Background: Acute kidney injury (AKI) affects about 10–15% of individuals hospitalized, and its occurrence escalates to over 50% among those in intensive care units, with ischemia-reperfusion injury (IRI) being one of the primary etiologies. Neutrophils initially infiltrate the area of injury after ischemia-reperfusion, however, the chemotactic mechanism of injured tubular cells for neutrophils remains unclear.
Methods: We analyzed single-cell transcriptomics from AKI mouse models to elucidate the interactions between injured tubular cells and neutrophils. Subsequently, we verified the spatial expression locations of Spp1 and Cd44 through spatial transcriptomics. Finally, mRNA and protein expression were validated in both AKI mouse and hypoxia/reoxygenation (H/R) cell models, with Transwell assays confirming that injured tubular cells induce chemotaxis in neutrophils via the Spp1/Cd44 axis.
Results: Upon analyzing 53,652 cells, we observed a higher number of neutrophils and identified injured tubular cells in AKI mouse kidneys compared to sham controls. Trajectory analysis showed differentiation of normal tubular cells towards injured and pro-fibrotic states, with differential genes enriched in neutrophil chemotaxis and migration pathways. Cell-cell communication analysis suggested that elevated Spp1 in injured tubular cells promoted neutrophil migration through Spp1-Cd44 interactions.

Co-expression and spatial transcriptomics analyses corroborated the synchronous expression of Spp1 and Cd44. RT-qPCR and Western blot analysis confirmed that in the AKI and H/R models, the mRNA and protein expressions of Spp1, Cd44, and Havcr1 were upregulated. Immunohistochemistry indicated increased Spp1 expression in the kidneys of AKI mouse compared to the sham control. Cell migration assays suggested an increase in neutrophil migration under H/R conditions, which was reduced upon inhibition of Spp1 activity with 5-ASA.

Conclusions: During the onset of AKI, the injured tubular cells upregulate the expression of Spp1, which interacts with the Cd44 receptor on the surface of neutrophils, inducing neutrophil migration and exacerbating kidney injury.