4^-CD8⁺T, CD4⁺CD8^-T and, CD4⁺CD8⁺T cells. However, the level of serum Flt3L among Tcrα^ko mice showed no difference with wild type mice after IR-AKD. Conversely, Flt3L-positive cells scattered within tubules in outer stripe of the outer medulla (OSOM). Both mRNA and protein increased within 24 hours after IRI-AKD in renal tissue, which were confirmed by immunohistochemistry staining, in situ hybridization staining, PCR and, RNA-sequencing data of IR kidneys. This was accompanied by reduced peripheral blood DC but increased kidney DC in mice after IR-AKD. The numbers of type I conventional dendritic cell (cDC1) and CD64⁺DC in kidney were significantly decreased in gilteritinib-treated IR-AKD mice, which were associated with more severe tubular injury. With reduced kidney cDC1, IRF8^KO mice also showed worsen kidney injury and aggravated functional failure upon IR-AKD. Therapeutic administration of rFlt3L significantly increased kidney cDC1 upon IR-AKD in wild type but not IRF8^KO mice. This was associated with significantly reduced tubular injury, enhanced proliferation activity of tubular cells, and decreased expressions of tubular injury markers (Ngal, Timp-2, Igfbp-7 and Hgf) in kidney of wild type mice. In addition, pre-treatment of wild type mice with rFlt3L increased kidney cDC1 and protected the mice from severe IR-AKD.

CONCLUSION: Flt3L is upregulated in humans and mice during IR-AKD and probably produced by tubules in OSOM. It fosters the accumulation of kidney cDC1 and, thereby limiting the severity of kidney injury in mice. The current study implies the possibility of Flt3L-dependent DC-based immunotherapy for treatment of AKD.

No relevant conflicts of interest to declare.